Semivolatile organic compounds in indoor environments
Introduction
Organic molecules that can have meaningful abundances in both the gas phase and condensed phases are commonly referred to as semivolatile organic compounds (SVOCs). For the purpose of this paper, we consider SVOCs to be organic compounds with vapor pressures between 10−14 and 10−4 atm (10−9 to 10 Pa), which corresponds to saturated mixing ratios of 0.01 ppt to 100 ppm, assuming 1 atm of total air pressure. For a species with a molecular mass of 250 g mol−1, the corresponding saturated gaseous mass concentrations (at T = 293 K) would span a vast range, from 0.1 ng m−3 to 1 g m−3.
The indoor occurrence, fates, and consequences of SVOCs are emerging as important research topics, but SVOCs have not been nearly so widely studied as certain other classes of indoor pollutants, such as VOCs, inorganic gaseous pollutants, and airborne particles. This situation should not be interpreted to mean that SVOCs are relatively less important than these other classes. Rather, the high degree of analytical challenges in measuring SVOCs has impeded progress in studying them.
Some SVOCs, such as polycyclic aromatic hydrocarbons (PAHs) originating from combustion, have long histories of contributing to human exposure. Other SVOCs, primarily manmade compounds, have been produced in large quantities only within the last half-century or so. Indeed, since the end of World War II, the mix of dominant indoor SVOCs has shifted on a decadal time scale owing to evolving practices, concerns and tastes (Weschler, 2008). Reflecting their extensive use in commercial products, many SVOCs are commonly present at higher abundance indoors than outside. SVOCs occur as active ingredients in pesticides, cleaning agents and personal care products, and as major additives in materials such as floor coverings, furnishings and electronic components. Because of their slow rate of release from sources and because of their propensity to partition into sorbed states, SVOCs can persist indoors for years after they are introduced. Parallels can be drawn between indoor persistent SVOCs and outdoor persistent organic pollutants (POPs).
The health effects of a given SVOC depend on its chemical nature, as well as the extent to which humans are exposed. Given the differences between indoor and outdoor concentrations, coupled with humans spending much more time indoors than outdoors, total exposure to a given SVOC may be strongly influenced by conditions and processes occurring indoors. Overall, exposures can occur via inhalation, ingestion and dermal pathways. Inhalation exposures depend on the airborne concentrations of SVOCs, both gaseous and sorbed to suspended particles. Dermal and incidental ingestion depends on surface concentrations of SVOCs, which are influenced by airborne concentrations. Dermal uptake may also occur by direct transfer from the air to the skin, potentially mediated by clothing. Even direct ingestion can be influenced by airborne SVOCs that sorb to food in contact with indoor air prior to consumption.
Although SVOC exposures are documented to occur indoors, specific knowledge regarding the associated health effects is limited. Some SVOCs are known to be toxic (e.g., dioxins, benzo[a]pyrene, pentachlorophenol). Many SVOCs have been removed from commercial use because of their demonstrated or suspected health effects (polybrominated biphenyls (PBBs), tris(2,3-dibromopropyl)phosphate (tris-BP), certain pentabromodiphenyl ethers (pentaBDE)). For others, there are emerging indicators of concern. Large-scale longitudinal biomonitoring studies of the US population's blood and urine samples have revealed measurable body burdens for more than a hundred SVOCs, and for some of these compounds, the average body burden has been observed to increase from one survey to the next (Blount et al., 2000, CDC, 2003, CDC, 2005). Various phthalate esters have been associated with allergic symptoms in children (Øie et al., 1997, Bornehag et al., 2004), retarded male reproductive development (Adibi et al., 2003, Swan et al., 2005), and altered semen quality (Hauser et al., 2006, Hauser et al., 2007). Elevated levels of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) in cord blood have been negatively associated with birth weight (Apelberg et al., 2007). Certain SVOCs have chemical structures that are similar to those of human hormones and can either mimic or block endocrine activity. These endocrine disrupters have been suggested as potential contributors to neurodevelopment and behavioral problems ranging from autism to attention deficit disorder (e.g., Howdeshell, 2002, Colborn, 2004), although the evidence supporting such links is incomplete and, in some cases, controversial (Hileman, 2007). Examples of SVOCs for which studies suggest a link to endocrine disrupting activity include PCBs (Jacobson and Jacobson, 1996, Iwasaki et al., 2002), brominated flame retardants (Birnbaum and Staskal, 2004), di-2-ethylhexyl phthalate (Sharp, 2005), bisphenol A (Newbold et al., 2007, Hileman, 2007), nonylphenol (Soto et al., 1995), and certain pesticides (Howdeshell, 2002).
To the extent that they have been studied in indoor environments, SVOCs have tended to be investigated as independent contaminant classes. In preparing to write this paper, we compiled a bibliography of almost 700 articles relevant to indoor SVOCs published through late 2006. The literature is young, with a median publication date of 2002. There were substantial numbers of articles on combustion byproducts (∼125 articles on polycyclic aromatic hydrocarbons, nicotine in environmental tobacco smoke, dioxins and furans with a median publication date of 2000) and on biocides (∼150 on pesticides, herbicides, and insecticides; median date 2001). Substantial literatures are also emerging on flame retardants (∼100, mainly related to polybrominated diphenyl ethers; median date 2004) and on phthalates and other plasticizers (∼75; median date 2004). We identified no published articles that have attempted to broadly synthesize the current state of understanding of SVOCs in indoor environments, although a fugacity-based mass-balance model has been developed for indoor pesticides (Bennett and Furtaw, 2004).
This article examines indoor occurrence, abundance and dynamics of SVOCs to better identify key factors that influence human exposure to these ubiquitous compounds. In many indoor settings, temperature, relative humidity and ventilation rates change diurnally or even hourly; steady-state conditions are not achieved, and SVOC concentrations are influenced both by thermodynamic properties and by kinetic processes, including mass transport. We articulate a physical-science-based framework that can be used to help interpret and generalize from the empirical data to better understand the spatial and temporal distribution of SVOCs among various compartments in indoor environments. This framework can also be used to estimate the lifetime and fate of an indoor SVOC. Better understanding of the dynamic behavior of SVOCs in indoor environments can enable more robust exposure and risk estimates. Furthermore, improved understanding of indoor SVOC dynamics can facilitate the design of effective control strategies, including avoiding applications that result in elevated SVOC abundances that, in turn, may lead to exposures for years or even decades following a product's initial introduction into indoor environments.
Section snippets
Occurrence and sources
Table 1 lists compounds representative of SVOCs encountered indoors. Grouped by product or chemical class, SVOCs occurring indoors include biocides/preservatives, combustion products, degradation products, flame retardants, heat-transfer fluids, microbial emissions, personal care products, pesticides, plasticizers, stain repellents, surfactants, and waxes/polishes. The specific compounds in the table have been chosen because of their ubiquity or notoriety and because, collectively, they span
Equilibrium partitioning of SVOCs among air, water and octanol
In environmental systems, an SVOC can be distributed among many states, and the partitioning relationships exhibit both equilibrium and kinetic aspects. Fig. 2 illustrates the equilibrium relationships between a pure, condensed-phase SVOC and its presence in three other phases that are relevant for indoor environments: air, water and octanol. The respective equilibrium-partitioning coefficients are the saturation vapor pressure, Ps, the saturation concentration in octanol, Co_sat, and the
Kinetics and equilibrium in indoor environments
The preceding discussion of SVOC partitioning was based on equilibrium conditions. Strictly, equilibrium is guaranteed to be satisfied as the steady-state condition only in a system that is isolated from its surroundings. Indoor environments are inherently dynamic, continuously exchanging mass and energy with their surroundings (Nazaroff and Alvarez-Cohen, 2001). Equilibrium concepts may still apply in an open system such as an indoor environment, but the extent to which they do must be
Mass-balance considerations: applied SVOCs, such as pesticides
When an SVOC is introduced into an indoor environment, there is a tendency for it to redistribute from its initial location to all indoor surfaces. The principle of material balance can be used to construct a conceptual model of SVOC levels in indoor air during different stages of this redistribution process. Typically, the SVOC level in indoor air will be largely determined by the net effects of two source terms – the rate of escape of SVOC molecules from the source (Re) and the rate of
Applying the framework to better understand human exposure
We have articulated a framework, based on key physical and chemical concepts, for understanding the concentrations and dynamic behavior of SVOCs in indoor environments. Such a framework can be applied to better understand the processes and pathways that govern human exposures to SVOCs. To illustrate, this section explores the potential for dermal exposure to SVOCs via air-to-human transport indoors.
Consider the transport of SVOCs from bulk air to exposed skin, hair and clothing of indoor
Conclusions
Semivolatile organic compounds are ubiquitous components of the indoor environment. There are many sources: constituents of indoor materials (e.g., plasticizers, flame retardants and antioxidants), products used indoors (pesticides), processes that occur indoors (combustion), plus intrusion from outdoor air. Specific SVOCs have physical and chemical properties that span vast ranges; both vapor pressures and octanol–air partition coefficients vary across 10 orders of magnitude. Measurement of
Acknowledgements
The authors thank Hal Levin, John Little and Tunga Salthammer for valuable suggestions. The paper was initiated during a sabbatical visit of WWN to the Technical University of Denmark as an Otto Mønsted visiting professor.
References (123)
- et al.
DEHP metabolites in urine of children and DEHP in house dust
International Journal of Hygiene and Environmental Health
(2004) A new detailed chemical model for indoor air pollution
Atmospheric Environment
(2007)- et al.
Measuring partition and diffusion coefficients for volatile organic compounds in vinyl flooring
Atmospheric Environment
(2001) - et al.
Aqueous solubility and Henry's law constant data for PCB congeners for evaluation of quantitative structure–property relationships (QSPRs)
Chemosphere
(1988) - et al.
Octanol–air partition coefficient as a predictor of partitioning of semi-volatile organic chemicals to aerosols
Atmospheric Environment
(1997) - et al.
Occurrence of organotin compounds in house dust in Berlin (Germany)
Chemosphere
(2005) - et al.
Polychlorinated biphenyls suppress thyroid hormone-induced transactivation
Biochemical and Biophysical Research Communications
(2002) - et al.
Sorption behavior of volatile organic compounds on material surfaces—the influence of combinations of compounds and materials compared to sorption of single compounds on single materials
Environment International
(1999) - et al.
Polycyclic aromatic hydrocarbons (PAHs) in indoor dust matter of Palermo (Italy) area: extraction, GC–MS analysis, distribution and sources
Atmospheric Environment
(2008) - et al.
Gas/particle distribution of polycyclic aromatic hydrocarbons in coupled outdoor/indoor atmospheres
Atmospheric Environment
(2003)