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Atmospheric Environment 41 (2007) 3217–3224

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The influence of humidity on the emission of di-(2-ethylhexyl)phthalate (DEHP) from vinyl flooring in the emission

cell ‘‘FLEC’’

Per Axel Clausena,�, Ying Xub, Vivi Kofoed-Sørensena,John C. Littleb, Peder Wolkoffa

aIndoor Environment Group, National Institute of Occupational Health, Lersø Parkalle 105, DK-2100 Copenhagen Ø, DenmarkbDepartment of Civil and Environmental Engineering, 418 Durham Hall, Virginia Tech, Blacksburg, VA 24061, USA

Received 16 March 2006; received in revised form 23 June 2006; accepted 29 June 2006

Abstract

Asthma in children appears to be associated with both phthalate esters and dampness in buildings. An impor-

tant question is whether the concentrations of phthalate esters correlate with dampness (expressed as relative humidity—

RH) in indoor air. The objective was to study the influence of RH on the specific emission rate (SER) of di-(2-

ethylhexyl)phthalate (DEHP) from one type of vinyl flooring in the well characterized Field and Laboratory Emission

Cell (FLEC). The vinyl flooring with ca. 17% (w/w) DEHP as plasticizer was tested in 6 FLECs at 22 1C. The RH

in the 6 FLECs was 10%, 30%, 50% (in triplicate) and 70%. The RH was changed after 248 d in 2 of the 50%-FLECs

to 10% and 70%, and to 50% in the 10%-and 70%-FLECs. The data show that the SER of DEHP from vinyl floor-

ing in FLECs during a 1 yr period is independent of the RH. A new physically based emission model for semivolatile

organic compounds was found to be consistent with the experimental data and independent of the RH. The model helps

to explain the RH results, because it appears that RH does not significantly influence any of the identified controlling

mechanisms.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Vinyl flooring; Emission; DEHP; Humidity; Plasticizer

1. Introduction

The prevalence of allergic airway diseases hasincreased in Western Europe and North America(Lundback, 1998). This increase may be associatedwith vinyl flooring containing phthalate esters thatare suspected to possess adjuvant effects that

e front matter r 2006 Elsevier Ltd. All rights reserved

mosenv.2006.06.063

ing author. Tel.: +453916 5273;

201.

ess: pac@ami.dk (P.A. Clausen).

enhance the health damaging potential of commonallergens. Polyvinyl chloride (PVC) is a majorconstituent of vinyl flooring and phthalate estersare commonly used as plasticizers in PVC. Anepidemiological study has shown that the total areaof vinyl flooring in homes was associated withdevelopment of bronchial obstruction in smallchildren (Jaakkola et al., 1999). This study not onlyshowed that the case homes had significantly morevinyl flooring but also significantly more dampnessproblems. These two building characteristics were

.

ARTICLE IN PRESSP.A. Clausen et al. / Atmospheric Environment 41 (2007) 3217–32243218

the only identified ones with significant differencebetween case and control homes. The phthalateester adjuvant hypothesis has been supportedthrough other studies. It has been proposed thatdeposition of the common PVC plasticizer, di(2-ethylhexyl)phthalate (DEHP), in the lungs increasesthe risk of inducing inflammation, which is acharacteristic of asthma (Øie et al., 1997). A recentepidemiological study has also suggested a weakassociation between DEHP and asthma in smallchildren (Bornehag et al., 2004). Both DEHP(Larsen et al., 2001b) and its expected metabolitemono(2-ethylhexyl)phthalate (Larsen et al., 2001a)have been shown to posses an adjuvant effect withsimultaneous injection of the allergenic ovalbuminin mice. Dampness in buildings has been found tobe a strong risk indicator for health effects such asasthma, although the mechanisms are unknown(Bornehag et al., 2001). An important question iswhether there is a causal association betweendampness (represented by relative humidity—RH)and the concentrations of phthalate esters in indoorair.

Phthalate esters used as plasticizers in PVC areslowly emitted as vapors (Uhde et al., 2001; Clausenet al., 2004; Afshari et al., 2004). They are commonpollutants in indoor air (Weschler, 1984; Weschlerand Fong, 1986; Sheldon et al., 1993; Wilson et al.,2001; Fromme et al., 2004) and surface dust(Wilkins et al., 1993; Øie et al., 1997; Pohneret al., 1997; Butte et al., 2001; Kersten and Reich,2003; Clausen et al., 2003). The presence ofphthalate esters in indoor air may be due toresuspension of sedimented dust (Øie et al., 1997)and/or emission from building products containingphthalates (e.g. vinyl flooring), furniture and officeequipment. The sorption properties of phthalateesters may be similar to those of other semivolatileorganic compounds (SVOCs). For example, it hasbeen shown that SVOCs in the gas phase weresorbed by cotton (Gebefugi, 1989) and in anotherstudy it was found that polychlorinated bi-phenyls(PCBs) in laboratory air (5–8 ngm�3) were sorbedby soil samples (Alcock et al., 1994). It is not onlyporous materials that sorb SVOCs, stainless steeldoes as well. In stainless steel chamber experimentsit has been found that more than 99% of therecovered nicotine (Van Loy et al., 1997), and that50% of emitted DEHP (Clausen et al., 2004) wasadsorbed to the chamber walls. In addition to theabove, only a few studies have focused on theinteractions of SVOCs with chamber and indoor

surfaces (Jayjock et al., 1995; Van der wal et al.,1998; Sparks et al., 1999a, b; Van Loy et al., 2001;Wilke et al., 2003). This is probably due todifficulties associated with sampling and analysisof SVOCs. Consequently, little is known aboutfactors governing mass transport of SVOCs in testchambers and in the indoor environment. We areaware of only three chamber studies on the emissionof DEHP published in peer-reviewed journals(Uhde et al., 2001; Clausen et al., 2004; Afshariet al., 2004). Recently, however, Xu and Little(2006) have successfully modelled emission ofDEHP from vinyl flooring. This model reveals thatthe mechanisms governing the rate of emissions ofDEHP from vinyl flooring into test chambers arepartitioning into the gas phase, the convective masstransfer, and adsorption onto interior chambersurfaces. Despite this improved understanding, itis not yet known if humidity influences the specificemission rate (SER) of phthalates from vinylfloorings.

The objective of this research was to study theeffect of RH on the emission of DEHP from vinylflooring in the test cell FLEC and to use the newphysically based SVOC-emissions model to evaluatethe impact of humidity on the individual mechan-isms that control the SER.

2. Materials and methods

2.1. Chemicals

For calibration and identification DEHP (99.5%(GC)) was purchased from Riedel de Haen. Acetone(pro analysi, 99%) and methanol (Lichrosolv99.6%) used for cleaning and as solvent in thestandard solutions were purchased from Merck.

2.2. Test piece

The test piece was a vinyl flooring with athickness of 2.0mm and containing 17% (w/w)DEHP as the only plasticizer. Other phthalate esterswere detected by gas chromatography and massspectrometry in trace amounts. It was termedhomogeneous polyurethane reinforced PVC floor-ing. The PVC flooring was delivered as a roll(1m� 2m) wrapped in paper directly from thefactory. A few days after receipt, the middle sectionof the test piece was cut into 0.25m� 0.25m testspecimens. These were placed in the test cells at timeequal to zero.

ARTICLE IN PRESSP.A. Clausen et al. / Atmospheric Environment 41 (2007) 3217–3224 3219

2.3. The test cell FLEC

The emission of DEHP was measured in sevenField and Laboratory Emission Cells (FLECs)(CHEMATEC) with the properties shown inTable 1 (Wolkoff, 1996; Wolkoff et al., 1991). Beforethe FLECs were placed on the test pieces they werecleaned with acetone and methanol. Then they wereplaced on cleaned glass plates to measure thebackground level of DEHP that was used as theconcentration at time equal to zero. Finally they wereplaced on the test pieces at time equal to zero. TheRH, flow, and temperature were measured in theexhaust of the FLECs with traceable calibratedinstruments. The temperature and RH were measuredwith a Testo 650 instrument (Testo), and the flow wasmeasured with a Gilian Gilibrator (Gilian).

2.4. Experimental design

FLECs with RHs of 10%, 30%, 50% and 70%were used. The 50% test was carried out in triplicate.In addition, one empty FLEC was placed on a glassplate in order to monitor the background level ofDEHP over time. The concentration of DEHP wasfollowed for a period of about 1yr. After 248d, bywhich time the concentrations were expected to havereached a steady state, the RH was changed in 2 of the50%-FLECs to 10% and 70%, and in the 10%-, and70%-FLECs to 50% in both of them. Then the DEHPconcentrations were followed for another 119d.

2.5. Sampling of DEHP

When SVOCs are sampled from air both thegas phase and the particle phase are usually

Table 1

Properties of the FLEC

Parameter FLECa

Volume (L) 0.035

Air exchange rate (h�1) at a

flow of 450mLmin�1771

Air velocity at test piece

surface (m s�1) at a flow of

450mLmin�1

0.016b

Area of test piece (m2) 0.0177

Internal surface area (m2) 0.018

Chamber surface material Polished stainless steel

The specific test conditions are shown in Table 3.aWolkoff (1996).bEstimated on the basis of geometry and airflow.

collected (Clausen and Wolkoff, 1997). However,the amount of particles in outlet air of the FLECswas assumed to be insignificant, because they weresupplied with filtered air from the air supply unit.Therefore DEHP was sampled directly on TenaxTA tubes with pumps (Gillian 5 from Gillian orFLEC Air Pump 1001 from CHEMATEC) cali-brated to a nominal flow rate of 200mLmin�1. Asampling time of 24 h (�volume of 288L) waschosen for the experiments. All samples werecollected in duplicate. Sampling was performedafter 66, 99, 123, 233, 269, 298, 331, and 367 d,respectively.

2.6. Analysis of the DEHP samples

A thermal desorber (TD) (Perkin Elmer ATD400) was connected to a gas chromatograph (GC)(Perkin Elmer Autosystem XL) with flame ioniza-tion detector (FID). The Tenax TA tubes weredesorbed for 20min at 300 1C using a He flowof 50mLmin�1 and a cold trap temperature of�30 1C. The cold trap was narrow bore (LowFlow Trap Tube) packed with a small piece ofsilylated glass wool. Flash heating of the cold trapto 350 1C transferred the analytes through thevalves at 250 1C and the transfer line at 225 1C tothe GC. The GC-FID had a constant pressureof He (carrier gas) of 24.5 psi resulting in a flow ofabout 1mLmin�1 at 120 1C (calculated) and wasequipped with 60-m� 0.25mm-i.d. Chrompack CPSil 8 CB Low Bleed/MS (0.25 mm film thickness)column. The temperature program was 120 1C,held for 2min, increased to 300 1C at 15 1Cmin�1,and held for 8min. The FID temperature was275 1C. No internal standard was used and nointerference appeared to come from other sub-stances. The analytical limit of detection (LD)was 0.02 mg/tube (�0.07 mgm�3 for a 288L sample)estimated as 3 times the standard deviationof 13 low standards. The standards were injectedonto the Tenax tubes as methanol solutions.The Tenax TA tubes were cleaned in a streamof nitrogen at 275 1C with a sample tube condition-ing apparatus (TC-20, Markes International, Eng-land). Randomly selected clean tubes were usedto estimate the background in the samplingand analytical system. Six-point calibrationcurves were made for each analysis series. Nocalibration curves had intercepts significantlydifferent from 0 and r2 was between 0.99 and0.999.

ARTICLE IN PRESSP.A. Clausen et al. / Atmospheric Environment 41 (2007) 3217–32243220

2.7. Data treatment

Turbochrom (Perkin Elmer) was used for treat-ment of the chromatographic data. Two single mea-surements from the FLEC with constant 50% RHat 269 d and from the FLEC with 70% RH at 331 dwere considered outliers and omitted from the dataset.

3. SVOC emission model

The model developed previously for emissions ofSVOCs from polymeric materials (Xu and Little,2006) was used here. A schematic representation ofthe idealized polymeric material slab placed in a testchamber is shown in Fig. 1.

3.1. SVOC SER

Based on the model depicted in Fig. 1, theanalytical solution for the SER per unit area attime t is (Xu and Little, 2006)

_mðtÞ ¼ �DqCðx; tÞ

qx

����x¼L

¼ DX1m¼1

sin2ðbmLÞ2ðb2m þH2Þ

Lðb2m þH2Þ þH

� ðC0 � Kyð0ÞÞe�Db2mt þ

Z t

0

e�Db2mðt�tÞK dyðtÞ� �

,

ð1Þ

where H ¼ hm=KD and bm (m ¼ 1, 2, y) are thepositive roots of

bm tanðbmLÞ ¼ H. (2)

In the preceding equations, C(x, t) is the material-phase concentration of the DEHP and x is thedistance from the base of the slab of vinyl flooring.The material-phase diffusion coefficient D isassumed to be independent of concentration. L is

Fig. 1. Schematic of vinyl flooring slab in experimental chamber.

the thickness of the slab, C0 the initial material-phase DEHP concentration, hm the convective masstransfer coefficient, y0(t) the concentration of theDEHP in the air immediately adjacent to thesurface, and y(t) the gas-phase DEHP concentrationin the well-mixed chamber air. K, the material/airpartition coefficient, is also assumed to be indepen-dent of concentration (Xu and Little, 2006).

3.2. Chamber surface adsorption

A nonlinear, instantaneously reversible Freun-dlich equilibrium relationship is assumed to existbetween the exposed interior chamber surface areaAi and the chamber air, or

q ¼ Ksyn, (3)

where q is the adsorbed DEHP surface concentra-tion and Ks and n are the Freundlich isothermparameters.

3.3. Chamber mass balance

With reference to Fig. 1, the accumulation ofDEHP in the chamber obeys the following massbalance:

dyðtÞ

dtV ¼ QyinðtÞ � Ai

dqðtÞ

dtþ As _mðtÞ �QyðtÞ. (4)

The DEHP emissions model that incorporatesinteraction with the chamber surfaces is obtainedby combining Eqs. (1)–(4).

3.4. Model parameters

With the exception of RH, the test conditions,shown in Table 1, are the same as for the DEHPemissions experiments carried out previously in theFLEC (Clausen et al., 2004). The emission modelwas fitted to the data from those experiments toobtain the model parameters shown in Table 2 (Xuand Little, 2006). This set of model parameters wasused to predict the new data collected in this study,although the model was not changed in any way toreflect the presence of the humidity.

4. Results and discussion

4.1. Impact of RH on SER of DEHP

The results in Table 3 show that the experimentalconditions were well controlled, with relatively

ARTICLE IN PRESSP.A. Clausen et al. / Atmospheric Environment 41 (2007) 3217–3224 3221

small variations. The DEHP concentration versustime data for the six FLECs with vinyl flooring anda blank at different RHs are shown in Fig. 2. Thedata confirm the repeatability of the experimentalprocedure (DEHP concentration, RH, temperatureand air exchange rate), because they are basicallythe same as previously reported, where data werecollected under identical conditions with the excep-tion of the unvarying RH at 50% (Clausen et al.,2004). The RH appears not to influence the SER ofDEHP in the FLEC since the concentrationsappeared to be the same, although there was somesmall variation in the steady-state condition. TheSVOC emission model was used to predict the newdata without any adjustments to the model para-meters, as shown in Fig. 2. Despite the very goodresults, they do not constitute a formal validation ofthe model, because the experimental conditions areidentical to those used when the model parameterswere obtained from the old data (Table 2).

Table 2

Model parameters for DEHP emissions in FLEC (Xu and Little,

2006)

Parameter FLEC Comments

C0 (mgm�3) 2.6� 1011 Known

D (m2, s�1) 1.0� 10�13 Estimated

hm (m s�1) 1.4� 10�3 Estimated

K (dimensionless) 2.3� 1011 Calculated

y0 (mgm�3) 1.06 Fitted

Ks (mgm�2)/((mgm�3)n) 6000 Fitted

n (dimensionless) 0.47 Fitted

Table 3

The mean, coefficient of variation CV (%), and the minimum and maxi

the FLECs

Mean (CV) [min/max]

FLEC–RH (%) 1st

period (2nd period)

Period (d) RH (%)

10 (50) 0–248 10.5 (7) [9.4/11.6]

249–367 51.2 (5) [46.6/54.3]

30 0–367 29.9 (5) [26.1/32.0]

50 (10) 0–248 49.9 (6) [39.0/52.0]

249–367 10.9 (10) [8.0/12.3]

50 0–367 49.9 (6) [39.9/54.4]

50 (70) 0–248 50.1 (4) [47.3/53.7]

249–367 69.9 (2) [66.5/73.0]

70 (50) 0–248 69.9 (1) [68.8/71.0]

249–367 50.7 (4) [47.1/54.4]

Blank 0–367 27.1 (29) [26.1/32.8]

These parameters were measured approximately every second week res

4.2. Impact of RH on individual model parameters

In the previous modeling study (Xu and Little,2006) emissions of DEHP from vinyl flooring in theFLEC were shown to depend on partitioning intothe gas phase, the convective mass-transfer coeffi-cient, and adsorption onto interior surfaces of thetest chambers. Because the mass of the DEHP thatis lost from the vinyl flooring is negligible relative tothe initial amount present, the material-phaseDEHP concentration remains effectively constantduring the entire emission process. The vinyl floor-ing essentially functions as a source of DEHP that ismaintained at a constant concentration. This in turnmeans that the mechanisms that affect the SER arethose that are ‘‘external’’ to the vinyl flooring.Referring to Fig. 1, these mechanisms are embodiedin the parameters K, the material/air partitioncoefficient, which is effectively constant (Xu andLittle, 2006), hm, the convective mass-transfercoefficient which controls that rate of mass transferthrough the gas-phase boundary layer in contactwith the vinyl flooring, and Ks and n, the Freundlichisotherm parameters which govern the extent ofadsorption (a sink effect) to the interior stainlesssteel chamber surfaces. We now consider thepotential effect of changes in RH on each of theseparameters in turn.

It has been shown that K the material/airpartition coefficient for phenol and vinyl flooringis not significantly influenced by RH (Cox et al.,2001), so it seems reasonable that K for DEHP andvinyl flooring is also uninfluenced by RH. From a

mum values of the relative humidity RH, temperature and flow in

Temperature (1C) Flow (mLmin�1)

21.8 (2) [21.0/22.8] 450 (1) [443/466]

21.8 (3) [21.0/23.1] 450 (1) [454/444]

21.7 (3) [20.4/23.5] 452 (2) [432/474]

21.8 (1) [21.2/22.3] 451 (1) [442/464]

21.7 (5) [20.5/23.5] 453 (5) [442/465]

21.9 (3) [20.6/23.3] 450 (1) [435/463]

21.9 (2) [21.2/22.8] 450 (1) [440/461]

21.9 (4) [20.7/23.6] 450 (2) [438/466]

22.1 (2) [22.2/23.4] 451 (1) [441/458]

21.7 (4) [20.6/23.3] 452 (1) [440/464]

21.9 (3) [20.6/23.6] 453 (6) [432/474]

ulting in ca. 30 observations over the entire period.

ARTICLE IN PRESS

Fig. 2. Concentration vs. time data for emission of DEHP from vinyl flooring at different relative humidities (RH) in FLECs. The errors

bars show the standard deviations of the duplicate measurements in the FLECs.

P.A. Clausen et al. / Atmospheric Environment 41 (2007) 3217–32243222

theoretical perspective, RH is not expected to have asignificant impact on hm, the mass transfer coeffi-cient, because it would not change the degree ofturbulence in the gas-phase boundary layer. It issomewhat surprising, however, that the Freundlichisotherm parameters (Ks and n) for DEHP alsoappear to be independent of the RH. It is knownthat adsorption isotherms for some VOC/surfacecombinations are influenced by RH, for examplesorption of trichloroethylene on soil mineral sur-faces (Ong and Lion, 1991). Also, the concentrationversus time curves of certain VOCs from differentbuilding materials tested in FLECs were stronglyinfluenced by the RH (Wolkoff, 1998). In thesecases, it appears that the water vapor moleculescompete for the surface sites and displace the VOCs.However, the present results suggest that DEHP ismore strongly bound to the surface sites than watervapor, and that displacement by water vapor isnegligible. This is supported by the fact that DEHPis strongly bound to the stainless-steel surfaces ofthe FLEC (Clausen et al., 2004).

The fact that RH does not influence the SER ofDEHP from vinyl flooring into a stainless-steelchamber does not necessarily mean that RH willhave no effect on the SER of DEHP into a regularindoor environment. For example, sorption ofDEHP by dust has been shown to increase theSER of DEHP from vinyl flooring (Clausen et al.,2004). It is conceivable that sorption of water vaporby dust may change the capacity of the dust to sorbDEHP by occupation of sites and by the hydro-

phobic nature of DEHP. If sorption of DEHP bydust is indeed decreased by humidity, then humiditycould decrease the SER of DEHP from vinylflooring into indoor environments containing sub-stantial amounts of dust. This final rather spec-ulative point illustrates the inherent difficultiesassociated with the use of test chambers to predict‘‘externally’’ controlled SERs of SVOCs frombuilding materials in indoor environments. SERscontrolled by ‘‘external’’ phenomena are influencedby the air velocity over the surface and the gas-phase concentrations of the emitted compounds inboth test chambers and indoor environments.Furthermore, the SER is also strongly influencedby uptake on the interior surfaces (Xu and Little,2006) of the test chamber, as shown in Eq. (4). Inthe test chambers, the interior surface is usuallyentirely comprised of stainless steel or glass, but aregular indoor environment has many other types ofsurface that will sorb SVOCs to different extentsthan stainless steel or glass. Thus, the SERmeasured in a test chamber may not be representa-tive of the actual SER from the exact same materialinto a regular indoor environment.

5. Conclusion

Emission testing of vinyl flooring in the FLEC forone year showed that the SER of DEHP is notinfluenced by the RH. The model prediction wasalso consistent with the experimental data andindependent of the RH. Although the model has

ARTICLE IN PRESSP.A. Clausen et al. / Atmospheric Environment 41 (2007) 3217–3224 3223

not been validated with the new data, it does help toexplain the RH results, because it appears that RHhas no significant influence on any of the threecontrolling mechanisms. Thus, the associationbetween the development of bronchial obstructionin children and both vinyl flooring and dampnessproblems in their homes (Jaakkola et al., 1999) maynot be due to an association or co-variation of RHand the presence of DEHP in the air.

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