ASSESSMENT REPORT ON

BBEENNZZEENNEE FOR DEVELOPING

AMBIENT AIR QUALITY

OBJECTIVES

ASSESSMENT REPORT ON

BENZENE

FOR DEVELOPING AMBIENT AIR QUALITY OBJECTIVES

Prepared by Toxico-Logic Consulting Inc.

for Alberta Environment

February 2006

ISBN: 978-1-4601-0472-9 (Print) ISBN: 978-1-4601-0473-6 (Online) Web Site: http://www.environment.alberta.ca/

Although prepared with funding from Alberta Environment (AENV), the contents of this report/document do not necessarily reflect the views or policies of AENV, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.

Any comments, questions, or suggestions regarding the content of this document may be directed to:

Air Policy Alberta Environment and Sustainable Resource Development 9th Floor, Oxbridge Place 9820 – 106th Street Edmonton, Alberta T5K 2J6

Additional copies of this document may be obtained by contacting:

Information Centre Alberta Environment and Sustainable Resource Development Phone: (780) 427-2700 Fax: (780) 422-4086 Email: env.infocent@gov.ab.ca

mailto:env.infocent@gov.ab.cahttp:http://www.environment.alberta.ca

FOREWORD

Alberta Environment maintains Ambient Air Quality Objectives to support air quality

management in Alberta. Alberta Environment currently has ambient objectives for more than

thirty substances and five related parameters. These objectives are periodically updated and new

objectives are developed as required.

With the assistance of the Clean Air Strategic Alliance, a multi-stakeholder workshop was held

in October 2004 to set Alberta’s priorities for the next three years. Based on those

recommendations to Alberta Environment, a three-year work plan was developed to review four

existing objectives, and create three new objectives.

This document is one in a series of documents that presents the scientific assessment for these

substances.

Laura Blair

Project Manager

Air Policy

Assessment Report on Benzene for Developing Ambient Air Quality Objectives i

ACKNOWLEDGEMENTS

The authors of this Assessment Report for Benzene would like to thank Laura Blair of the

Environmental Policy Branch of Alberta Environment for her input. Toxico-Logic Consulting

Inc. would also like to acknowledge and thank the following authors who participated in the

completion of this report:

Dr. Selma Guigard

Edmonton, Alberta

Dr. Warren Kindzierski

WBK & Associates Inc.

St. Albert, Alberta

Jason Schulz

Calgary, Alberta

Dr. John Vidmar

Edmonton, Alberta

Colleen Purtill

Toxico-Logic Consulting Inc.

Calgary, Alberta

Assessment Report on Benzene for Developing Ambient Air Quality Objectives ii

TABLE OF CONTENTS

FOREWORD.................................................................................................................... i ACKNOWLEDGEMENTS............................................................................................... ii LIST OF TABLES ........................................................................................................... v LIST OF FIGURES.......................................................................................................... v ABBREVIATIONS AND ACRONYMS........................................................................... vi SUMMARY................................................................................................................... viii

1.0 INTRODUCTION .................................................................................................. 1

2.0 GENERAL SUBSTANCE INFORMATION........................................................... 3 2.1 Physical, Chemical and Biological Properties.......................................................3

2.2 Emissions Sources and Ambient Levels ...............................................................6

2.2.1 Natural Sources........................................................................................6

2.2.2 Anthropogenic Sources.............................................................................6

2.2.3 Ambient Levels .........................................................................................7

3.0 ATMOSPHERIC CHEMISTRY AND FATE.......................................................... 9

4.0 EFFECTS ON HUMANS AND ANIMALS .......................................................... 10 4.1 Overview of Chemical Disposition.....................................................................10

4.1.1 Absorption..............................................................................................10

4.1.2 Distribution ............................................................................................10

4.1.3 Metabolism.............................................................................................11

4.1.4 Elimination.............................................................................................13

4.1.5 Physiologically Based Pharmacokinetic (PBPK) Models........................14

4.1.6 Mechanism of Toxic Action.....................................................................15

4.2 Genotoxicity ......................................................................................................16

4.3 Acute Toxicity ...................................................................................................17

4.3.1 Acute Toxicity in Humans .......................................................................18

4.3.2 Acute Toxicity in Animals .......................................................................19

4.4 Subchronic and Chronic Toxicity .......................................................................22

4.4.1 Subchronic and Chronic Toxicity in Humans..........................................22

4.4.2 Subchronic and Chronic Toxicity in Animals ..........................................23

4.5 Developmental and Reproductive Toxicity.........................................................28

4.5.1 Developmental and Reproductive Toxicity in Humans ............................29

4.5.2 Developmental and Reproductive Toxicity in Animals.............................29

4.6 Carcinogenicity..................................................................................................33

4.6.1 Human Carcinogenicity Studies..............................................................33

4.6.2 Animal Carcinogenicity Studies..............................................................34

5.0 EFFECTS ON VEGETATION............................................................................. 36 5.1 Plant Uptake and Transformation of Benzene.....................................................36

Assessment Report on Benzene for Developing Ambient Air Quality Objectives iii

6.0 EFFECTS ON MATERIALS............................................................................... 37

7.0 AIR SAMPLING AND ANALYTICAL METHODS.............................................. 38 7.1 Reference Methods ............................................................................................38

7.1.1 US EPA Compendium Method TO-1.......................................................38

7.1.2 US EPA Compendium Method TO-2.......................................................39

7.1.3 US EPA Compendium Method TO-14A...................................................39

7.1.4 US EPA Compendium Method TO-15.....................................................40

7.1.5 US EPA Compendium Method TO-17.....................................................40

7.1.6 NIOSH Method 1501 ..............................................................................41

7.1.7 NIOSH Method 2549 ..............................................................................41

7.1.8 NIOSH Method 3700 ..............................................................................41

7.1.9 NIOSH Method 3800 ..............................................................................42

7.1.10 OSHA Method 7 .....................................................................................42

7.1.11 OSHA Method 12....................................................................................42

7.1.12 OSHA Method 1005................................................................................43

7.2 Alternative, Emerging Technologies ..................................................................43

7.2.1 Passive Sampling....................................................................................43

7.2.2 Active Sampling......................................................................................44

7.2.3 Automated Samplers...............................................................................44

7.2.4 Differential Optical Absorption Spectroscopy.........................................45

8.0 AMBIENT OBJECTIVES IN OTHER JURISDICTIONS..................................... 48 8.1 Benzene Air Quality Guidelines and Objectives .................................................48

8.1.1 Canada...................................................................................................48

8.1.2 United States Air Quality Guidelines and Objectives ..............................48

8.1.3 International Air Quality Guidelines and Objectives...............................49

9.0 REFERENCES................................................................................................... 51

APPENDIX A ................................................................................................................ 61

APPENDIX B ................................................................................................................ 77

Assessment Report on Benzene for Developing Ambient Air Quality Objectives iv

LIST OF TABLES

Table 1 Identification of Benzene (Lewis, 2004; Genium, 1999).......................................4

Table 2 Physical and Chemical Properties of Benzene ......................................................5

Table 3 Environmental Fate of Benzene............................................................................9

Table 4 On-site releases of Benzene in Canada and Alberta According to the NPRI

Database (in tonnes).............................................................................................7

Table 5 Acute Effects Following Human Exposure to Benzene.......................................18

Table 6 Acute Effects Following Animal Exposure to Benzene ......................................20

Table 7 Subchronic and Chronic Effects Reported Following Human (Occupational)

Exposure to Benzene..........................................................................................24

Table 8 Subchronic and Chronic Effects Following Animal Exposure to Benzene ..........26

Table 9 Reproductive and Developmental Effects Following Animal Exposure to Benzene

..........................................................................................................................31

Table 10 Cancer Effect Levels (CELs1) for Humans Exposed to Benzene.........................34

Table 11 Cancer Effect Levels (CELs1) for Animals Exposed to Benzene.........................35

Table 12 Advantages and Disadvantages of Sampling and Analytical Methods.................46

Table 13 Summary of Ambient Air Quality Objectives and Guidelines for Benzene .........50

LIST OF FIGURES

Figure 1 Metabolism of Benzene (ATSDR, 2005)............................................................12

Assessment Report on Benzene for Developing Ambient Air Quality Objectives v

ABBREVIATIONS AND ACRONYMS

AENV Alberta Environment

ANLL Acute nonlymphocytic leukemia

ANR Agency of Natural Resources

ATSDR Agency for Toxic Substances and Disease Registry

BP British Petroleum

CAS Chemical Abstract Services

CASA Clean Air Strategic Alliance

CEQ Commission on Environmental Quality

CNS Central nervous system

CMS Carbon molecular sieve

d day

DEM Department of Environmental Management

DEP Department of Environmental Protection

DEQ Department of Environmental Quality

DES Department of Environmental Services

DNR Department of Natural Resources

DOAS Differential Optical Absorption Spectroscopy

DOE Department of Ecology

DOH Department of Health

EC Environment Canada

ECD Electron Capture Detector

ENR Environment and Natural Resources

EPHC Environment Protection and Heritage Council

FID Flame Ionization Detector

FTIR Fourier Transform Infrared Spectrometry

GC/MS Gas Chromatography/Mass Spectrometry

hr hour

HSDB Hazardous Substances Data Bank

IARC International Agency for Research on Cancer

IPCS International Programme on Chemical Safety

IRIS Integrated Risk Information System

LOAEL Lowest Observable Adverse Effect Level

mg m -3 milligram per cubic metre

MRL Minimum Risk Level

mo month

MOE Ontario Ministry of the Environment

NAPS National Air Pollution Surveillance Network

Assessment Report on Benzene for Developing Ambient Air Quality Objectives vi

NIOSH National Institute for Occupational Safety and Health

NOAEL No Observable Adverse Effect Level

NPD Nitrogen-Phosphorous Detector

NPRI National Pollutant Release Inventory

OEHHA Office of Environmental Health Hazard Assessment (California)

OSHA Occupational Safety and Health Administration

PBPK Physiologically Based Pharmacokinetic Models

PID Photo-Ionization Detector

POI Point of Impingement

ppm part per million

RIVM Rijksinstituut Voor Volksgezondheid En Milieu (Netherlands National Institute of Public Health and the Environment)

RTECS Registry of Toxic Effects of Chemical Substances

STEL Short-Term Exposure Limit

TC05 Tumorigenic Concentration05

TDLAS Tunable Diode Laser Absorption System

TLV Threshold Limit Value

TWA Time Weighted Average

µg m -3

microgram per cubic metre

UN United Nations

US EPA United States Environmental Protection Agency

VOC Volatile Organic Compound

WHO World Health Organization

wk week

yr year

Assessment Report on Benzene for Developing Ambient Air Quality Objectives vii

SUMMARY

Benzene is a clear, colourless, volatile, highly flammable liquid with a sweet aromatic odour.

It is primarily derived from petroleum and used in the manufacturing of chemical intermediates

and as an additive in gasoline. The historic use of benzene as a universal solvent and degreaser

was stopped in the early 1920s following reported deaths due to benzene exposure. The

majority of non-fuel use of benzene in the United States and Canada is attributed to chemical

manufacturing. The amount of benzene added to gasoline in Canada is regulated to below

1.5%.

Airborne benzene exists almost exclusively in the vapour phase and is transformed primarily by

reaction with hydroxyl radicals, resulting in a residence time ranging from 2 hours (at higher

hydroxyl radical concentrations) to 8 days (at lower hydroxyl radical concentrations). Benzene

occurs in petroleum at levels ranging from 1 to 4% and consequently may occur naturally in

water near petroleum and natural gas deposits. Natural sources of atmospheric emissions of

benzene include volcanoes, forest fires and plant volatiles. Major anthropogenic sources of

benzene in Canada include transportation, natural gas dehydrators, petroleum distribution and

refining, steel industry, chemical industry, and wood combustion. Vehicle emissions provide

the greatest anthropogenic contribution of benzene to the Canadian environment (over 80% in

urban areas).

In Canada, anthropogenic emissions of benzene are tracked by the Environment Canada

National Pollutant Release Inventory program. The industrial sectors that contribute to

benzene emissions in Alberta are principally the crude petroleum and natural gas sector

(~60%), the wood industries sector (~14%), the chemical and chemical products sector (~9%),

the petroleum products industries (~5%), and the refined petroleum and coal products sector

(~3%). Ambient air concentrations of benzene were reported for rural and urban sites in

Canada with annual mean benzene concentrations ranging from 0.6 to 5.5 µg m -3 in 2003 and from 0.2 to 1.9 µg m -3 in 2004. In Alberta, annual average ambient benzene concentrations measured in 2000 at three monitoring stations in urban areas (Edmonton and Calgary) ranged

from 2.05 to 2.21 µg m -3 .

Acute exposure to very high benzene concentrations (44,662,000 to 65,200,000 µg m -3) is fatal to animals and humans. Causes of death in humans include asphyxiation, respiratory arrest,

central nervous system (CNS) depression and cardiac collapse. Haematological, neurological,

and respiratory effects were reported in humans following acute exposure to non-lethal benzene

air concentrations (up to 978,000 µg m -3). Acute exposure of mice and rats to benzene (>33,000 µg m -3) decreased production of white and red blood cells, and reduced bone marrow cellularity. Chromosomal damage was induced in mice acutely exposed to 68,000 µg m -3

benzene.

Acute, subchronic and chronic exposure of animals to relatively low levels of benzene produces

measurable depression of one or more types of circulating blood cells which can produce

haematotoxic and immunotoxic effects. A more severe effect of benzene exposure is aplastic

anaemia in which the bone marrow is unable to function and stem cells do not mature. The

Assessment Report on Benzene for Developing Ambient Air Quality Objectives viii

progression of aplastic anaemia can result in acute myelogenous leukemia. There are

inconclusive data to suggest that occupational exposure to benzene may result in reproductive

(menstrual disorders, reduced fertility) or developmental (chromosomal aberrations) toxicity in

humans, however, reproductive (ovary and testicular) and developmental (fetal mortality,

skeletal, and haematological effects) were observed following controlled exposures of animals

to high concentrations of benzene. The International Agency for Research on Cancer (IARC)

has classified benzene as a Group I human carcinogen. Although limited by confounding

exposures to other chemicals and lack of precise exposure monitoring, occupational studies

demonstrate a consistent increase in the risk of leukemia with exposure to benzene. Studies of

controlled animal exposure to benzene have also reported leukemia as well as non-Hodgkin’s

lymphoma, and tumours in the lung, liver, mammary gland, and Zymbal gland.

There were very few reports on the phytotoxicity of benzene on plants and those available

reported acute effects in plant species at benzene air concentrations in excess of 10,000,000 µg

m-3

. The available literature focused on the ability of plants to bioconcentrate benzene and the

utilization of plants for detoxification of atmospheric benzene. Blackberry, cucumber, and

apples were shown to accumulate benzene in the leaves and fruits with no phenotypic effects

following atmospheric exposure to 1,000 µg m-3

benzene. The removal of benzene from

ambient air was also demonstrated with several indoor plant species. Air-to-leaf transfer is the

major pathway of benzene uptake; once inside the plant benzene is metabolized to both organic

and amino acids.

The residence time for benzene in ambient air is too short to have substantial effects on

materials. Widely employed and accepted reference air monitoring methods for benzene have

been developed, tested and reported by the United States Environmental Protection Agency

(US EPA), National Institute of Occupational Safety and Health (NIOSH), and Occupational

Safety and Health Administration (OSHA). Standard collection methods include:

charcoal/glass/metal sorbent tubes; multi-bed adsorbent tubes; carbon molecular sieve (CMS)

adsorption; stainless steel canisters; Tedlar bags, and; direct reading sample pumps. Analysis

can be conducted by gas chromatography/mass spectrometry (GC/MS), flame ionization

detector (FID), nitrogen-phosphorous detector (NPD), electron capture detector (ECD), and

photo-ionization detector (PID). Emerging technologies for analyses include automatic

sampling using on-line GC/FID and Differential Optical Absorption Spectroscopy (DOAS), an

open path optical measuring technique.

An ambient (outdoor) objective of 30 µg m-3

as a 1-hr average has been developed for benzene

by Alberta Environment. British Columbia MOE, Manitoba Conservation, and Ontario MOE

currently do not have ambient air objective limits for benzene. Ambient air objectives for

benzene were identified from several international jurisdictions, including Australia, New

Zealand, Netherlands, UK, European Commission, and the WHO. Objectives ranged from 1 to

16 µg m-3

corresponding to an averaging time period of one year. Three jurisdictions (New

Zealand, UK, and European Commission) have committed to lowering their guidelines by

2010.

The US Agency for Toxic Substances and Disease Registry (ATSDR) has developed acute,

intermediate, and chronic minimal risk levels of 29, 19, and 10 µg m-3

for exposure to benzene

Assessment Report on Benzene for Developing Ambient Air Quality Objectives ix

ranging in durations of

1.0 INTRODUCTION

Alberta Ambient Air Quality Objectives (AAQO) are established by Alberta Environment under

Section 14 (1) of the Environmental Protection and Enhancement Act (EPEA) (AENV, 2007).

The purpose of this assessment report is to provide a review of scientific and technical

information to assist in evaluating the basis and background for an ambient air quality objective

for benzene. The following aspects were examined as part of the review:

• Physical-chemical properties and environmental fate;

• Existing and potential anthropogenic emissions sources in Alberta;

• Effects on humans, animals, and vegetation;

• Effects on materials and air monitoring techniques, and;

• Ambient air guidelines and objectives in other jurisdictions.

The physical and chemical properties identified for benzene include chemical structure,

molecular weight, melting and boiling points, water solubility, density, vapour density, organic

carbon partition coefficient, octanol water partition coefficient, vapour pressure, Henry's Law

constant, bioconcentration factor, and odour threshold. A discussion of the behaviour of benzene

in the environment was also presented. Existing and potential natural and anthropogenic sources

of benzene emissions in Alberta were examined. Benzene is a reportable substance on

Environment Canada’s National Pollutant Release Inventory.

Scientific information on the effects of benzene on humans, animals, and vegetation were

identified. Toxicity and epidemiology studies were located in peer reviewed evaluations by the

Agency for Toxic Substances and Disease Registry, the U.S. Environmental Protection Agency,

and the World Health Organization. The effects of benzene on vegetation were identified

following a comprehensive search of the Web of Science database and using data from the

Canadian Environmental Protection Act Priority List Substance Assessment Report for Benzene

and the Agency for Toxic Substances and Disease Registry.

There was a lack of literature for the effects of airborne benzene on materials as atmospheric

benzene reacts quickly with other chemicals and decomposes within a few days. Air sampling

and analytical methods for benzene used by regulatory agencies were identified and reviewed for

this assessment. The monitoring of benzene for regulatory purposes requires methods suitable to

measure very low concentrations with a sufficiently high accuracy. Widely employed and

accepted reference air monitoring methods for benzene reported by the United States

Environmental Protection Agency (US EPA), National Institute of Occupational Safety and

Health (NIOSH), and Occupational Safety and Health Administration (OSHA) were reviewed

and air monitoring methods used by Alberta Environment and Environment Canada identified.

A few unique and alternate technologies used to conduct point and line measurements of ambient

benzene concentrations were also identified from recent literature.

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 1

Ambient air guidelines for benzene have been established by a number of jurisdictions in North

America, Europe, and Australia for different averaging time periods. The basis for how these

guidelines were developed and used by different jurisdictions was investigated in this report.

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 2

2.0 GENERAL SUBSTANCE INFORMATION

2.1 Physical, Chemical and Biological Properties

Benzene is a clear, colourless, volatile, highly flammable liquid (O’Neil, 2006) with a

characteristic sweet aromatic odour (O’Neil, 2006; Genium, 1999). Benzene was first obtained

from the pyrolysis of coal; however, with the development of new technologies, benzene is now

primarily derived from petroleum (Folkins, 2005) by catalytic reforming, hydrodemethylation,

transalkylation or disproportionation of toluene and steam cracking of heavy naphthas or light

hydrocarbons (Fruscella, 2002).

In the past, benzene was primarily used as a universal solvent and degreaser (Fruscella, 2002).

With reported deaths due to benzene exposure in the early 1920s, benzene was replaced, for the

most part, by other solvents such as toluene and aliphatics (Fruscella, 2002). Benzene is now

commonly used in the manufacturing of chemical intermediates and as an additive in gasoline.

Approximately 86% of benzene use (non-fuel uses) in the United States is attributed to the

manufacturing of ethylbenzene (50%), cumene (24%) and cyclohexane (12%) (Fruscella, 2002).

In Canada, both produced and imported benzene are also primarily used for the manufacturing of

ethylbenzene (Health Canada, 2007). Ethylbenzene is mainly used to produce styrene, which is

in turn used to make plastics and elastomers (ATSDR, 2005). Cumene is used in the

manufacturing of phenol and acetone, which are in turn used to make resins, nylon intermediates,

pharmaceuticals, or in the case of acetone, as a solvent (ATSDR, 2005). Cyclohexane is used in

the manufacturing of nylon resins (ATSDR, 2005). Other non-fuel uses of benzene include the

manufacturing of aniline (6%), alkylbenzene (2%), chlorobenzene (1%) and other compounds

(5%) (Fruscella, 2002). Benzene is also a component of gasoline although the amount of benzene

in gasoline is regulated (Folkins, 2005). For example, in Canada, it is prohibited to supply

gasoline that contains benzene at a concentration greater than 1.0% by volume or to sell gasoline

that contains benzene at a concentration greater than 1.5% by volume (EC, 1998). Additional

uses of benzene include in photogravure printing and in veterinary medicine (Genium, 1999)

Table 1 provides a list of important identification numbers and common synonyms for benzene.

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 3

Table 1 Identification of Benzene (Lewis, 2004; Genium, 1999)

Property Value

Formula C6H6

Structure

CAS Registry Number 71-43-2

RTECS number CY1400000

UN Number UN 1114

Common Synonyms/Trade names (6)annulene; benzeen; benzene; benzin; benzine; benzol1; benzol 90; benzole; benzolene; benzolo; bicarburet of hydrogen; carbon

oil; coal naphtha; cyclohexatriene; EPA Pesticide Chemical Code

008801; fenzen; mineral naphtha; motor benzol; NCI-C55276;

nitration benzene; phene; phenyl hydride; polystream;

pyrobenzol; pyrobenzole; RCRA waste number U019

1 name used to designate a compound or material containing benzene as its major component (Folkins, 2005; Fruscella, 2002)

The physical and chemical properties of benzene are summarized in Table 2.

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 4

Table 2 Physical and Chemical Properties of Benzene

Property Value Reference

Molecular weight 78.1 g mol-1 Lide, 2007; Verschueren, 2001; O’Neil,

2006

Physical state Clear colourless volatile liquid Lide, 2007; Verschueren, 2001; O’Neil,

2006

Melting Point 5.49 ºC Lide, 2007; Verschueren, 2001; O’Neil,

2006

Boiling Point 80.09 ºC Lide, 2007; Verschueren, 2001; O’Neil,

2006

Density (liquid) 0.8765 g ml-1 (at 20ºC) Lide, 2007

Density (gas) 2.77 (air=1) Verschueren, 2001

Vapour pressure 12.7 kPa (at 25ºC) Lide, 2007

Solubility in water 1780 mg.L-1 (at 20ºC)

0.188 % w/w (at 23.5ºC)

Verschueren, 2001

Budavari, 1989

Solubility miscible with most organic

solvents1 IPCS, 1993

Henry’s Law Constant 5.55 10-3 atm.m3.mol-1 Mackay et al., 1979 (cited in SRC, 2007)

Octanol water partition coefficient

(log Kow)

Organic carbon partition coefficient

(Koc)

2.13

1.56 to 2.15

Koc 38 to 53 at 1.0 mg.L-1

log Koc 1.8 to 1.9

Sangster, 1989 (cited in Lide, 2007)

IPCS, 1993

Seip et al., 1986 (cited in Verschueren,

2001)

IPCS, 1993; Kenaga, 1980 (cited in

ATSDR, 2005)

Bioconcentration factor (log BCF) 0.48 to 1.49

1.48, 1.72

SRC, 2007

Veith and Kosian, 1982 (cited in

Verschueren, 2001)

Flash Point -11ºC Lide, 2007; O’Neil, 2006

Explosive limits 1.2 to 7.8% 1.4 (lower) to 8.0% (upper)

Lide, 2007 Lewis, 2000

Autoignition temperature 498ºC Lide, 2007

Odour threshold 4.68 ppm Verschueren, 2001

STEL 2.5 ppm Lide, 2007

TWA 0.5 ppm Lide, 2007

Conversion factors for vapour (at

25 °C and 101.3 kPa) 1 mg.m

-3= 0.307 ppm

1 ppm= 3.26 mg.m-3 Verschueren, 2001

1 see Lide (2007) for detailed list

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 5

2.2 Emissions Sources and Ambient Levels

2.2.1 Natural Sources

Benzene occurs naturally in the environment (IPCS, 1993). Benzene occurs in petroleum at

levels ranging from 1 to 4% (IARC, 1989, cited in IPCS, 1993). As a result, benzene may also

occur naturally in water near petroleum and natural gas deposits (Reynolds and Harrison, 1982,

cited in IPCS, 1993). Atmospheric emissions of benzene from natural sources include volcanoes,

forest fires and plant volatiles (IARC, 1982 and Graedel, 1978, cited in Howard, 1990).

2.2.2 Anthropogenic Sources

Anthropogenic sources of benzene include the combustion of gasoline and diesel fuels for

vehicles and residential heating, emissions during benzene production, emissions during primary

iron and steel production, and emissions during gasoline marketing (Health Canada, 2007).

Environment Canada (EC) (2007a) lists the main sources of benzene emissions in Canada as

wood combustion, transportation, natural gas dehydrators, the steel industry, petroleum

distribution and refining; and the chemical industry, with transportation (more specifically

vehicle emissions) being the largest anthropogenic source to the environment (EC, 2004a).

According to Environment Canada (1998), in 1995, approximately 56% of Canadian benzene

emissions were from vehicles. More specifically, in urban areas, vehicles represented over 80%

of the benzene emissions (EC, 1998). Other anthropogenic sources include tobacco smoke,

wastewater treatment plants, the petrochemical and petroleum industries (Edgerton and Shah,

1992 as cited in ATSDR, 2005), landfill emissions, oil spills (Bennett, 1987, as cited in ATSDR,

2005; Hazardous Substances Data Bank (HSDB), 2005 as cited in ATSDR, 2005; Wood and

Porter 1987, as cited in ATSDR, 2005) and off-gases from particle board (Glass et al. 1986, as

cited in ATSDR, 2005). Benzene is also produced indirectly in coke ovens and during

nonferrous metal manufacturing, ore mining, wood processing, coal mining and textile

manufacturing (HSDB, 2007).

Industrial emissions of benzene in Canada are provided by the National Pollutant Release

Inventory. The data for on-site releases of benzene in Canada and more specifically for Alberta

since 1995 are summarized in Table 3 (EC, 2007b). More detailed emissions data are also

presented for Alberta for 2005 in Appendix A (EC, 2007c). The 2005 data were used since, at the

time of writing this report, the 2006 data were preliminary and unreviewed. Appendix table A-1

summarizes the benzene emissions to air, land and water and appendix table A-2 provides details

specifically related to air emissions of benzene.

The results in Appendix A show that, in Alberta, the NPRI reported emissions of benzene are

almost exclusively to air, and that these air emissions are predominantly the result of stack or

point source emissions and fugitive emissions. The industrial sectors that contribute to benzene

emissions in Alberta are principally the crude petroleum and natural gas sector (approximately

60% of the emissions), the wood industries sector (approximately 14%), the chemical and

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 6

chemical products sector (approximately 9%), the petroleum products industries (approximately

5%), the refined petroleum and coal products sector (approximately 3%).

Table 3 On-site releases of Benzene in Canada and Alberta According to the NPRI

Database (in tonnes)

Canada Alberta

Benzene Benzene

Emissions Number of Facilities Emissions Number of Facilities

Year (in tonnes) Reporting (in tonnes) Reporting

2006 915.9 217 281.9 98

2005 975.6 252 255.6 108

2004 861.8 253 251.2 105

2003 995.3 252 257.6 112

2002 863.2 205 230.9 98

2001 1052.0 136 307.1 78

2000 1138.0 125 353.7 77

1999 1341.5 118 321.2 71

1998 1464.9 112 352.0 68

1997 1686.2 115 357.0 64

1996 2029.8 105 406.2 66

1995 2148.1 104 490.0 65

2.2.3 Ambient Levels

Extensive ambient air concentration data for benzene are presented in the HSDB (2007), ATSDR

(2005), EC (1993), IPCS (1993) and Howard (1990). Ambient benzene concentrations at several

rural and urban Canadian sites are available from the annual reports of the National Air Pollution

Surveillance (NAPS) Network (see http://www.etc-cte.ec.gc.ca/NAPS/index_e.html). Benzene

has been monitored as part of this program since 1989. In 2003, benzene concentrations were

reported for 52 sites (14 rural sites and 38 urban sites in 18 different cities) (EC, 2004b). Annual

mean benzene concentrations at the 38 urban sites ranged from 0.6 to 5.5 µg m -3, and, at 33 of the 38 urban sites, the annual mean concentration was below 2.0 µg m -3 (EC, 2004b). In 2004, annual mean benzene concentrations were reported for 27 sites (14 rural sites and 13 urban sites

in 10 different cities), and the concentrations ranged from 0.2 to 1.9 µg m -3 (EC, 2007d).

In a recent report summarizing NAPS Network data from 2001 and trends from 1990 to 2001,

EC (2004a) notes that the highest ambient air concentrations (median values) over the studied

time period were observed in Saint John, Oakville, Edmonton, Montreal and Vancouver, while

the lowest concentrations were observed in Peterborough, Sarnia, London, and Kitchener. The

high concentrations observed in Saint John, Oakville, Edmonton, Montreal and Vancouver were

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 7

http://www.etc-cte.ec.gc.ca/NAPS/index_e.html

attributed to the influence of nearby industries (refineries and others) emitting significant

amounts of benzene (EC, 2004a). Results from the NAPS Network data also showed a seasonal

variability of ambient benzene concentrations, with lower concentrations in the summer months

versus the winter months (EC, 2004a). EC (2004a) suggests that the lower concentrations of

benzene are most likely due to the presence of higher concentrations of oxidants and greater

mixing depths in the summer months (EC, 2004a). In considering the NAPS Network data over

the period of 1990 to 2003, EC (2004b) notes that urban annual mean benzene concentrations

decreased by 65% from 1990 to 2002 but increased slightly from 2002 to 2003 in 30 of the 38

urban sites. No explanation for this increase is provided. Rural annual mean benzene

concentrations remained relatively unchanged (of the order of 0.5 µg m -3) over the period of 1994 to 2002 (EC, 2004b). It should be noted that complete rural data were not available prior to

1994 and therefore were not considered in the trend analysis (EC, 2004b).

In Alberta, there are three monitoring stations that measure ambient air benzene concentrations

in urban areas: Edmonton East, Edmonton Central and Calgary Central (AENV, 2001). Benzene

ambient air concentrations are available from the Clean Air Strategic Alliance (CASA). CASA

(2007) presents the long-term trend for ambient air benzene concentrations in Alberta over the

period of 1991 to 2000. The data show that over this time period, the benzene concentrations

have decreased by 51% at the east Edmonton monitoring station and by 30% at the downtown

Calgary monitoring station. Data for the downtown Edmonton monitoring station were only

available for the period of 1992 to 2000 and indicate a 52% decrease in benzene concentrations

(CASA, 2007). Alberta Environment (AENV) (2001) presents similar data for ambient air

benzene concentrations over the period of 1995 to 2000 and states that the benzene

concentrations have decreased by 24%, 29% and 53% in Edmonton East, Edmonton Central and

Calgary Central, respectively (AENV, 2001). The annual average ambient benzene

concentrations in 2000 were 2.21, 2.19 and 2.05 µg m -3 in Edmonton East, Edmonton Central and Calgary Central, respectively (AENV, 2001).

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 8

3.0 ATMOSPHERIC CHEMISTRY AND FATE

The environmental fate of benzene presented by Howard (1990) is summarized in Table 4.

Briefly, benzene emitted to air will exist almost exclusively in the vapour phase and will be

transformed primarily by reaction with hydroxyl radicals (ATSDR, 2005). Reaction with

photochemically produced hydroxyl radicals results in a benzene atmospheric residence time

ranging from approximately 2 hours (at higher hydroxyl radical concentrations characteristic of

polluted atmospheres) to 8 days (at lower hydroxyl radical concentrations) (ATSDR, 2005). A

detailed discussion is also presented in ATSDR (2005).

Table 4 Environmental Fate of Benzene

System Fate Reaction rates

Surface water • rapidly volatilizes • not expected to significantly absorb to

sediment, bioconcentrate in aquatic

organisms, or hydrolyze

• may biodegrade • may photodegrade in situtations where

biodegradation is not favoured (low

temperatures, low nutrient levels for

example)

• volatilization half-life ranging from 2.7 hours (in a river at 20ºC) to 5.2

hours (in a wind-wave tank)

• half-lives for evaporation from seawater of 3.1, 13 and 23 days in the

summer, winter and spring,

respectively with biodegradation

playing a major role in the spring and

summer

• biodegradation half-life of 16 days in a river test under aerobic conditions

• photodegradation half-life of 17 days

Soil • rapidly volatilizes at soil surface • half-life for volatilization (without • benzene that does not volatilize will be water evaporation) of 7.2 days at 1cm

highly to very highly mobile and may leach depth and 38 days at 10cm depth

into groundwater • 47% biodegradation in 10 weeks in a • may biodegrade silty soil

Groundwater • may biodegrade in shallow, aerobic groundwater

Air • • •

•

exists predominantly in the vapour phase

will not be subject to direct photolysis

will react with photochemically produced

hydroxyl radicals

will form products such as phenol,

•

•

half-life of 13.4 days by reaction with

photochemically produced hydroxyl

radicals

half-life of 4 to 6 hours by reaction in

polluted atmospheres with nitrogen

nitrophenols, nitrobenzene, formic acid and oxides or sulphur dioxide

• peroxyacetyl nitrate

will be removed from the atmosphere by rain

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 9

4.0 EFFECTS ON HUMANS AND ANIMALS

The chemical disposition and adverse health effects reported following the inhalation of benzene

by humans and experimental animals were reviewed and are summarized below. Literature

sources identified for this review include the Agency for Toxic Substances and Disease Registry

(ATSDR, 2005), the U.S. Environmental Protection Agency (US EPA, 1998; 2002; IRIS, 2002),

and the World Health Organization (IPCS, 1993; WHO, 2000).

4.1 Overview of Chemical Disposition

Inhalation is the major route of human exposure to benzene (ATSDR, 2005). A summary of the

chemical disposition of benzene (absorption, distribution, metabolism and elimination) in

humans and animal systems following inhalation is provided below.

4.1.1 Absorption

Studies in mice, rats, and humans suggest that benzene is rapidly absorbed following inhalation

(IPCS, 1993; ATSDR, 2005). Approximately 50% of inhaled benzene (50 to 100 ppm; 163 to

326 mg m-3

) was absorbed following acute controlled exposure studies in humans (Nomiyama &

Nomiyama and Snyder et al., cited in IPCS, 1993). A higher absorption rate occurred during the

first 5 minutes of exposure to benzene with absorption dropping to approximately 50% following

prolonged exposure (15 minutes to 3 hours) (Srbova et al., cited in ATSDR, 2005; Nomiyama &

Nomiyama, cited in US EPA 2002). A recent study reported an average absorption of 64%

following exposure to benzene (32 to 69 ppm; 104 to 225 mg m-3

) produced from burning

cigarettes (Yu and Weisel, cited in ATSDR, 2005); however, this study was limited by a low

number (n=3) of exposed individuals.

Absorption was dose-dependent in acute inhalation studies conducted on rats and mice.

Increasing exposure to benzene from 10 to 100 ppm (33 to 326 mg m-3

) resulted in decreasing

retention, from 50 to 10% (Sabourin et al., cited in ATSDR, 2005). The uptake and retention of

inhaled benzene varied according to species, with mice absorbing and retaining a greater dose

than rats or monkeys as a result of a higher respiratory rate and faster metabolism (Henderson,

cited in US EPA 2002).

4.1.2 Distribution

Benzene distributes to fatty tissue throughout the body upon absorption into the bloodstream

(ATSDR, 2005). Animal studies suggest that the tissue distribution of benzene depended on

lipid content and the rate of blood perfusion of the tissues (IPCS, 1993; US EPA 2002; ATSDR,

2005). The inhalation exposure of pregnant animals to very high concentrations of benzene

(2,000 ppm; 6,520 mg m-3

) for 10 minutes resulted in the detection of benzene in lipid rich (brain

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 10

and fat) and perfused (liver and kidney) tissues as well as in the placenta and fetuses immediately

after exposure (Ghantous and Danielsson, cited in ATSDR, 2005).

Benzene was detected in the fat at levels of 16.4 mg% (where mg% equals mg of benzene per

100 mL of blood or mg per 100 g of tissue), in the bone marrow (3.8 mg%), blood (1.2 mg%)

and to a lesser extent in the kidney, lung, liver, brain, and spleen of rats exposed by inhalation for

6 hours to 500 ppm (1,630 mg m-3

). Concentrations of some benzene metabolites (e.g., phenol,

catechol, and hydroquinone) were higher in the bone marrow compared to the blood (Rickert et

al., cited in ATSDR, 2005).

Case studies of human individuals exposed to lethal benzene concentrations reported preferential

distribution in the brain (lipid rich), followed by the blood (plasma proteins) and liver (Tauber

and Winek & Collom, cited in ATSDR, 2005). Benzene will cross the placenta and was reported

in cord blood at concentrations equal to or greater than maternal blood concentrations (Dowty et

al., cited in ATSDR, 2005). Benzene has also been detected in samples of human breast milk

(Fabietti et al., cited in ATSDR, 2005).

4.1.3 Metabolism

The toxicity of benzene is attributed to multiple benzene metabolites yet the metabolism of

benzene is not thoroughly understood (US EPA, 2002). The data available suggests that benzene

metabolites produced in the liver are responsible for toxic effects in the bone marrow; however,

in vitro studies have demonstrated benzene metabolism can also occur within the bone marrow

itself (ATSDR, 2005).

The metabolic pathways reported for benzene are summarized in Figure 1. Benzene is initially

catalyzed by cytochrome P450 2E1 (CYP2E1) to form the epoxide benzene oxide (Lindstrom et

al., cited in ATSDR, 2005). Benzene oxide can then be metabolized via several alternative

pathways, the predominant pathway being nonenzymatic rearrangement to form phenol (Jerina et

al., cited in ATSDR, 2005). The epoxide may also react with glutathione to form S

phenyl-mercapturic acid (Nebert et al.; Sabourin et al.; Schafer et al.; Schlosser et al.; Schrenk et

al., and; van Sittert et al., cited in ATSDR, 2005). Benzene oxide can also form trans,trans

muconic acid via the highly reactive trans,trans-muconaldehyde (MUC) produced by an iron-

catalyzed, ring-opening reaction (Bleasdale et al.; Nebert et al.; Ross, and; Witz et al., cited in

ATSDR, 2005). Benzene oxide may also be acted on by epoxide hydrolase (EH) to form

benzene dihydrodiol (Nebert et al.; Snyder et al., cited in ATSDR, 2005), which is converted to

catechol by dihydrodiol dehydrogenase (DHDD) (Nebert et al.; Snyder et al., cited in ATSDR,

2005).

The major metabolic pathway for benzene is oxidation by CYP2E1 and rearrangement to form

phenol. Further oxidation of phenol by CYP2E1 produces catechol or hydroquinone; CYP2E1

catalysis of these products produces the reactive metabolite 1,2,4-benzenetriol. Alternatively,

catechol or hydroquinone can undergo oxidation by myeloperoxidase (MPO) to form 1,2- or 1,4

benzoquinone, respectively. These benzoquinones can be reduced back to catechol or

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 11

hydroquinone via NAD(P)H:quinone oxidoreductase (NQ01) (Nebert et al., cited in ATSDR,

2005).

Figure 1 Metabolism of Benzene (ATSDR, 2005)

The role of CYP2E1 in the metabolism and toxicity of benzene has been well documented in

mice studies. No signs of benzene toxicity (genotoxicity and cytotoxicity) were observed in

transgenic CYP2E1 mice that did not express hepatic CYP2E1 (Valentine et al., cited in

ATSDR, 2005). CYP2E1 is likely a major catalyst in benzene metabolism but other CYPs (e.g.,

CYP2B1 and CYP2F2) may also play a role (Gut et al.; Powley and Carlson; Sheets and

Carlson; Sheets et al.; Snyder et al., cited in ATSDR, 2005). Pretreatment of mice with CYP

inhibitors reduced benzene metabolite formation (Andrews et al.; Gill et al.; Ikeda et al., and;

Tuo et al., cited in ATSDR, 2005) and genotoxicity (Tuo et al., cited in ATSDR, 2005). In

contrast, induction of CYP increased the metabolism and clastogenicity (ability to cause

chromosomal breaks) of benzene (Gad-el-Karim et al., cited in ATSDR, 2005).

The majority of CYPs are located in the liver which is believed to be the primary site for benzene

metabolism (ATSDR, 2005). However, results of in vitro studies on human and animal

pulmonary microsomes (Powley and Carlson; Sheets et al., cited in ATSDR, 2005) and on bone

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 12

marrow from rabbits (Andrews et al.; Schnier et al., cited in ATSDR, 2005), rats (Irons et al.

cited in ATSDR, 2005), and mice (Ganousis et al. cited in ATSDR, 2005) have indicated CYP-

catalyzed benzene metabolism can also occur in the lung and bone marrow, indicating the

potential for production of toxic metabolites at these locations.

Phenol is believed to be a major product of benzene metabolism; however, the administration of

phenol does not produce the same toxicity as benzene. Differences in urinary metabolites

produced by mice following oral exposure to phenol compared to benzene (e.g., less

hydroquinone produced from phenol administration compared to benzene administration) were

attributed to differences in the zonal distribution of metabolic enzymes within the liver (Kenyon

et al. cited in ATSDR, 2005). Enzymes capable of conjugating phenol for urinary excretion are

located in zone 1 (periportal area) of the liver, the first region to absorb xenobiotics. CYP

enzymes (including CYP2E1) are located in zone 3 (pericentral area) of the liver and metabolism

by these enzymes occurs only after a xenobiotic has passed through zone 1. Conjugation of

orally administered phenol would be extensive in zone 1 of the liver which would greatly reduce

the potential for oxidation of free phenol to hydroquinone by CYPs in zone 3. The metabolism

of orally administered benzene to phenol would not occur until zone 3 of the liver, where CYPs

are located. Unlike the conjugation and removal of phenol in zone 1 of the liver, phenol

produced in zone 3 by the metabolism of benzene is preferentially converted to hydroquinone

due to the high CYP content in this zone (Kenyon et al. cited in ATSDR, 2005).

It is likely that several metabolites are responsible for the toxic effects of benzene. Animal

studies have reported large variances in metabolite production between species (Sabourin et al.,

cited in ATSDR, 2005). There is still uncertainty as to which animal model best describes

benzene metabolism in humans as there is limited data available to define the proportion of

benzene metabolite production in humans (US EPA, 2002). Further details on studies of the

metabolism of benzene and associated toxicity can be found in ATSDR, 2005 and US EPA,

2002.

4.1.4 Elimination

Animal studies have reported that exposure to very high air concentrations of benzene (>850

ppm or 2,718 mg m-3

) saturates metabolic processes and results in the exhalation of greater

concentrations of unchanged benzene (48% in rats and 14% in mice). Exposure of animals to

lower air concentrations of benzene (10 ppm or 32 mg m-3

) resulted in the exhalation of lower

concentrations of unchanged benzene (

benzene for 8 hours or 99 ppm (316 mg m-3

) for 1 hour, Sherwood (cited in US EPA, 2002)

demonstrated a greater proportion of the inhaled benzene dose was excreted in the urine versus

exhalation. This is supported by occupational and cross-sectional studies which suggest that

benzene and benzene metabolites in urine are important biomarkers for benzene exposure.

4.1.5 Physiologically Based Pharmacokinetic (PBPK) Models

Physiologically based pharmacokinetic models (PBPK) utilize mathematical equations to

describe the uptake and disposition of a chemical substance, to predict chemical concentrations

in target tissues, and, to describe the relationship between target tissue dose and toxic end points

(ATSDR, 2005). The ability of these models to predict the dose-response characteristics of a

chemical is dependent on how accurately mechanisms for chemical absorption, transport, and

metabolism are described. In general, the models assume that biochemical processes identified

in animal models will simulate the human toxicological response (US EPA, 2002).

As described below, PBPK models have been developed and used to describe benzene

metabolism, to relate benzene metabolites to toxicity endpoints, and, to a limited extent, to

extrapolate data on benzene metabolism in animals to predict human metabolism of benzene

(ATSDR, 2005).

Medinsky and colleagues developed a multicompartmental PBPK model (blood, bone

marrow, fat, liver, lung, slowly-perfused tissues, and rapidly-perfused tissues) to describe the

absorption and distribution of benzene in the human, mouse, and rat (ATSDR, 2005). The model

has been used to predict benzene metabolite formation in rats and mice after inhalation

exposures. When compared to rats, mice were predicted to have a higher rate of benzene

metabolism and produce more hydroquinone glucuronide and muconic acid (metabolites

associated with toxic effects of benzene). Although this model has been used to simulate the

formation of major metabolites in mice, rats, and humans, it is limited by the assumption that all

metabolism occurs in the liver. The model also excludes bone marrow as a target tissue for

benzene metabolites (ATSDR, 2005). Sun and colleagues incorporated into the Medinsky model

the formation of haemoglobin adducts in the liver by benzene oxide following oral and inhalation

exposure of mice and rats to benzene (ATSDR, 2005). This model is limited in that it attributes

the formation of haemoglobin adducts to benzene oxide concentrations in the liver with no

consideration of potential haemoglobin adduct formation by other benzene metabolites (i.e.,

hydroquinone, phenol, muconaldehyde) (ATSDR, 2005).

Travis and colleagues also developed a multicompartmental PBPK model (blood, bone marrow,

fat, liver, lung, and other slowly- and rapidly-perfused tissues) to describe the absorption and

disposition of benzene in humans, mice, and rats (ATSDR, 2005). In this model benzene

metabolism is assumed to occur both in the liver and bone marrow. The model has been applied

to humans to predict benzene metabolism and benzene concentrations in expired air and blood

following inhalation exposures. An internal dose-response relationship for humans was derived

for benzene using this model (Cox, cited in ATSDR, 2005). The Travis et al. model was

extended by Fisher and colleagues to predict the partitioning of benzene into breast milk and

subsequent exposure of breast feeding infants (ATSDR, 2005). Sinclair and colleagues also

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 14

extended the Travis et al. model to include dermal absorption of benzene and first-order urinary

excretion of total benzene metabolites and phenol, in order to predict benzene levels in exhaled

air and urinary phenol concentrations following occupational exposure to benzene (ATSDR,

2005).

Bois and colleagues developed PBPK models for benzene which included metabolic pathways in

the liver and bone marrow, phenol conjugation in the lung and gastrointestinal tract, and

endogenous production of phenolic metabolites (ATSDR, 2005). Their model was used to

predict benzene and phenol metabolite formation in rats during inhalation exposures equivalent

to the OSHA PEL for benzene. The results suggest that dose rate (dose and time) is an important

factor in bone marrow toxicity as a greater amount of toxic metabolites (hydroquinone, catechol,

and muconaldehyde) were produced in rats exposed for 15 minutes to a 32 ppm (104 mg m-3

)

benzene compared to rats exposed for 8 hours to 1 ppm (3 mg m-3

) benzene (ATSDR, 2005).

Brown and colleagues developed a PBPK model for determining benzene concentrations in

blood following inhalation by humans; the model incorporates gender-specific partition

coefficients and predicts a higher blood:air partition coefficient and subsequently a higher

metabolic rate for benzene in females compared to males (ATSDR, 2005).

Cole and colleagues developed a multicompartmental PBPK model (blood, fat, liver, and other

slowly- and rapidly-perfused tissues) to describe the absorption and disposition of benzene in the

mouse following oral or inhalation exposures (ATSDR, 2005). This model included simulation

of the disposition of major metabolites; metabolic parameters were derived empirically from in

vitro studies versus model optimization. The model is limited by a lack of simulation of

metabolic processes in the bone marrow. Furthermore, data on tissue dosimetry for benzene

metabolites indicate the rat may be a better model for humans than the mouse (Seaton et al.,

cited in US EPA, 2002).

4.1.6 Mechanism of Toxic Action

Haematotoxicity and leukemia are considered to be the most critical effects of benzene exposure

and thus numerous studies have been conducted to determine the mechanisms by which benzene

produces these effects. Benzene also affects reproduction, development, and the nervous system;

however, the mechanism for these endpoints has not been studied in as much detail (ATSDR,

2005; IPCS, 1993; US EPA, 1998; 2002).

Studies conducted in mice and rats by Sammett et al. and Valentine et al. have shown that the

metabolism of benzene by CYP2E1 was necessary for the expression of haematotoxicity (US

EPA, 2002). The haematotoxicity of benzene has been attributed to the transport of phenolic

metabolites (phenol, catechol, hydroquinone, 1,2,4-benzenetriol, and 1,2- and 1,4-benzoquinone)

to the bone marrow and potentially the metabolism of benzene within the bone marrow (ATSDR,

2005). Phenolic metabolites in the bone marrow can be metabolized by myeloperoxidase to

form highly reactive semiquinone radicals and quinones that stimulate production of reactive

oxygen species and lead to DNA damage; damage to stem or early progenitor cells would be

expressed as hematopoietic and leukemogenic effects (Smith, cited in ATSDR, 2005).

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 15

The US EPA (2002) cited several different mechanisms for benzene toxicity including: the

formation of covalent adducts with cell proteins and DNA; chromosomal aberrations; oxidative

stress/formation of reactive oxygen species in target tissues, and; the inhibition of cytokine IL-1

formation, which is essential for the development of blood cells (haematopoiesis).

4.2 Genotoxicity

Chromosomal aberrations were reported in peripheral blood lymphocytes and bone marrow

following occupational exposure (dermal and inhalation) to benzene (ATSDR, 2005). In

workers chronically exposed to benzene, Zhang and colleagues reported monosomy of

chromosomes 5, 7, and 8 and trisomy and/or tetrasomy of chromosomes 1, 5, 7, and 8 in

peripheral lymphocytes (mean time weighted average (TWA) exposure of 30 ppm or 98 mg m-3

benzene); Sasiadek et al. reported breaks in chromosomes 2, 4, and 9 gaps in chromosomes 1

and 2; Kasuba et al. reported dicentric chromosomes and unstable aberrations, and; Smith et al.

reported increased hyperploidy of chromosomes 8 and 21 and translocations between

chromosomes 8 and 21 (mean TWA exposure of 31 ppm or 102 mg m-3

benzene) (ATSDR,

2005).

A 10 year study (1990 to 2000) on the hprt loci of peripheral blood lymphocytes collected from

oil refinery workers exposed to benzene (annual peak concentrations from 1.5 to 43.8 ppm; 5 to

143 mg m-3

) reported a reduction in the incorporation of tritiated thymidine and an increase in

the frequency of mutations in the hprt loci, suggesting the potential for benzene to produce

mutations (Major et al., cited in ATSDR, 2005).

Liu et al. (cited in ATSDR, 2005) assessed DNA damage in peripheral blood lymphocytes from -3 -3

workers exposed to low (2.46 mg m or 0.78 ppm), medium (103 mg m or 32.2 ppm), or high

(424 mg m-3

or 133 ppm) benzene concentrations. Oxidative DNA damage (formation of 8

hydroxy-2-deoxyguanosine or 8-OHdG) and micronuclei formation increased in a dose-related

manner with increasing benzene concentrations. A dose-related increase in urinary levels of 8

OHdG and DNA single-strand breaks were reported for male gasoline station workers exposed to

breathing zone benzene concentrations ranging from 0.003 to 0.6 ppm (mean 0.13 ppm or 0.42

mg m -3

) by Nilsson et al. (cited in ATSDR, 2005). These studies suggest the induction of

reactive oxygen species and oxidative DNA damage by benzene metabolites (ATSDR, 2005).

Numerous in vivo mammalian studies have provided convincing evidence of the genotoxicity of

benzene (ATSDR, 2005). Chromosomal aberrations have been reported in bone marrow of

mice, rats, Chinese hamsters and rabbits (Tice et al.; Siou et al.; Meyne and Legator; Shelby and

Witt; Giver et al.; Styles and Richardson; Anderson and Richardson; Philip and Jensen; Fujie et

al.; Hoechst; Kissling and Speck; cited in ATSDR, 2005) and in spleen lymphocytes, lymphoid

cells, and myeloid cells of mice (Rithidech et al.; Au et al.; Giver et al., cited in ATSDR, 2005)

following in vivo exposure to benzene. Benzene exposure was also associated with an increase

in sister chromatid exchange in mice (bone marrow, lymphocytes, fetal liver cells) and rats

(lymphocytes) (Tice et al.; Erexson et al.; Sharma et al., cited in ATSDR, 2005).

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 16

An increase in micronuclei frequency was reported in fetal liver cells, bone marrow, lung

fibroblasts, and peripheral and bone marrow erythrocytes of benzene exposed mice (Farris et al.;

Siou et al.; Erexson et al.; Healy et al.; Ranaldi et al.; Diaz et al.; Choy et al.; Barale et al.; Hite

et al.; Meyne and Legator; Au et al.; Harper et al.; Toft et al.; Ciranni et al.; Suzuki et al.;

Rithidech et al.; Luke et al., cited in ATSDR, 2005). The increase in micronuclei frequency in

erythrocytes was exposure-dependent and required exposure concentrations of 100 ppm (326 mg

m-3

) or greater; this response was not observed by Farris and colleagues at air concentrations of 1

or 10 ppm (3 to 33 mg m-3

) benzene (ATSDR, 2005).

In vivo exposure to benzene produced DNA mutations (spleen lymphocytes and lung tissue)

(Mullin et al.; Ward et al., cited in ATSDR, 2005) and strand breaks (peripheral blood

lymphocytes) (Tuo et al., cited in ATSDR, 2005) in mice, inhibited DNA and/or RNA synthesis

(bone marrow, liver mitochondria) in mice and rats (Lee et al; Kissling and Speck; Kalf et al.,

cited in ATSDR, 2005) and resulted in oxidative DNA damage in rats (Liu et al., cited in

ATSDR, 2005). Equivocal results were reported for the formation of DNA adducts following in

vivo exposure to benzene, with negative responses reported for mouse bone marrow and

mammary gland by Reddy et al. (ATSDR, 2005) and positive responses reported for mouse bone

marrow and white blood cells, and rat liver cells by Pathak et al.; Levay et al., and; Lutz and

Schlatter (ATSDR, 2005). One in vivo study in mice reported sperm head abnormality following

benzene exposure (Topham, cited in ATSDR, 2005).

In vitro studies of benzene confirm the results of in vivo genotoxicity studies and suggest that

genotoxicity is primarily the result of benzene metabolites (ATSDR, 2005). Several studies

reported positive results for gene mutation in Salmonella typhimurium (Glatt et al.; Kaden et al.;

Seixas et al., cited in ATSDR, 2005), sister chromatid exchange in human lymphocytes

(Morimoto, cited in ATSDR, 2005), altered DNA synthesis in rat hepatocytes (Glauert et al.,

cited in ATSDR, 2005), altered RNA synthesis in rat liver mitoplasts and rabbit and cat bone

marrow mitoplasts (Kalf et al., cited in ATSDR, 2005), and DNA adduct formation in rat liver

mitoplasts (Rushmore et al., cited in ATSDR, 2005) only in the presence of endogenous (within

the cell) or exogenous (added to the cellular preparation) metabolic activators of benzene.

In summary, benzene is considered a potent clastogen with evidence of numerical and structural

chromosomal aberrations, sister chromatid exchanges, and micronuclei following in vivo

exposure of experimental animals and humans. In vitro tests results suggest that benzene

metabolites are responsible for gene mutations, DNA adducts, and altered DNA and RNA

synthesis. Details on in vivo and in vitro studies of benzene genotoxicity are provided in

ATSDR, 2005.

4.3 Acute Toxicity

Acute exposure to very high benzene concentrations (44,662 to 65,200 mg m-3

) has been fatal to

animals and humans. Haematological (reduced blood cell count), neurological, and respiratory

effects occurred in humans following acute exposure to non-lethal benzene air concentrations (up

to 978 mg m-3

). The acute exposure of mice and rats to benzene decreased production of white

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 17

blood cells (leucocytes, granulocytes, and lymphocytes), red blood cell production (erythrocytes

and CFU-E), bone marrow cellularity, and induced chromosomal damage.

4.3.1 Acute Toxicity in Humans

Exposure of humans to 20,000 ppm (65,200 mg m-3

) benzene for 5 to 10 minutes is considered

fatal (Flury et al., cited in ATSDR, 2005). The cause of death from acute overexposure to

benzene has been reported to result from asphyxiation, respiratory arrest, CNS depression or

cardiac collapse (Avis and Hutton; Greenburg; Hamilton; Winck and Collom; Winck et al.; cited

in ATSDR, 2005). Brief exposure (30 minutes) to 300 ppm (978 mg m-3

) benzene produced

drowsiness, dizziness and headaches in exposed workers (Flury et al., cited in ATSDR, 2005).

Occupational exposure of males to benzene air concentrations >60 ppm (196 mg m-3

) for up to 3

weeks (2.5 to 8 hours/day) during the removal of residual fuel from shipyard tanks produced

respiratory effects (mucus membrane irritation and dyspnea), reduced blood cell counts

(leukocytes, erythrocytes, and thrombocytes), and neurological effects (dizziness, nausea,

headache, fatigue) (Midzenski et al., cited in ATSDR, 2005). Uncertainty in exposure levels and

duration, the potential for confounding exposures to other chemicals, and lack of corresponding

control groups, limit the use of data collected from an occupational setting; however, the ATSDR

has identified well conducted occupational studies with effects linked to specific benzene

exposure concentrations. Adverse health effects reported in well conducted human studies

following the acute inhalation of benzene and the air concentration at which they are predicted to

occur are summarized in Table 5.

Table 5 Acute Effects Following Human Exposure to Benzene

Effect Exposure Period Air Concentration

ppm (mg m-3

) Reference

1

Death 5 to 10 minutes 20,000 (65,200) Flury et al. 1928

Neurological: drowsiness,

dizziness, headaches 30 min 300 (978) Flury et al. 1928

Neurological: dizziness,

headaches, nausea, fatigue

(males)

1-21 d, 2.5-8 hr/d 60 (196) Midzenski et al.

1992

Respiratory: mucus membrane

irritation and dyspnea (males).

Hematological: leucopenia, anemia, and thrombocytopenia

(males).

1-21 d, 2.5-8 hr/d 60 (196) Midzenski et al.

1992

1cited in ATSDR, 2005

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 18

4.3.2 Acute Toxicity in Animals

Acute inhalation of high benzene concentrations has resulted in death of experimental animals

with a 4 hour LC50 value of 13,700 ppm (44,662 mg m-3

) was reported for Sprague-Dawley rats

(Drew and Fouts, cited in ATSDR, 2005). Exposure of C57B1/6BNL mice to lower benzene air

concentrations (400 ppm or 1304 mg m-3

) for longer duration (2 weeks) did not cause death

(Cronkite et al., cited in ATSDR, 2005).

The acute exposure of mice and rats to benzene resulted in decreased production of white blood

cells (leucocytes, granulocytes, and lymphocytes), decreased red blood cell production

(erythrocytes and CFU-E), decreased bone marrow cellularity, and chromosomal damage

(ATSDR, 2005). The lowest observed adverse effect levels (LOAELs) for decreases in white

blood cells in mice or rats acutely exposed (6-24 hours/day for 1 to 14 days) to benzene ranged

from 10.2 to 400 ppm (33 to 1304 mg m-3

), while the no observed adverse effect levels

(NOAEL) for these responses ranged from 3 to 50 ppm (10 to 163 mg m-3

) benzene (Toft et al.;

Rosenthal and Snyder; Gill et al.; Wells and Nerland; Green et al.; Rozen et al.; Li et al.;

Cronkite et al.; Chertkov et al.; Ward et al.; Aoyama et al, cited in ATSDR, 2005). The

LOAEL values reported for decreased red blood cell production in mice acutely exposed (6

hours/day for 5 to 14 days) to benzene ranged from 10 to 400 ppm (33 to 1304 mg m-3

), while

the NOAELs for these responses in mice ranged from 25 to 30 ppm (82 to 98 mg m-3

) of benzene

(Dempster and Snyder; Rozen et al.; Cronkite et al.; Neun et al.; Ward et al., cited in ATSDR,

2005). LOAELs for decreased bone marrow cellularity in acutely exposed mice (6 to 24

hours/day for 1 to 14 days) ranged from 21 to 400 ppm (68 to 1304 mg m-3

)(Toft et al.; Gill et

al.; Green et al.; Cronkite et al.; Neun et al.; Chertkov et al., cited in ATSDR, 2005) with one

NOAEL of 9.9 ppm (32 mg m-3

) reported for this response in mice (Green et al., cited in

ATSDR, 2005).

An increased frequency of micronucleated polychromatic erythrocytes, indicative of

chromosomal damage, was reported in male NMRI mice following a series of experiments

conducted by Toft et al. (cited in ATSDR, 2005) exposing the mice to benzene for 8 to 24

hours/day over 1 to 14 days. A LOAEL of 21 ppm (68 mg m-3

) and NOAEL values ranging

from 10.5 to 14 ppm (34 to 46 mg m-3

) were reported for this response in mice (ATSDR, 2005).

A minimum risk level (MRL) for acute inhalation exposure (14 days or less) to benzene was

established by the ATSDR (2005) based on the lowest reported LOAEL (10.2 ppm or 33 mg m-3

)

for decreased white blood cell production in mice. Exposure of male C57B1/6J mice to 10.2

ppm (33 mg m-3

) benzene for 6 hours per day over 6 days resulted in decreased lymphocyte

production (Rozen et al., cited in ATSDR, 2005). An MRL of 0.009 ppm (29 µg m-3

) was

recommended based on this LOAEL following adjustment for intermittent exposure, conversion

to a human equivalent concentration, and division by a 300-fold uncertainty factor to account for

use of a LOAEL, extrapolation from animals to humans, and variations in human sensitivity.

This MRL is an estimate of daily human exposure to benzene that is likely to be without an

appreciable risk of adverse effects (non carcinogenic) over a continuous exposure period of 14

days or less (ATSDR, 2005). A summary of the acute effects of benzene inhalation in animals,

the exposure period and concentration at which they occurred, and the species studied is

provided in Table 6.

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 19

Table 6 Acute Effects Following Animal Exposure to Benzene

Effect Exposure Period Air Concentration

ppm (mg m-3

) Species Reference

1

4 hr LC50 = 13,700 (44,662) Rat

Sprague-Dawley

Drew and Fouts,

1974 Death

2 weeks NOAEL = 400 (1304) Mouse C57B1/6BNL

Cronkite et al. 1985

1-10 d, 24 hr/d LOAEL = 21 (68) Mouse, male

NMRI Toft et al. 1982

1-12 d, 6 hr/d LOAEL = 30 (98)

NOAEL = 10 (33)

Mouse, male

C57B1/6

Rosenthal and

Snyder, 1985

2-8 d, 24 hr/d LOAEL = 100 (326) Mouse, male

C57B1/6 Gill et al. 1980

5 d, 6 hr/d LOAEL = 25 (82)

NOAEL = 3 (10)

Mouse, male

Swiss Webster

Wells and Nerland,

1991

5 d, 6 hr/d LOAEL = 103 (336)

NOAEL = 9.9 (32)

Mouse, male

CD-1 Green et al. 1981

6 d, 6 h/d LOAEL = 10.2 (33) Mouse, male

C57B1/6J Rozen et al.1984

7 d, 8 hr/d LOAEL = 100 (326)

NOAEL = 50 (163)

Rat, female

Wistar Li et al. 1986

11 d, 5 d/wk, 6 hr/d LOAEL = 400 (1304) Mouse, male

Hale-Stoner Cronkite et al., 1982

Decreased WBC:

leucopenia,

granulocytopenia,

lymphopenia.

2 wk, 5 d/wk, 6 hr/d LOAEL = 25 (82)

NOAEL = 10 (33)

Mouse

C57B1/6 BNL,

CBA/Ca

Cronkite et al. 1985;

1986

2 wk, 5 d/wk, 6 hr/d LOAEL = 300 (978) Mouse, male

DBA/2

Chertkov et al.,

1992

2 wk, 5 d/wk, 6 hr/d LOAEL = 300 (978)

NOAEL = 30 (98)

Rat, male

Sprague-Dawley

Mouse

CD-1

Ward et al., 1985

2 wk, 5 d/wk, 8 hr/d LOAEL = 21 (68)

NOAEL = 10.5 (34)

Mouse

NMRI Toft et al. 1982

14 d, 6 h/d LOAEL = 48 (156) Mouse, male

BALB/c Aoyama et al., 1986

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 20

Table 6 Acute Effects Following Animal Exposure to Benzene (continued)

Air Concentration Effect Exposure Period -3 Species Reference

1

ppm (mg m )

Mouse Dempster and 5 d, 6 h/d LOAEL = 10 (33)

DBA/2J Snyder, 1991

Mouse, male 6 d, 6 h/d LOAEL = 100 (326) Rozen et al., 1984

C57B1/6J

Mouse, male Cronkite et al., Decreased RBC: 11 d , 5 d/wk, 6 hr/d LOAEL = 400 (1304)

Hale-Stoner 1982; 1989 anaemia, decreased

Mouse, male erythrocytes, reduced 2 wk, 4 d/wk, 6 h/d LOAEL = 300 (978) Swiss Webster, Neun et al., 1982 CFU-E.

C57B1/6J

LOAEL = 100 (326) Mouse 2 wk, 5 d/wk, 6 hr/d Cronkite et al., 1985

NOAEL = 25 (82) C57B1/6BNL

LOAEL = 300 (978) Mouse 2 wk, 5 d/wk, 6 hr/d Ward et al. 1985

NOAEL = 30 (98) CD-1

Mouse 1-10 d, 24 hr/d LOAEL = 21 (68) Toft et al. 1982

NMRI

Mouse, male 2-8 d, 24 hr/d LOAEL = 100 (326) Gill et al. 1980

C57B1/6

LOAEL = 103 (336) Mouse, male 5 d, 6 hr/d Green et al. 1981

NOAEL = 9.9 (32) CD-1 Decreased bone

Mouse, male marrow cellularity 11 d, 5 d/wk, 6 hr/d LOAEL = 400 (1304) Cronkite et al., 1982 Hale-Stoner

Mouse, male

2 wk, 4 d/wk, 6 h/d LOAEL = 300 (978) Swiss Webster, Neun et al., 1982

C57B1/6J

Mouse, male Chertkov et al., 2 wk, 5 d/wk, 6 hr/d LOAEL = 300 (978)

DBA/2 1992

Mouse, male Chromosomal: 1-10 d, 24 h/d LOAEL = 21 (68) Toft et al. 1982 NMRI increased frequency

of micronucleated Mouse, male 1 wk, 24 hr/d NOAEL = 14 (46) Toft et al. 1982

polychromatic NMRI erythrocytes (MN LOAEL = 21 (68) Mouse, male PCEs). 2 wk, 5 d/wk, 8 h/d Toft et al. 1982 NOAEL = 10.5 (34) NMRI

LC50: lethal concentration to 50% of the exposed study population 1cited in ATSDR, 2005

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 21

4.4 Subchronic and Chronic Toxicity

Similar to the effects reported following acute exposures, subchronic and chronic exposure to

relatively low levels of benzene produced measurable depression of one or more circulating

blood cells, resulting in haematotoxic and immunotoxic effects. Subchronic and chronic studies

in humans and animals have reported pancytopenia or the reduction in number of all major blood

cells, including leukocytes (white blood cells), erythrocytes (red blood cells), and thrombocytes

(platelets). Blood cells are produced by the bone marrow and therefore pancytopenia is a

condition that results from the inability of the bone marrow to adequately produce mature blood

cells. A more severe effect of benzene exposure is aplastic anaemia in which the bone marrow is

unable to function and stem cells do not mature. The progression of aplastic anaemia can result

in acute myelogenous leukemia, or cancer of the myeloid line of white blood cells (ATSDR,

2005).

The ability of benzene to induce carcinogenic effects via the same mechanism as haematotoxic

and immunotoxic effects has not yet been demonstrated. It has been well demonstrated in human

and animal studies that chronic exposure to benzene depressed bone marrow function however,

there is no suitable animal model for the induction of leukemia following benzene exposure.

Furthermore, benzene-induced haematotoxicity/immunotoxicity leads to health effects apart

from potential induction of leukemia (US EPA, 2002).

4.4.1 Subchronic and Chronic Toxicity in Humans

Pancytopenia was reported in workers occupationally exposed to benzene concentrations ranging

from 3 to 210 ppm (10 to 685 mg m-3

) over periods of 4 months to 3 years (Askoy and Erdrem;

Askoy et al.; Doskin; Erf and Rhoads, Rothman et al., cited in ATSDR, 2005). Decreased

production of white blood cells (leucocytes and lymphocytes) occurred in workers

occupationally exposed for 1 to 21 years to benzene concentrations ranging from 0.57 to 75 ppm

(1.86 to 245 mg m-3

)(Cody et al.; Xia et al.; Tsai et al.; Kipen et al.; Qu et al.; Rothman et al.,

cited in ATSDR, 2005). Decreased red blood cell counts and anaemia were reported following

subchronic and chronic occupational exposure to benzene concentrations ranging from 2.26 to 29

ppm (7.37 to 95 mg m-3

) (Yin et al.; Goldwater; Greenburg et al.; Tsai et al.; Kipen et al.; Qu et

al., cited in ATSDR, 2005).

There was a lack of observed adverse effects on blood cells in male refinery workers exposed to

0.53 ppm (1.73 mg m-3

) benzene for 1-21 years (Tsai et al., cited in ATSDR, 2005). This

exposure level was selected by the California Office of Environmental Health Hazard

Assessment and adjusted for continuous exposure and variation in human sensitivity to develop a

chronic reference exposure level (REL) of 0.02 ppm or 60 µg m-3

(OEHHA, 1999).

The study reporting the lowest air concentration at which white blood cell (lymphocyte) levels

were reduced was selected by the ATSDR (2005) for the development of the minimum risk level

(MRL) for chronic inhalation exposure (>365 days) to benzene. Significant decreases in B-

lymphocyte counts were reported for male shoe manufacturing workers in Tianjin, China

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 22

exposed to 0.57 ppm (1.86 mg m-3

) benzene for an average of 6.1 years (Lan et al., cited in

ATSDR, 2005). A chronic MRL of 0.003 ppm (0.01 mg m-3

) was determined using benchmark

dose (BMD) modeling and adjusting from occupational to continuous exposure. A 10-fold

uncertainty factor was also applied to account for variations in human sensitivity (ATSDR,

2005).

The US EPA developed a Reference Concentration (RfC) also based on a study reporting

decreased lymphocyte counts following occupational exposure to 7.6 ppm (24 mg m-3

) benzene

(Rothman et al., cited in US EPA, 2002). The US EPA used benchmark dose modeling and

adjusted for human variability, subchronic-to-chronic exposures, and database deficiencies to

arrive at an RfC of 30 µg m-3

for lifetime chronic human exposure to benzene (US EPA, 2002).

The California OEHHA, the ATSDR, and the US EPA have all developed chronic exposure

guidelines for benzene based on effects (or lack thereof) on blood cell counts following

occupational exposures. However, the US EPA (2002) cautions that there is inherent uncertainty

as to when a change in a parameter that has inherent variability, such as hematologic parameters,

will translate into an adverse effect.

Selected key effects reported in humans following subchronic and chronic inhalation exposure to

benzene and the air concentrations at which these effects occurred are summarized in Table 7.

4.4.2 Subchronic and Chronic Toxicity in Animals

Subchronic and chronic exposure of rats and mice to 200 or 300 ppm (652 to 978 mg m-3

)

benzene resulted in death or a shortened lifespan (Maltoni et al.; Cronkite et al.; Farris et al.;

Green et al.; Snyder et al., cited in ATSDR, 2005). Pancytopenia occurred in mice

subchronically exposed (13 weeks) to 300 ppm (978 mg m-3

) benzene (Ward et al., cited in

ATSDR, 2005) and chronically exposed (lifetime) to 100 ppm (326 mg m-3

) benzene (Snyder et

al., cited in ATSDR, 2005). A NOAEL of 30 ppm (98 mg m-3

) was reported for pancytopenia in

mice following subchronic exposure to benzene (Ward et al. cited in ATSDR, 2005).

The lowest LOAELs reported for decreases in white blood cell count (leukocytes, granulocytes,

and/or lymphocytes) following subchronic or chronic benzene exposure ranged from 100 to 400

ppm (326 to 1304 mg m-3

) in mice (Cronkite et al.; Green et al.; Farris et al.; Toft et al.; Cronkite

et al.; Green et al.; Snyder et al., cited in ATSDR, 2005) and from 100 to 500 ppm (326 to 1630

mg m -3

) in rats (Dow; Ward et al.; Wolf et al.; Snyder et al., cited in ATSDR, 2005). LOAELs

of 80, 88 and 100 ppm (261, 287, and 326 mg m-3

) were reported for decreased white blood cell

counts in chronically exposed rabbits, guinea pigs and Duroc Jersey pigs, respectively (Wolf et

al.; Dow, cited in ATSDR, 2005).

LOAEL values for reductions in red blood cell counts in mice subchronically or chronically

exposed to benzene ranged from 10 to 400 ppm (33 to 1304 mg m-3

) benzene (Baarson et al.;

Vacha et al.; Farris et al.; Toft et al.; Plappert et al.; Seidel et al.; Cronkite et al.; Luke et al.;

Green et al.; Snyder et al., cited in ATSDR, 2005). NOAEL values ranging from 9.6 to 100 ppm

(31 to 326 mg m-3

) were reported for reduced red blood cell counts following subchronic and

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 23

chronic exposure of mice to benzene (Farris et al.; Toft et al.; Plappert et al.; Green et al., cited

in ATSDR, 2005).

Table 7 Subchronic and Chronic Effects Reported Following Human (Occupational)

Exposure to Benzene

Effects Reported Exposure Period Air Concentration

ppm (mg m-3

) Reference

1

4 mo – 1 yr 150 (489) Askoy and Erdem, 1978

4 mo – 1 yr 210 (685) Askoy et al. 1972

Pancytopenia: decreased

RBC, WBC and

platelets.

1 – 3 yr

Chronic

(not specified)

6.3 yr (average)

3 (10)

24 (78)

31 (101)

Doskin, 1971

Erf and Rhoads, 1939

Rothman et al. 1996a;

1996b

Decreased WBC:

leucopenia, lymphopenia..

1 yr

> 1 yr

1 – 21 yr

1 – 25 yr

4.5 – 9.7 yr

6.1 yr (average)

6.3 yr (average)

40 (130)

0.69 (2.25)

NOAEL = 0.53 (1.73)

75 (245)

2.26 (7.4)

0.57 (1.86)

7.6 (24)

Cody et al. 1983

Xia et al. 1995

Tsai et al. 1983

Kipen et al. 1989

Qu et al. 2002; 2003

Lan et al, 2004

Rothman et al. 1996a;

1996b

3.5 mo – 19 yr 29 (95) Yin et al. 1987

0.5 to 5 yr 11 (36) Goldwater 1941;

Greenburg et al. 1939

Decreased RBC 1 – 21 yr NOAEL = 0.53 (1.73) Tsai et al. 1983

1 – 25 yr 75 (245) Kipen et al. 1989

4.5 – 9.7 yr 2.26 (7.37) Qu et al. 2002; 2003

1cited in ATSDR, 2005

A reduction in bone marrow cellularity was reported for mice and rats subchronically and

chronically exposed to benzene air concentrations ranging from 100 to 400 ppm (326 to 1304 mg

m-3

) (Farris et al.; Cronkite et al.; Ward et al.; Cronkite et al.; Snyder et al., cited in ATSDR,

2005) with NOAELs for this response reported to range from 10 to 30 ppm (33 to 98 mg m-3

)

benzene (Farris et al.; Toft et al.; Ward et al.; Cronkite et al., cited in ATSDR, 2005). In

Assessment Report on Benzene for Developing Ambient Air Quality Objectives 24

contrast, granulocytic hyperplasia (increased WBC production) was induced in the bone marrow

of mice subchronically exposed (16 weeks) to 300 ppm (978 mg m-3

) benzene (Farris et al., cited

in ATSDR, 2005).

A decrease in spleen cells and