Semivolatile organic compounds (SVOCs): phthalates and flame retardants
Abstract:
Among the many chemicals found indoors, semivolatile organic compounds (SVOCs) constitute an important class. While certain SVOCs are associated with adverse health effects, exposure is strongly influenced by the types of materials and products in which these SVOCs occur. This chapter begins with a brief summary of phthalates and flame retardants, two important types of SVOCs. Chamber experiments used to characterize the emissions process are then reviewed. A simple emission model that can be used to predict the steady-state indoor gas-phase SVOC concentration is described. Knowing the long-term concentration in the indoor air, the potential exposure via inhalation of air and airborne particles, ingestion of dust, and dermal absorption can be calculated using general relationships.
5.1 Semivolatile organic compounds (SVOCs) in the indoor environment
Potential risks to human health arise from the manufacture and use of thousands of chemicals (Collins et al., 2008; Rudén and Hansson, 2010). Although indoor chemicals are not regulated (Sarigiannis et al., 2011), building materials and furnishings contain a vast array of compounds that may be released indoors (Weschler, 2009) where people spend most of their time (Klepeis et al., 2001; Schweizer et al., 2007). Among the many chemicals found indoors, semivolatile organic compounds (SVOCs) constitute an important class (Weschler and Nazaroff, 2008; Weschler, 2009; Wang et al., 2010; Król et al., 2011) with examples including chlorpyrifos (a pesticide), triclosan (a biocide), bisphenol-A (BPA, a residual monomer in polycarbonate plastics), 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47, a flame retardant), and di(2-ethylhexyl) phthalate (DEHP, a plasticizer). In this chapter, we will focus on phthalates and flame retardants, both of which may be found as additives that are present in polymeric materials and products.
As reported by Rudel and Perovich (2009), phthalates are common plasticizers used in polyvinyl chloride (PVC). By weight, they may contribute 10–60% of plastic products. They are added because of their ability to increase flexibility and transparency. In 2004 worldwide production of phthalates was estimated to be 6,000,000 tons per year. Phthalates are found in a wide variety of products including vinyl upholstery, shower curtains, food containers and wrappers, toys, floor tiles, lubricants, sealers, and adhesives. Phthalates are also used as solvents in cosmetics such as perfume, eye shadow, moisturizer, nail polish, hair spray, and liquid soap. Because there is no covalent bond between phthalates and the plastics into which they are mixed, they may be slowly released into the environment. Phthalates are subject to various degradation processes and generally do not persist in the outdoor environment. Phthalates are ubiquitous in the indoor environment, with indoor air concentrations generally higher than outdoor concentrations. More volatile phthalates such as diethyl phthalate (DEP), dimethyl phthalate (DMP) and dibutyl phthalate (DBP) are present at higher concentrations in air, while heavier, less volatile phthalates such as di(2-ethyl-hexyl) phthalate (DEHP) and benzyl butyl phthalate (BBP) are more prevalent on interior surfaces and dust.
Rudel and Perovich (2009) also summarized the situation for polybrominated diphenyl ethers (PBDEs), which are used as flame retardants and are found in a variety of consumer products such as plastics, upholstery, construction materials, and electrical appliances. PBDEs are lipophilic and hydrophobic compounds that tend to persist in the environment. There are 209 PBDE congeners, named according to the total number of bromines off the phenyl rings. Generally the lower-brominated congeners (mono through penta) are thought to be more harmful to humans. While PBDEs tend to distribute between the aerosol and the gas phase in air, as the congeners get heavier (more bromine atoms) they are increasingly found in the particulate phase. PBDEs are sold as three commercial mixtures: penta-bromodiphenyl ether (penta-BDE), octa-bromodiphenyl ether (octa-BDE), and deca-bromodiphenyl ether (deca-BDE). About 67,000 tons of PBDEs were produced worldwide in 2001. Deca-BDE is the most widely used, with an estimated 56,100 tons produced worldwide in 2001, compared to 7500 tons of penta-BDE produced in the same year. While penta-BDE and octa-BDE are being phased out of production, many sources of exposure remain as these PBDEs are prevalent in a number of common products. Traditionally, penta-BDE was dominated by the BDE 47 and BDE 99 congeners and used in polyurethane foam, foam products, bedding, vehicle interiors, and furniture. Octa-BDE contained a mixture of hepta, octa and nona congeners while deca-BDE consists primarily of BDE 209. Both octa- and deca-BDE can be found in electronics such as computers and televisions and deca- BDE is also often in textiles. Because PBDEs are additives mixed into polymers, and are not chemically bound to the polymer matrix, they also tend to leach out of products and into the indoor air.
SVOCs are of concern because they are ubiquitous, abundant and often persistent contaminants, found in indoor air, in airborne particles, in settled dust, on indoor surfaces, and even in clothing, on human hair and on human skin. Biomonitoring has shown that people have high and increasing body burdens of some SVOCs (Rudel et al., 2003, 2010; Weschler and Nazaroff, 2008; Rudel and Perovich, 2009; Weschler, 2009). Associations between adverse health effects and exposure to certain SVOCs have been identified, with several SVOCs categorized as endocrine-disrupting compounds (Weschler and Nazaroff, 2008; Rudel and Perovich, 2009). Although the specific sources and exposure pathways are not yet clear, an understanding of the indoor occurrence, transport and exposure to SVOCs is beginning to emerge (Bennett and Furtaw, 2004; Schettler, 2006; Xu and Little, 2006; Weschler and Nazaroff, 2008, 2010; Weschler et al., 2008; Rudel and Perovich, 2009; Webster et al., 2009; Zhang et al., 2009).
Exposures are strongly influenced by the types of materials and products in which chemicals occur, and the ways in which the materials and products are used. For example, certain phthalates are used as plasticizers in soft polyvinyl chloride (PVC) products, while other phthalates are used as solvents in personal care products such as perfume, eye shadow, moisturizer, and nail polish (Rudel and Perovich, 2009). Exposure to phthalates in PVC products occurs following emission from the source into air and subsequent migration to different media such as dust and other indoor surfaces, including human skin, hair and clothing (Xu et al., 2009, 2010), while exposure to phthalates present in personal care products is more likely a consequence of dermal absorption and/or accidental ingestion (Wormuth et al., 2006; Wittassek et al., 2011) and therefore largely controlled by human behavior. Phthalates are also found in food as a result of contact with materials used in processing and packaging (Wormuth et al., 2006), with exposure to certain phthalates strongly controlled by diet (Dickson-Spillmann et al., 2009).
5.2 Emission of semivolatile organic compounds (SVOCs) from building materials and consumer products
Only a few studies have been carried out to experimentally characterize emissions of SVOCs from building materials and consumer products, mainly due to the difficulties associated with sampling and analysis of the very low volatility chemicals. In this section, we briefly review some of the studies carried out in test chambers and then describe a simple model that has been developed to predict gas-phase emissions of SVOCs present in materials and products as additives.
5.2.1 Characterizing emissions of SVOCs in small chambers
Uhde et al. (2001) measured phthalate concentrations emitted from 14 PVC-coated wall coverings in regular emission chambers for 14 days and suggested that the chamber concentration of phthalates with lower boiling point tends to be higher than those with higher boiling point. Afshari et al. (2004) tested dibutyl phthalate (DBP) and di(2-ethylhexyl) phthalate (DEHP) emissions from several materials such as wallpaper, PVC flooring and electric wire in the Chamber for Laboratory Investigations of Materials, Pollution and Air Quality (CLIMPAQ) as well as the Field and Laboratory Emission Cell (FLEC), and found that the chamber concentration of DEHP reached steady state after about 150 days and that sorption by chamber surfaces had a strong effect on gas-phase chamber concentrations. Fujii et al. (2003) developed a passive-type sampler to measure the emission rate of phthalates from synthetic leather, wallpaper and vinyl flooring and found that emission rates of several phthalates increased significantly at higher temperature. Schripp et al. (2010) tested wall paints and pure phthalate liquid in two different chambers and measured concentrations of DEHP and DBP in the chamber air and in the dust placed in the chamber. Kawamura et al. (2011) estimated the emission rate of DEHP from building materials through micro-chamber studies with variable surface area.
5.2.2 Effect of sorption to chamber surfaces
Xu et al. (2011) designed a special stainless steel chamber that maximized the surface area of the vinyl flooring source and minimized the surface area of the internal sink (the interior stainless steel surface of the chamber) thereby reducing the time to reach steady state, compared to previous studies using conventional chambers such as CLIMPAQ and FLEC. In addition, three precision-ground stainless steel rods, matched to the interior stainless steel chamber surface, were inserted into the chamber and then periodically removed so that the adsorbed surface concentration could be measured. This allowed the instantaneous gas-phase concentration in the chamber to be related to the adsorbed surface concentration at that point in the chamber run. Because equilibrium was not fully established as the gas-phase concentration built up to higher levels, the partition coefficients obtained under these conditions were somewhat underestimated. In a second set of experiments, the rods were introduced into the chamber after the gas-phase concentration had reached steady state. It then took approximately 60 days for the rods to reach equilibrium with the chamber air, allowing the equilibrium between the stainless steel surface and the air in the chamber to be accurately quantified.
5.2.3 Effect of air flow rate
Clausen et al. (2010) measured emission of DEHP from vinyl flooring as a function of applied air flow rate in the FLEC. Initially, the air flow rate through all FLECs was 450 ml/min. After about 2 years the air flow rates were increased to 1000, 1600, 2300, and 3000 ml/min, respectively, in four of the FLECs, and maintained at 450 ml/min in the fifth FLEC. Air samples were collected from the effluent air at regular intervals. Because the flow within the FLEC cavity is laminar, diffusion determines how fast DEHP can be transferred from the vinyl flooring emission surface into the air. Higher air flow rates increase the air exchange rate and reduce the residence time of the air so that there is less time for vertical diffusion to take place, resulting in a lower gas-phase concentration within the FLEC cavity. Because the concentration of DEHP in the air adjacent to the emission surface is known to be constant, the vertical air concentration gradient is therefore larger at the higher flow rate, which in turn increases the concentration-gradient-driven diffusion and the emission rate. With low flow rates, the residence time is long enough to allow vertical diffusion to create a uniform gas-phase concentration in the FLEC chamber. Below a flow rate of 1000 ml/min in the FLEC the air exchange rate is the limiting step and the emission rate is linearly dependent on the flow rate. However, with increasing flow rate the vertical diffusion becomes the limiting step of the emission process and a concentration gradient builds up in the FLEC. When the flow rate is increased by about seven times (from 450 to 3000 ml/min) the emission rate is increased almost six times. Although increased flow rate introduces more dilution, the increased emission rate driven by the concentration gradient almost compensates for the decrease in gas-phase concentration so that the gas-phase concentration does not drop substantially. The system therefore maintains a relatively constant bulk gas-phase concentration despite the variation in the air exchange rates.
5.2.4 Effect of humidity
Clausen et al. (2007) measured the influence of relative humidity (RH) on the emission rate of DEHP from vinyl flooring in the well-characterized FLEC. The vinyl flooring with DEHP as plasticizer was tested in six FLECs at 22 °C. The RH in the six FLECs was 10%, 30%, 50% (in triplicate) and 70%. The RH was changed after 250 days in two of the 50%-FLECs to 10% and 70%, and to 50% in the 10%- and 70%-FLECs. The data show that the emission rate of DEHP from vinyl flooring during a one-year period was independent of the RH. A physically based emission model for SVOCs (Xu and Little, 2006) helps to explain the RH results, because it appears that RH does not significantly influence any of the identified controlling mechanisms.
5.2.5 Effect of temperature
Clausen et al. (2011) measured emissions of DEHP from vinyl flooring in the FLEC. The gas-phase concentration of DEHP versus time was measured at an air flow rate of 450 ml/min at five different temperatures: 23 °C, 35 °C, 47 °C, 55 °C, and 61 °C. The experiments were terminated two weeks to three months after steady state was reached, and the interior surface of all the FLECs was rinsed with methanol to determine the surface concentration of DEHP. The most important findings were as follows:
1. DEHP steady-state concentrations increased substantially with increasing temperature (0.9 ± 0.1 μg/m3, 10 ± 1 μg/m3, 38 ± 1 μg/m3, 91 ± 4 μg/ m3, and 198 ± 5 μg/m3, respectively).
2. Adsorption to the chamber walls decreased substantially with increasing temperature (measured partition coefficient between FLEC air and walls: 640 ± 146 m, 97 ± 20 m, 21 ± 5 m, 11 ± 2 m, and 2 ± 1 m, respectively).
3. The gas-phase DEHP concentration in equilibrium with the vinyl flooring surface is close to the vapor pressure of pure DEHP liquid.
4. With an increase of temperature in a home from 23 °C to 35 °C, the amount of DEHP in the gas- and particle-phase combined is predicted to increase almost 10-fold. The amount in the gas-phase increases by a factor of 24 with a corresponding decrease in the amount on the airborne particles.
5.2.6 Effect of aerosol particles
Benning et al. (2011) measured the partitioning of gas-phase DEHP emitted from vinyl flooring onto to airborne particles. An experimental chamber was used to measure emissions of DEHP from vinyl flooring, and ammonium sulfate particles were introduced to examine their influence on emissions. When particles were introduced to the chamber at concentrations of between 100 and 245 μg/m3 the total (gas + particle) DEHP concentrations increased by a factor of three to eight due to the enhanced emission rate. The measured DEHP partition coefficient was 0.032 ± 0.003 m3/μg. The DEHP-particle sorption equilibration time was shown to be less than 1 min, which agrees well with predictions from the literature. This study provided the first known measurements of the particle–gas partition coefficient for DEHP with results showing that emissions of SVOCs from materials will be enhanced in the presence of airborne particles.
5.2.7 Models to predict emissions and transport of SVOCs in the indoor environment
The mechanisms governing emissions of SVOCs from a solid material in which it is present as an additive (for example, DEHP in vinyl flooring or BDE-47 in polyurethane foam) are illustrated in Fig. 5.1 (Xu and Little, 2006). The variables are defined as follows: V is the room or compartment volume, A is the surface area of the source, Q is the ventilation rate, and y is the bulk gas-phase concentration of the SVOC. The SVOC in the source, at a material-phase concentration of C0, is assumed to be in equilibrium with the SVOC in the air in contact with the source, which has a gas-phase concentration of y0. SVOCs partition strongly to interior surfaces, and the sorbed SVOC concentration, qs, on the interior surface, As, can be assumed to be in equilibrium with ys, the gas-phase SVOC concentration in the air in contact with the surface. A boundary layer exists between the source and the bulk air in the room, with a mass transfer coefficient h, and between the bulk air and the interior surface, with a mass transfer coefficient hs. The concentration of suspended particles in the room is TSP, and a partition coefficient, Kp, describes the equilibrium between the air in the room and the suspended particles.
5.1 Schematic showing mechanisms governing emissions of SVOCs, present as additives in materials and products, into a room or compartment (see text for symbols). For simplicity, we assume that no SVOC enters in the influent air.
For SVOCs present as additives, the depletion of the source occurs so slowly that C0 and hence y0 are usually effectively constant, which simplifies conditions so that the emission rate is given by
where the product of h, the convective mass transfer coefficient, and (y0 − y), the concentration driving force, determines the rate at which the SVOC moves through the boundary layer of air into the bulk air in the room. SVOCs have low volatility (by definition), and consequently they partition strongly to surfaces in contact with the air. The resulting mass transfer between the bulk air and all interior surfaces (for example, walls, ceilings, windows, carpets, curtains, airborne particles, dust, clothing, human hair and skin) strongly influences the rate of change of the gas-phase concentration, y, and this in turn affects the emission rate, as shown in equation 5.1. A simple linear and reversible equilibrium relationship is assumed to exist between the exposed interior surface area As and the gas-phase concentration of the SVOC in immediate contact with the surface, or
where qs is the sorbed SVOC concentration on the surface, Ks is the surface/air partition coefficient, and ys is the gas-phase SVOC concentration in the air in immediate contact with the surface. As with emission from the source, there is a boundary layer through which the SVOC must transfer to get either to or from the surface, and Es, the mass transfer rate, is given by
where hs is the convective mass transfer coefficient associated with the surface. A linear and instantaneously reversible equilibrium relationship is also assumed to exist between the airborne particles and SVOCs in the air, or
Here, qp is the sorbed SVOC particle phase concentration, Kp is the particle/air partition coefficient, and TSP is the total suspended particle concentration. Because airborne particles are small, the rate of mass transport to particles can usually be neglected for common indoor-air residence time scales, which are of the order of an hour (Weschler and Nazaroff, 2008). For emissions of DEHP from vinyl flooring, major elements of the model have been validated (Xu and Little, 2006; Clausen et al., 2010) or are in the process of being validated. The model is illustrated here for DEHP in vinyl flooring placed in an idealized room. The model parameters used for the simulation are provided in Table 5.1 with predictions shown in Fig. 5.2.
Table 5.1
Parameters used to estimate DEHP concentrations in indoor environments
aKoa at 25 °C is used to estimate Kp and Kdust in the room at 25 °C; Kow and Kwa at 32 °C are used to estimate permeability through the skin at 32 °C.
5.2 Predicted gas-, particle- and interior surface-phase concentrations of DEHP emitted from vinyl flooring.
According to the simple model, during the first 100 days, approximately 150 mg of DEHP is emitted from the vinyl flooring. Of this, about 10% leaves the room in the air, 60% leaves the room sorbed to the particles suspended in the air, and the remaining 30% accumulates on the interior surfaces. Insight into SVOC persistence can be gleaned by comparing the amounts in the various compartments. On day 300, for example, there would be ~ 10 μg of DEHP in the air, ~ 50 μg present on the airborne particles in the air, and ~ 58,000 μg present on the interior surfaces. Table 3 in Weschler (2003) provides similar information for a wide range of compounds. If, in an effort to clean the SVOC from the indoor environment, the windows in the room were opened, the air in the room would be rapidly replaced with fresh air and the 60 μg of airborne DEHP would be ventilated away. However, shortly after the windows were shut again, continued volatilization from the vinyl flooring (at ~ 1.2 μg/min) plus desorption from the interior surfaces (at ~ 1.4 g/min initially) would rapidly restore the airborne levels to the same value as before opening the windows. Because of the large reservoirs associated both with primary sources and with secondary sorbed SVOCs, the system tends towards homeostasis, restoring itself to the former conditions after transient perturbations.
Clearly, the processes taken into account in Fig. 5.1 constitute a highly idealized representation of reality. For example, all interior surface interactions are considered to be on ‘hard’ surfaces with no diffusion of the DEHP into the many soft or porous materials that are commonly found in indoor environments. In addition, a large variety of interior surfaces are lumped into a single surface represented by a ‘typical’ DEHP partition coefficient of 2500 m (Xu et al., 2009). Although this idealization may seem extreme, it has been suggested that indoor environments age in a way that leads to thin organic films with fairly constant properties covering indoor surfaces (Weschler and Nazaroff, 2008). The relatively narrow range of partition coefficients estimated for different interior surfaces (Xu et al., 2009) supports this view. The assumed interior surface area of 120 m2 is only about twice the nominal surface area associated with walls and ceilings, and the actual interior surface in many indoor environments may be higher than this value. Finally, it is well known that DEHP sorbs strongly to dust (Weschler et al., 2008; Weschler and Nazaroff, 2010). Such partitioning is not taken into account in this simple model, but will be accounted for in the exposure assessment.
5.3 Exposure to semivolatile organic compounds (SVOCs) emitted from building materials and consumer products
For SVOCs present as additives, the depletion of the source occurs so slowly that C0 and hence y0 are effectively constant. At steady state, the amount emitted from the source must equal the amount removed from the room, or
The parameter Q* is an ‘equivalent’ air flow rate that includes the amount of SVOC present on suspended particles in the air. Thus, y is largely determined by y0, A, and other parameters that are relatively easy to estimate (h, Q, Kp, and TSP). The convective mass transfer coefficient, h, can be estimated using correlations based on the near surface flow velocity (Axley, 1991) with typical values of h indoors available in the literature (Huang et al., 2004; Lin et al., 2004).
In addition to inhaling SVOCs in indoor air, exposure via inhalation of airborne particles (Weschler and Nazaroff, 2008; Xu et al., 2009, 2010), ingestion of dust (Weschler et al., 2008; Xu et al., 2009, 2010; Weschler and Nazaroff, 2010), and dermal absorption (Kissel, 2011) may play significant roles, contributing to total human intake rates. For SVOCs present as additives, equation 5.6 can be used to roughly estimate the gas-phase concentration, and emissions are considered to persist for the entire time in which the source is present in the indoor environment. We illustrate the exposure analysis approach here using DEHP as an example. With an estimate of the gas-phase concentration from equation 5.6, the particle-phase concentrations and dust-phase concentrations are estimated based on Kp and Kdust (i.e., the partition coefficients between particles and air and between dust and air, respectively). The amount sorbed through the skin is estimated in relation to overall permeability, p, based on mass transfer from air. Values for Kp, Kdust and p were obtained using the relationships listed in Table 5.2, with the required chemical properties for DEHP reported in Table 5.1. This information was then used to calculate exposure via inhalation, oral ingestion of dust, and dermal absorption for a three-year old child, as summarized in Tables 5.3 and 5.4.
Table 5.2
Relationships used to estimate exposure
| Parameter | Units | Equation (source)a |
| Particle/air partition coefficient (Kp) | m3/μg | Kp = fom_part × Koa/ρpart (Weschler and Nazaroff, 2010) |
| Dust/air partition coefficient (Kdust) | m3/g | Kdust = fom_dust × Koa/ρdust (Weschler and Nazaroff, 2010) |
| Skin/water permeability (pskin/water) |
cm/s | log(pskin/water) = 0.7log(Kow) – 0.0722 MW2/3 – 5.25 (Mitragotri, 2002) |
| Skin/air permeability (pskin/air) | m/h | log(pskin/air) = log(pskin/water) + log(Kwa) |
| Overall permeability (p) | m/h | p = [(1/ pair) + (1/ pskin/air)]− 1 |
aMW is the molecular weight in units of g/mol; fom_part and fom_dust are the volume fractions of organic matter associated with airborne particles and settled dust (estimated to be 0.4 and 0.2), respectively; ppart and pdust are density of airborne particles (1 × 106 g/m3) and settled dust (2 × 106 g/m3), respectively.
Although it has been demonstrated that a compound’s octanol–air partition coefficient (Koa) is a strong predictor of its abundance in settled dust based on the gas-phase concentration (Weschler and Nazaroff, 2010), for SVOCs with high Koa values the concentration in dust may not have sufficient time to equilibrate with the gas phase. The same is true for SVOCs with high Koa values partitioning to human skin. Hence, exposure via ingestion of dust or direct air-to-skin transfer may be overestimated in these examples for the lower volatility SVOCs. We note here that the relationships that are used to estimate exposure are based on Koa and Kow (the octanol–water partition coefficient), which are generalized parameters that are readily available for many SVOCs (Weschler and Nazaroff, 2008, 2010).
Once exposure to SVOCs present in building materials and consumer products has been estimated, a toxicokinetic model (also referred to as a physiologically based, pharmacokinetic or PBPK model) is needed to account for the transport and metabolism of the compounds in the human body. For example, a toxicokinetic model for di(n-butyl) phthalate (DnBP) and its active metabolite, mono(n-butyl) phthalate (MnBP), as well as the glucuronide of MnBP, has been developed for the rat. The rodent-based model was evaluated by comparison with the only human kinetic study of DnBP reported in the literature, with good results (Clewell et al., 2008). This suggests that human toxicokinetic models can be developed using allometric scaling of animal models, although in vitro metabolism data will be necessary to ensure the predictive ability of the human model to estimate target tissue dose (Clewell et al., 2008).
Although a mechanistic understanding of the ultimate toxic mode of action remains elusive for many SVOCs in commerce, rapid methods are being developed and applied to evaluate hundreds of these compounds for potential hazard. The ToxCast™ program uses high-throughput screening, computational chemistry, and toxicogenomic techniques to profile bioactivity and prioritize toxicity testing for large numbers of chemicals (Dix et al., 2007; Judson et al., 2010). In vitro assays quantitatively characterize the pharmacodynamics of a chemical in concentration-response mode and new high-throughput methods are applied to estimate the corresponding pharmacokinetics of a potential toxicant (Rotroff et al., 2010). In addition to this toxicological information, there is a strong need for screening-level procedures to estimate human exposure to SVOCs, such as those being developed in the ExpoCast™ program (Cohen Hubal et al., 2010). Combined with rapid estimates of toxicity, the development of efficient exposure tools would enable a more complete risk-based prioritization of chemicals.
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