Emission of Di-2-ethylhexyl Phthalate from PVC Flooring into Air and

Mar 26, 2004 - The emission of di-2-ethylhexyl phthalate (DEHP) from a PVC flooring was studied for up to 472 days in both the FLEC (Field and Laborat...
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Environ. Sci. Technol. 2004, 38, 2531-2537

Emission of Di-2-ethylhexyl Phthalate from PVC Flooring into Air and Uptake in Dust: Emission and Sorption Experiments in FLEC and CLIMPAQ P E R A X E L C L A U S E N , * ,† V I V I H A N S E N , † LARS GUNNARSEN,‡ ALIREZA AFSHARI,‡ AND PEDER WOLKOFF† National Institute of Occupational Health, Lersø Parkalle 105, DK-2100 Copenhagen Ø, Denmark, and Danish Building and Urban Research, P.O. Box 119, DK-2970 Hørsholm, Denmark

The emission of di-2-ethylhexyl phthalate (DEHP) from a PVC flooring was studied for up to 472 days in both the FLEC (Field and Laboratory Emission Cell) and the CLIMPAQ (Chamber for Laboratory Investigations of Materials, Pollution, and Air Quality). The loading of the CLIMPAQs was varied but was constant in the FLECs. The sorption properties of FLEC and CLIMPAQ were investigated using different methods. In addition, the uptake of DEHP by office floor dust on the PVC flooring was studied in CLIMPAQ experiments. The concentration versus time curves in both FLECs and CLIMPAQs increased slowly over about 150 days and reached a quasi-static equilibrium at 1 µg m-3. The main conclusions were that (i) the emission rate of DEHP was limited by gas-phase mass transport and (ii) the dust layer increased the emission rate by increasing the external concentration gradient above the surface of the PVC. These conclusions were based on the facts that the specific emission rate was inversely proportional to the loading and that the dust had sorbed about four times as much DEHP over a 68-day period as emitted in the gasphase experiments. About one-half of the emitted DEHP was deposited on the internal surfaces of both the FLEC and the CLIMPAQ.

Introduction The prevalence of allergic airway diseases appears to be rapidly increasing in Western Europe and North America (1). This increase may be associated with phthalate esters that are suspected to possess adjuvant effects that enhance the health damaging potential of common allergens. An epidemiological study has shown that the total area of poly(vinyl chloride) (PVC) in homes was associated with development of bronchial obstruction in children (2). On the basis of another study, it has been proposed that deposition of the common PVC plasticizer di-2-ethylhexyl phthalate (DEHP) in the lungs increases the risk of inducing inflammation, which is characteristic of asthma (3). Both DEHP (4) and its expected metabolite mono-2-ethylhexyl phthalate (5) have * Corresponding author e-mail: [email protected]; phone: +45 39 16 52 73; fax: +45 39 16 52 01. † National Institute of Occupational Health. ‡ Danish Building and Urban Research. 10.1021/es0347944 CCC: $27.50 Published on Web 03/26/2004

 2004 American Chemical Society

been shown to possess an adjuvant effect with simultaneous injection of the allergenic ovalbumin in mice. Phthalate esters used as plasticizers in PVC are slowly emitted as vapors (6). They are common pollutants in indoor air (7-9) and surface dust (3, 10-14). The existence of phthalate esters in indoor air may be due to resuspension of sedimented dust (3) and/or emission from phthalatecontaining building products (e.g., PVC flooring), furniture, and office equipment. The sorption properties of phthalate esters may be similar to those of other semivolatile organic compounds (SVOCs). It has been demonstrated in chamber experiments that SVOCs via the gas phase were sorbed by cotton (15). In a study of PCB, it was found that PCB in the laboratory air (5-8 ng m-3) was sorbed by soil samples at a rate of 5 µg m-2 day-1 (16). It is not only porous materials that absorb SVOC, stainless steel does as well. In a stainless steel chamber experiment, it was found that more than 99% of the recovered nicotine, another SVOC, was sorbed to the chamber walls (17). In addition to the above, only a few studies have focused on the interactions of SVOCs with chamber and indoor surfaces (18-22). This is probably due to the difficulties associated with sampling and analysis of SVOCs. Consequently, little is known about factors governing mass transport of SVOCs in test chambers and indoor air. We are aware of only one chamber study on the emission of DEHP published in a peer-reviewed journal (6). The aim of this study was to increase our understanding of emission of SVOCs from building materials in the indoor environment, in general, using DEHP emission from a PVC flooring in an emission cell and an emission test chamber. The emission of DEHP from the PVC flooring was followed for nearly 500 days. The sorption behavior of DEHP in both the FLEC (Field and Laboratory Emission Cell) and the CLIMPAQ (Chamber for Laboratory Investigations of Materials, Pollution, and Air Quality) was investigated with different methods. An experiment with different loadings was performed as well. In addition, the uptake of DEHP in office floor dust soiled onto PVC flooring was studied in a chamber experiment.

Materials and Methods Chemicals. DEHP (Pestanal, 99.5%) was from Riedel-de Ha¨en, and methanol (pro analysis, >99.8%) was from Merck. Test Piece. A 2 mm thick PVC flooring containing about 17% (w/w) DEHP as the only plasticizer (other phthalate esters were only found in trace amounts by gas chromatography and mass spectrometry) was used. It was termed homogeneous polyurethane reinforced PVC flooring. It was delivered as a roll wrapped in plastic foil directly from the factory. A few days after receipt, it was cut into 0.25 m × 0.25 and 0.8 m × 0.2 m sheets. The last mentioned sheets were stapled together in pairs (back to back). Immediately after being cut, the test pieces were wrapped in aluminum foil for storing at room temperature. Approximately 3 weeks thereafter they were unwrapped and placed in the emission chambers and cells. Emission Chambers and Cells. Both an emission test chamber and an emission cell were used for the experiments, CLIMPAQ (23) and FLEC (Chematec) (24), respectively. The CLIMPAQ is made of panes of window glass assembled with low-emitting two-component epoxy glue. Other main surface materials are stainless steel (SS) and eloxated aluminum. The air exchange rate and air velocity over the test piece surface may be varied independently. One internal fan recirculates the air over the test pieces. The FLEC is a microemission cell made of hand polished SS that is positioned VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Sequence of the Emission and Sorption Experiments and References to Results day

FLEC 1

0 0-279 0-332 379 379-404 337 337-472 472 472 476+479 479 479-491 492 a

FLEC 12a

FLEC 2

placed on PVC

connected to FLEC 1b connected to FLEC 2 DEHP measd in outlet disconnected from disconnected from FLEC 1 FLEC 2 DEHP measd in outlet internal surfaces rinsed with methanol removed from PVC and put together to a double FLECc DEHP measd in outlet internal surfaces rinsed with methanol

results Figure 2

PVC removed DEHP measd in outlet

Figure 5

b

Figure 4 Figure 2 Table 3

Figure 5 Table 3

See Figure 2. c Bottom part against bottom part, see Figure 1.

TABLE 2. Test Conditions for CLIMPAQ and FLECsa parameter

CLIMPAQ

FLEC

temperature (°C) relative humidity (%RH) volume (L) air flow (L min-1) air exchange rate, N (h-1) air velocity at test piece surface (m s-1) area of test piece (m2) loading, L (m2 m-3) L /N internal surface area (m2) internal surface area/ volume (m2 m-3)

ca. 22 ca. 50 51 8.3-9.4 9.8-11 0.14-0.16b

20.1-23.6 47.9-52.0 0.035 0.444-0.465 760-796 0.016c

1.6 31 2.8-3.2 1.6d 59

0.0177 510 0.64-0.67 0.018e 514

a Minimum and maximum values observed during the entire test period. b Estimated by hotwire measurements. c Estimate based on geometry. d Calculated based on measurements of the dimensions of the internal surfaces (glass, SS, anodized aluminum, plastic fan, silicone gasket). e Calculated (31).

upon the test piece. The test piece and the inner surface of the FLEC form a cone-shaped cavity. Planar materials are sealed to the FLEC by means of an O-ring. The supplied air is distributed by a circular channel with a slit at the inner lower edge. The air flows from the circular slit at the inner periphery of the cell over the material surface and into the center, where it exits the FLEC outlet. Thus, the air velocity over the surface depends on the emission cell airflow (25). Emission of DEHP into Air. The emission tests were conducted in duplicate in two FLECs (FLEC 1 and FLEC 2 on two separate test pieces) and in one CLIMPAQ (see Table 1). An additional empty CLIMPAQ was the blank chamber. The test duration was up to 472 days. The five test pieces (0.2 m × 0.8 m) in the CLIMPAQ were placed vertically and were equally spaced in the chamber. Test conditions for both CLIMPAQ and FLEC are shown in Table 2. The CLIMPAQs and FLECs were cleaned, and background measurements were performed before testing. The FLECs were placed on cleaned glass plates for the background measurements. Temperature, humidity, and airflow through the CLIMPAQs and FLECs and, in addition, the air velocity in the CLIMPAQs were checked before and after each sampling. Sorption Experiments. Two FLECs in Series. The emission and sorption experiments are outlined in Table 1. FLEC 12 and FLEC 22 were clean (empty) FLECs with a clean handpolished SS plate as the bottom part (instead of a test piece). They were connected to the outlet of each of the test FLECs 9

CLIMPAQ PVC inserted DEHP measd in outlet

DEHP measd in outlet

Includes SS bottom part.

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(FLEC 1 and FLEC 2) that were still positioned upon the PVC flooring. For DEHP analysis, air was sampled from the outlet of the empty FLEC 12 and FLEC 22. After a 135-day period in which several air samples were collected, FLEC 12 and FLEC 22 were disconnected, and the SS bottom parts and the internal surfaces (excluding the air distribution channel and air supply tubes) were rinsed with about 45 mL of methanol. All glassware used was cleaned with methanol. Double FLEC. The measurement of DEHP in the outlet of FLEC 1 and FLEC 2 was resumed once after disconnection of FLEC 12 and FLEC 22. Then they were removed from the PVC flooring and joined (bottom part against bottom part). This resulted in an empty “double FLEC” (see Figure 1). DEHP was measured in the effluent air from the double FLEC, and after 13 days, the internal surfaces of the two FLECs were rinsed with methanol. A 50-µL portion of the methanol was directly injected into a Tenax tube and analyzed by thermal desorption as described below (14). CLIMPAQ. After air was sampled for the last data point, the PVC test pieces were removed from the CLIMPAQ, which was closed immediately again. Air was sampled three times from the outlet of this CLIMPAQ over a 25-day period. Loading Experiment. The experiment was conducted in four CLIMPAQs in which the test piece areas were 1.6 (the test piece area used in the other experiments), 0.8, 0.4, and 0.2 m2, respectively. An additional empty CLIMPAQ was the blank chamber. All other test chamber conditions were identical to the other CLIMPAQ experiments (Table 2). Air was sampled in duplicate from the CLIMPAQs after 0 (background), 110, 158, and 200 days and analyzed for DEHP as described in the following sections. DEHP Uptake by Dust on PVC Flooring. Three different scenarios were investigated to study the DEHP uptake: dust on PVC, dust on wet PVC washed with soap/water and airdried, and dust on wet PVC washed with pure water and air-dried. The experiments were conducted in three CLIMPAQs in which the five test pieces were placed horizontally in contrast to the emission experiments. The three upper test pieces in each chamber were soiled on the top side with about 0.5 g (∼3 g m-2) of the particle fraction of homogenized house dust with a known content of DEHP (14). The dust was sampled in an office building with a standard industrial vacuum cleaner, and the particle fraction was separated by sieving (500 µm). The dust was applied by a specially designed soiling equipment having a groove with a slit of 300 µm (26). All other test chamber conditions were identical to the other emission experiments. The chambers were opened after 5, 31, and 68 days; one of the test pieces

FIGURE 1. Setup of the FLEC sorption experiments. The double FLEC. was vacuumed with a specially designed SS surface dust sampler using cleaned glass fiber filters (27). The background of DEHP in the air was measured before insertion of the soiled PVC in the CLIMPAQs, and for all three sampling rounds, field blank filters for each dust sample were analyzed. The sample filters and blank filters were extracted with pressurized liquid extraction using a Dionex ASE 200 system (27), and 5-50-µL portions of the methanol extract were directly injected into a Tenax tube and analyzed by thermal desorption as described previously (14). Before each dust collection, air was sampled in duplicate from the CLIMPAQs and analyzed for DEHP as described in the following sections. Sampling of DEHP in the Effluent Air from CLIMPAQs and FLECs. When SVOCs are sampled from air, both the gas phase and the particle phase are usually collected (28). However, in this case the air of the CLIMPAQs and FLECs was assumed to contain an insignificant amount of particles since they were supplied with filtered air. Therefore, DEHP was sampled directly on Tenax TA tubes with a pump (Alpha1, Ametek) calibrated to a nominal flow of 200 mL min-1. In pilot emission tests conducted over a 2-month period, 11 sample tubes were connected to backup tubes to check for breakthrough and to estimate the specific emission rate (SER). The sampling volumes were between 130 l and 440 L. No backup tubes had DEHP concentrations above the detection limit. The observed SER was as low as ∼ 1 µg m-2 h-1; therefore, a sampling time of 24 h with a volume of 288 L was chosen for the experiments. All samples were in duplicate. TD-GC-MS/FID Analysis of DEHP on Tenax TA Tubes. A thermal desorber (TD) (Perkin-Elmer ATD 400) was connected to a gas chromatograph (GC) with mass spectrometric (MS) detector (Perkin-Elmer Autosystem XL/ TurboMass) or flame ionization detector (FID). The Tenax TA tubes were desorbed for 20 min at 300 °C, a He flow of 50 mL min-1, and a cold trap temperature of -30 °C. The cold trap was narrow bore (Low Flow Trap Tube) packed with a small piece of silylated glass wool. Flash heating of the cold trap to 350 °C transferred the analytes through the valves at 250 °C and the transfer line at 225 °C to the GC. The GCMS/FID had a constant pressure of He (carrier gas) of ∼20 psi resulting in a flow of about 1 mL min-1 at 120 °C (calculated) and was equipped with 60 m × 0.25 mm i.d. Chrompack CP Sil 8 CB Low Bleed/MS (0.25 µm film thickness) column. The temperature program was 120 °C, held for 2 min, increased to 300 °C at 15 °C min-1, held for 8 min, and finally increased to 320 °C at 20 °C min-1 and held for 4 min. The MS transfer line and the FID temperature were 275 °C. The mass spectrometer was operated in the

electron impact ionization mode (EI+, 70 eV) with a source temperature of 175 °C using full-scan mode (m/z 30-400). No internal standard was used, and ion m/z 149 was used for quantification since no interference appeared to come from other substances. The analytical limit of detection (LD) was 0.01 µg per tube (∼0.03 µg m-3 for a 288-L sample) estimated as three times the standard deviation of 13 low standards. The standards were injected onto the Tenax tubes as methanol solutions. All tubes were analyzed by two successive desorptions to test for complete desorption of both the tube and the TD system. The second desorption of the tubes showed in all cases concentrations below the detection limit, and these tubes were then considered as clean. Randomly selected clean tubes were used to estimate the background in the sampling and analytical system. Calibration curves (six points) were made for each analysis series. No calibration curves had intercepts significantly different from zero, and r 2 was between 0.99 and 0.999. Data Treatment. The Microsoft Excel spreadsheet has been used for all plots and calculations. All gas-phase concentrations (µg m-3) are means of duplicate samples. The GC-MS gave unreliable results in a period from day 72 to day 142 during the emission test in the two FLECs and one CLIMPAQ, and five data points for the FLECs and one data point for the CLIMPAQ were rejected. At day 259, the FLEC temperature was 2 °C below the mean temperature, and one data point was rejected.

Results Emission of DEHP into Air. The background concentrations in the empty CLIMPAQs and FLECs (time ) 0) and the highest concentration of the blank CLIMPAQ were about 10 times lower than the corresponding highest measured emission concentrations. The test conditions were relatively constant over the entire test period (Table 2). The concentration versus time data for the CLIMPAQs and FLECs are shown in Figure 2, and the corresponding SERs are shown in Figure 3. The SERs (µg m-2 h-1) were calculated for each concentration (C, µg m-3) assuming a constant source (see Table 2 for nomenclature):

SER ) C‚N/L

(1)

Sorption Experiments. The concepts used in Table 3 are italicized in the text, and the detailed calculations are given below. The DEHP sorbed by the internal surfaces of the FLECs was rinsed off with methanol (see Table 3). Generally, the amount of DEHP sorbed by CLIMPAQs and FLECs was VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Concentration (( 1 standard deviation of duplicate samples) vs time data for emission of DEHP from PVC flooring in two FLECs and one CLIMPAQ and additional background concentration for an empty CLIMPAQ.

FIGURE 3. Observed specific emission rates (SERs) (( 1 standard deviation of duplicate samples) vs time after a quasi-static equilibrium has been reached. The SERs were calculated for each concentration of DEHP. calculated by subtracting the amount exhausted (i.e., measured in the outlets) from the amount supplied either emitted by the PVC within FLEC 1, FLEC 2, and CLIMPAQ or supplied as air containing DEHP (FLEC 12 and FLEC 22). The amount of DEHP exhausted was calculated as the area under the curves in Figures 2, 4, and 5 multiplied by the flow through the CLIMPAQs and FLECs. The amount of DEHP exhausted between two measurements was calculated using

M ) F(t2 - t1)(C1 + C2)/2

(2)

where M is the mass of exhausted DEHP (µg) between two measured concentrations C1 (µg m-3) and C2 (µg m-3) at time t1 (h) and t2 (h), respectively. F (m3 h-1) is the flow through the CLIMPAQs or FLECs. Two FLECs in Series. The concentrations of DEHP over time in FLEC 12 and FLEC 22 are shown in Figure 4. DEHP was below the detection limit during the first month. The final gas-phase concentrations (the last measured) in the connected FLECs (e.g., FLEC 1 and FLEC 12) were relatively close indicating that “sinks” had reached a quasi-static equilibrium (see Table 3). The amount of DEHP that was sorbed by the internal surfaces of FLEC 12 and FLEC 22 was 2534

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calculated as the difference between the supplied amounts from FLEC 1 and FLEC 2 and the amounts exhausted from the outlets of the connected FLEC 12 and FLEC 22 (using eq 2). This amount should equal the amount of DEHP recovered with methanol rinse from the internal surfaces of FLEC 12 and FLEC 22 (see Table 3). However, the air distribution channel and air tubing are also sinks, and they were not rinsed with methanol. The minimum amount was estimated assuming a constant incoming concentration (at the circular slit) equal to the final concentrations in the outlets of FLEC 12 and FLEC 22. This is an underestimation since the final concentrations in FLEC 1 and FLEC 2 at 472 days were slightly higher and decreased from an even higher concentration measured at 332 days (see Figure 2). The maximum amount was estimated from the FLEC 1 and FLEC 2 measurements at 332 and 472 days, respectively, and the duration (135 days) of this sorption experiment (using eq 2). Double FLEC and CLIMPAQ. The DEHP concentration in the empty double FLEC was decreasing whereas it was constant in the empty CLIMPAQ from which the PVC had been removed (see Figure 5). The amount of DEHP emitted from (supplied by) the PVC within FLEC 1 and FLEC 2 and CLIMPAQ was calculated using the measured maximum concentrations (Figure 2) and assuming that the PVC was a constant source over the entire period that the PVC remained in the FLECs and the CLIMPAQ, 479 and 379 days, respectively (see Table 1). The amount adsorbed by the FLECs and the CLIMPAQ was calculated as the difference between the estimated amount emitted by the PVC and the amount exhausted. During the double FLEC experiment, more DEHP was removed from the surfaces of the FLECs, and this amount was calculated from the curve in Figure 5 using eq 2. This gave 5 µg of DEHP in total for the double FLEC; i.e., 2.5 µg per FLEC. Thus, the final amount of DEHP sorbed by the internal surface of FLEC 1 and FLEC 2 was calculated by an additional subtraction of the amount exhausted from the double FLEC. This amount should equal the amount of DEHP rinsed off the internal surfaces of FLEC 1 and FLEC 2 (see Table 3). Loading Experiment. The concentration versus time data for the loading experiment (including the blank CLIMPAQ) are shown in Figure 6. The background concentration in the empty CLIMPAQ was well below the concentrations in the other CLIMPAQs. We do not know the reason for the

TABLE 3. Measured and Roughly Calculated Amounts of DEHP Sorbed by Internal Surface of FLECs and CLIMPAQ FLEC 1 final gas-phase concn (µg m-3) rinsed off with methanol (µg) amt supplied (µg) amt exhausted (µg) amt exhausted from double FLEC (µg per FLEC) amt sorbed (µg) amt sorbed/amt supplied (%) internal surface concn (µg m-2)

FLEC 12

Measured 0.80 68 Calculated 338 258 2.5 78 23 3900d-4400e

0.78 36a 69-76 29 40-47 59-63 1000d-1300e

FLEC 2 0.71 75 329 234 2.5 93 28 4300d-5300e

FLEC 22

CLIMPAQ

0.55 ?a,b 49-69 20

5000c 2700c

29-49 59-70 1300e

2300c 46c 1400c

a Rinsed off the internal surface of the FLEC + the bottom part () 0.0357 m2). b This sample was strongly contaminated with DEHP. c Calculated from emission curve extrapolated to 379 days when PVC was removed from CLIMPAQ. d Based on amount rinsed off. e Based on the (largest) calculated value for amount sorbed.

FIGURE 4. Concentration of DEHP vs time data for the empty FLEC 12 and FLEC 22 (closed with a SS bottom) connected in series with the FLEC 1 and FLEC 2, respectively, both of which were positioned upon on PVC flooring.

FIGURE 6. Concentration vs time data for emission of DEHP from PVC flooring for different loadings in CLIMPAQs. Error bars are omitted for clarity. The average relative standard deviation was 16%. the soiled PVC. The lower initial SERs estimated from the gas-phase measurements may be due to a competing sorption effect of the dust. Using the calculation method described in the sorption experiment section and the maximum concentration in Figure 2 gives a total amount of DEHP of about 900 µg emitted to the CLIMPAQs over the 68-day duration of the experiment. The total uptake measured in the sampled dust was about 3700 µg in all three CLIMPAQs for the 68 days (i.e., a difference of 2800 µg).

Discussion FIGURE 5. Concentration of DEHP vs time data for the empty CLIMPAQ from which the PVC was removed and the two FLECs that have been removed from the PVC flooring and put together (bottom part agaist bottom part) to an empty double FLEC (see Figure 1). increased background concentration at 200 days. However, it did not appear to have influenced the other CLIMPAQs. DEHP Uptake by Dust on PVC Flooring. The recoveries of the soiling dust from the PVC test pieces by vacuuming varied from 27% (minimum) to 57% (maximum) and were on an average 45%. One sample had a recovery of only 4% and could not be used for estimation of the DEHP content (dust + soap scenario at 5 days). This was due to losses during the soiling process and to incomplete vacuuming. The DEHP concentrations in the CLIMPAQ air and the soiling dust are shown in Figure 7 for a 68-day period. The gas phase concentrations after 31 and 68 days were close to the concentrations emitted from the nonsoiled PVC (Figure 2) and thereby the observed SERs were also close. Thus, the uptake in the dust is apparently an additional emission from

Emission of DEHP into Air. The emission curves in Figure 2 show a slow increase of the concentrations over time and a tendency to stabilization of the levels after 150 days in both FLECs and CLIMPAQ. There appears to be a good agreement between the two FLECs and comparable concentration levels in the FLECs and the CLIMPAQ. However, this is inconsistent with about five times lower L/N ratio in the FLEC (Table 2) and may explain that the observed SERs using eq 1 are 5-10 times higher in the FLECs (see Figure 3). The comparable concentrations in both FLECs and CLIMPAQ were apparently due to saturation of the air. Loading Experiment. This experiment was conducted to test the hypothesis that the emission of DEHP from the PVC was suppressed at the observed DEHP gas phase concentrations. Also in this experiment the emission curves appeared to reach the maximum concentration at about 150 days (see Figure 6), and the different loadings gave different concentrations. However, the highest loading that was equal to that used in the emission experiment resulted in a concentration similar to that shown in Figure 2. The maximum concentrations were expected to be directly proportional to the loading VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. DEHP uptake of floor dust soiled on PVC flooring in CLIMPAQs for three different scenarios. The dust concentrations (the bars) have been plotted with error bars showing the analytical 95% confidence intervals. The gas-phase concentrations (the data points) are plotted together with the corresponding dust concentrations (see Experimental Section and Results for details). Error bars of the gas-phase concentrations are omitted for clarity. The average relative standard deviation was 25%. if the emission was independent of the gas-phase concentrations. In fact, the concentrations were higher than expected. Therefore, it may be concluded that the DEHP emission from the tested PVC was limited by diffusion in the boundary layer and that “equilibrium” between the PVC surface and the gas phase was approached. This may also explain that the maximum chamber concentrations for emission of DEHP from PVC wall coverings were below 1 µg m-3 (6). It might be possible that the chamber concentrations for the lower loadings will increase slowly to that of the highest loading if the emission is still not limited by internal diffusion in the PVC. The slow increase may be due to a lower emission rate that results in a slower saturation of the sinks. Sorption Experiments. The results in Table 3 and Figures 4 and 5 show that sorption was “substantial” in both the FLECs and the CLIMPAQ. The results in Table 3 also indicate that the wall sorption and the exhausted DEHP accounted for all DEHP emitted from the PVC. The concentration of DEHP in the empty CLIMPAQ after removal of the PVC appeared to remain constant over the 25-day period. The decreasing concentration in the double FLEC (see Figure 5) appeared to stabilize after 10 days. This behavior has also been observed for nicotine and phenanthrene in a SS chamber (22). The decreasing concentration in the double FLEC suggests a more “labile” sorption in the FLEC. However, this was not reflected by a shorter time to reach the maximum concentration in the FLEC (Figure 2). A first-order decay mechanism of the desorption from the surfaces in the FLEC combined with a relatively high surface concentration (Table 3) may explain the rapid concentration decrease. However, only a minor part of the sorbed DEHP was emitted from the double FLEC during the experiment (Table 3). The difference between the amounts sorbed for FLEC 1 and FLEC 12 and for FLEC 2 and FLEC 22 indicates that the sorption is not a linear process. In FLEC 12 and FLEC 22 half the amount of DEHP was sorbed (either rinsed off or calculated) during a period that was about one-fourth of the time elapsed for sorption in FLEC 1 and FLEC 2. The relative amount sorbed by the CLIMPAQ was calculated to be in between. This agrees with the similar concentrations and experimental period for PVC emission in the CLIMPAQ (379 days) as compared with 135 and 479 for the FLECs, respectively. This nonlinear sorption of SVOCs has also been shown both experimentally and theoretically for nicotine in a SS chamber (17). Figure 4 indicates that sorption works as 2536

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a chromatographic process and that the “retention time” of DEHP in the FLEC is more than 1 month. The strong sorption may partly explain the slow increase of the concentrations in Figure 2. DEHP Uptake by Dust on PVC Flooring. The uptake of DEHP in the soiling dust probably better reflects the development in the emission rate over time than SER since it is both sorption media and sampling sorbent. However, as shown in Figure 7 some DEHP “escaped” and was emitted to the air. This may be due to the fact that only 30% of the PVC area in the chambers was soiled, that the soiling was inhomogeneous, and that the soiled PVC was not fully covered (dust concentration ∼3 g m-2). The results indicate that the dust strongly increased the total emission from the PVC since only 900 µg of DEHP was emitted during the first 68 days in the first CLIMPAQ experiment whereas in total 3700 µg was sorbed by the dust over the 68-day period of the experiment. This may be due to the direct contact between the dust and the PVC surface since the boundary layer is partly removed and that the dust as an adsorbent removes DEHP and thereby increases the slope of the concentration gradient in the boundary layer. This may, in addition to the presence of PVC particles due to physical breakdown, explain the high concentration of DEHP measured in dust sampled 1 day after cleaning in schools (14). Apparently wet cleaning does not wash DEHP out of PVC as shown in Figure 7. Gas-phase concentrations of DEHP measured in the indoor environment are usually the sum of DEHP in the gas phase and the particle phase and typically lower than the maximum gas-phase concentrations observed in this study (7, 8). Weschler (29) has estimated based on a room temperature saturation vapor pressure of 1.9 × 10-10 atm (30) that about 80% of airborne DEHP in, for example, indoor settings is associated with particles (total suspended particles) in the air. Since DEHP emission from PVC is suppressed by its gas-phase concentration, the SER in the indoor environment may be higher than measured in this study because of the much lower indoor gas-phase concentrations. In addition, the direct contact between PVC (with DEHP) and dust may also increase the total emission (to air and dust). Thus, the emission of DEHP in the indoor environment must be considered as a leakage from the sources via low vapor concentrations to other parts of the indoor environment (building products, furniture, dust, etc.). On the basis of this study, we conclude that it was difficult to obtain reliable

emission data for DEHP from the PVC because of the sensitivity to the test conditions. The long period required to reach the quasi-static equilibrium also makes emission testing impractical. The relevance of gas-phase emission data of DEHP in relation to the indoor climate must be reevaluated since particles and dust on the PVC surface increase the emission. Thus, inhalation of resuspended dust may be the most important exposure route. However, frequent cleaning should reduce exposure.

Acknowledgments This work was a part of the research activities in the Center for the Environment and Respiratory System, which was supported by the Danish Environmental Research Program.

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Received for review July 21, 2003. Revised manuscript received February 16, 2004. Accepted February 23, 2004. ES0347944

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