Risk Assessment

Philip Bedient, Ph.D.

Rice University

Introduction

• The concepts of risk and hazard are inextricably intertwined

• Hazard - implies a probability of adverse effects in a particular situation

• Risk - measure of the probability• The use of the results of a risk assessment to

make policy decisions is called risk management

A Scientific Point of View• Scientists and

engineers use models to calculate an estimated risk, such as:Tornadoes

Hurricanes

Floods

Droughts

Risk Perception• Some risks are well quantified

Ex: Frequency and severity of auto accidents are well documented.

• In contrast, other hazardous activities, such as those resulting from the use of tobacco and alcohol are more difficult to document

Risk Perception

• Table 5-1 illustrates different perceptions of risk. Four different groups were asked to rate 30 activities & technologies according to the present risk of death from each.

• Table 5-1 shows how each group ranked the risk of of ten out of the original 30 topics.

Table 5-1. Ordering of Perceived Risk for 30 Activities and Technologies

Source of Risk Group 1

LOWV

College students

Active Club Members

Experts

Nuclear Power 1 1 8 20

Motor vehicles 2 5 3 1

Handguns 3 2 1 4

Smoking 4 3 4 2

Motorcycles 5 6 2 6

Alcoholic Beverages

6 7 5 3

General (private) aviation

7 15 11 12

Police Work 9 8 7 17

Pesticides 9 4 15 8

Surgery 10 11 9 5

• Putting risk perception in perspective, we can calculate the risk of death from some familiar causes.

Ex: In the U.S. in 2001, there were about 3.9 million deaths per year. Of these, about 541,532 were cancer-related.

The annual risk (assuming a 70-year life expectancy & ignoring age factors) is about:

€

541,532

3.9 ×106= 0.14

€

0.14

70= .002

• For comparison, Table 5-2 summarizes the risk of dying from some causes of death.

Cause of Death # of Deaths in Rep. Year Individual Risk per Year

Black lung disease 1,135 8 x 10-3, or 1/125

Heart attack 724,859 2.7 x 10-3, or 1/370

Cancer 541,532 2.0 x 10-3, or 1/500

Coal mining accident 180 1.3 x 10-3, or 1/770

Fire fighting -- 3 x 10-4, or 1/3,333

Motor vehicle 46,000 2.2 x 10-4, or 1/4,545

Truck driving 400 10-4, or 1/10,000

Falls 16,339 7.7 x 10-5, or 1/13,000

Football (averaged over participants)

4 x 10-5, or 1/25,000

Home accidents 25,000 1.2 x 10-5, or 1/83,333

Bicycling 1,000 10-5, or 1/100,000

Air Travel: 1 transcontinental trip/year

2 x 10-6, or 1/500,000

Risk Assessment

• In 1989, the EPA adopted a formal process for conducting a baseline risk assessment

• This process includes: Data collection and evaluation Toxicity assessment Exposure assessment Risk characterization

Data Collection and Evaluation

• This part of the process includes: Gathering background and site information Preliminary identification of potential human

and ecosystem exposure through sampling• To gather background information, it is

necessary to find: Possible contaminants Concentrations of contaminants in key

sources and media Characteristics of environmental setting

Toxicity Assessment

• Toxicity Assessment- the process of determining the relationship between the exposure to a contaminant and the increased likelihood of the occurrence or severity of adverse effects

• Hazard Identification- determines whether exposure to a contaminant causes increased adverse effects for humans and to what level of severity

• Dose- the mass of chemical received by the animal or exposed individual

Toxicity Assessment Cont.• Quantal- all-or-nothing responses

Ex.- mortality and tumor formation• Dose-response curve- statistical relationship

of organism response to dose• This is usually expressed as a cumulative-

frequency distribution

Example 5-1• An experiment was developed to ascertain

whether a compound has a 5% probability of causing a tumor. The same dose of the compound was administered to 10 groups of 100 test animals. A control group of 100 animals was, with the exception of the test compound, exposed to the same environmental conditions for the same period of time. The following results were obtained:

Group Number of Tumors

Group Number of Tumors

A 6 F 9

B 4 G 5

C 10 H 1

D 1 I 4

E 2 J 7

Example 5-1 Results

*No tumors were detected in the controls(not likely in reality)

Example 5-1 Solution• The average number of excess tumors is

4.9%. These results tend to confirm that the probability of causing a tumor is 5%.

• If, instead of using 1000 animals, only 100 animals were used, it is fairly evident from the data that, statistically speaking, some very anomalous results might be achieved. That is, we might find a risk from 1-10%.

• Note that a 5% risk is very high in comparison with the EPA’s objective of achieving an environmental contaminant risk of 10 -7 to 10 -4.

Toxicity Assessment Terms

• Linearized multistage model- a modification of the multistage model for toxicological assessment

• This model states that we can extrapolate from high doses to low doses with a straight line

• Slope factor- expressed in units of risk per unit dose• Integrated Risk Information- EPA toxicological data

base that provides background information on potential carcinogens

Slope Factors for Potential CarcinogensChemical CPS0

( kg*day*mg-1)

CPSi

(kg*day*mg -1)

Arsenic 1.5 15.1

Benzene 0.029 0.029

Benzo(a)pyrene 7.3 N/A

Cadmium N/A 6.3

Carbon Tetrachloride

0.13 0.0525

Chloroform 0.0061 0.08

Chromium (VI) N/A 42.0

DDT 0.34 0.34

1,1-Dichloroethylene

0.6 0.175

Dieldrin 16.0 16.1

Slope Factors for Potential Carcinogens Cont.

Heptachlor 4.5 4.55

Hecachloroethane 0.014 0.014

Methylene Chloride

0.0075 0.00164

Polychlorinated Biphenyls

7.7 N/A

2,3,7,8-TCDDb 1.5 * 105 1.16 * 105

Tetrachloroethyleneb

0.052 0.002

Trichloroethylenec w 0.006

Vinyl chlorideb 1.9 N/A

Chemical CPS0

( kg*day*mg-1)

CPSi

(kg*day*mg -1)

Limitations of Animal Studies

• Most of the effects on people can be produced in species

• Exceptions to this include: Toxicities dependent on immunogenic mechanisms

• Subtle toxicity is difficult to transfer from lab animals to people because of the effects of ancillary factors

Limitations of Epidemiological Studies

• There are four difficulties presented in this type of study:1) Large populations are required to detect a low

frequency of occurrence of a toxicological effect2) A long or highly variable latency period may be

needed between the exposure to the toxicant and a measurable effect

3) Competing causes of the observed toxicological response make it difficult to attribute a direct cause and effect

4) Epidemiological studies are often based on data collected in specific political boundaries that do not necessarily coincide with environmental boundaries such as those defined by an aquifer or the prevailing wind patters

Exposure Assessment• Total exposure assessment- evaluation of all major

sources of exposure • Elimination of a pathway of entry can be justified if:

The exposure from a particular pathway is less than that of exposure through another pathway involving the same media at the same exposure point

The magnitude of exposure from the pathway is low The probability of exposure is low and incidental risk

is not high• Reasonable maximum exposure- the highest exposure

that is reasonably expected to occur and is intended to be a conservative estimate within the range of possible exposures

Intake EquationCDI= C [ (CR) (EFD) ] (1)

BW AT

• CDI- chronic daily intake• C- chemical concentration, contacted over the

exposure period• CR- contact rate, the amount of contaminated

medium contacted per unit time• EFD- exposure frequency and duration• BW- body weight• AT- averaging time

Example 5-2• Problem: Estimate the chronic daily intake CDI of

benzene from exposure to a city water supply that contains a benzene concentration equal to the drinking water standard. The allowable drinking water concentration (maximum contaminant level, MCL) is 0.005 mg*L-1

• Solution: From Table 5-7, we note that five routes of exposure are possible from the drinking water medium: 1.) ingestion,dermal contact while 2.) Showering and 3.) swimming, 4.) inhalation of vapor while showering, and 5.) ingestion whie swimming.

• CDI= (0.005 mg*L-1)(2.0 L*day-1)(365 days*year-1)(70 years)

(70 kg)(70 years)(365 days*year-1)

• = 1.43 * 10-4mg*kg-1*day-1

Risk Characterization• For low-dose cancer risk:

Risk = (intake)(slope factor)• For high carcinogenic risk:

Risk = 1-exp[-(intake)(slope factor)]• The noncancer hazard quotient or

hazard index: HI= intake

RfD

Example 5-3 • Using the results from Example 5-2, estimate the risk

from exposure to drinking water containing the MCL for benzene

• Equation 5-21 in the form:• Total exposure risk: riskj may be used to estimate

the risk. Because the problem is only to consider one compound, namely benzene, i=1 and others do not need to be considered. Because the total exposure from Example 5-2 included each of the routes of concern for drinking water, that is, all j’s, the final sum may be used to compute risk. The risk is: Risk= (1.90 * 10-4 mg*kg -1*day -1)(2.9 * 10-2

kg*day*mg -1) =5.5 * 10 -6

Example 5-3 Cont.• This is the total lifetime risk (70 years) for

benzene in drinking water at the MCL. Another way of viewing this is to estimate the number of people that might develop cancer. For example, in a population of 2 million: (2*106) (5.5*10-6) = 11 people might develop

cancer• This risk falls within the EPA guidelines of 10-4

to 10-7 risk. It, of course, does not account for all sources of benzene by all routes. Nonetheless, the risk, compared with some other risks in daily life, appears to be quite small.

Risk Management• This is performed in order to decide the

magnitude of risk that is tolerable in specific circumstances

• If a very high certainty in avoiding risk is required, the costs in achieving low concentrations of the contaminant are likely to be high

• To reduce risk it is necessary to: Change the environment Modify the exposure Compensate for the effects