p-dichlorobenzene (PDCB) is a chlorinated volatile organic compound (VOC) that is widely used in essentially pure form (>99.8%) as a repellant against snakes, rats, mice, squirrels, bats, and insects, as a deodorizer for toilets, urinals, and diaper pails, as an insecticidal fumigant, and as an air freshener (ATSDR, 2006; Kelly, 2009; National Toxicology Information Program, 1993; Wilhide and Fletcher, 1995). When used as moth repellents, PDCB-containing products (in the form of crystals, flakes, or cakes) are typically placed in closed drawers, closets, and plastic bags where clothes, blankets, and other goods are stored. As a deodorizer, PDCB is often placed in a toilet, diaper pail, bathroom, attic, basement, garage, pet cage, vehicle, or other location where odor is a concern. There are no known natural sources of PDCB (IARC, 1999a). Outdoor emission sources include the volatilization of consumer and commercial products containing PDCB, waste sites, and manufacturing facilities (ATSDR, 2006). PDCB is relatively stable in the environment compared to other VOCs, and its estimated atmospheric half-life is 14–31 days (Howard, 1989; Mackay et al., 1992). Outdoor concentrations are usually low, generally below 1 μg/m3 (Hultin et al., 2010; Johnson et al., 2010; Sexton et al., 2004; Simon et al., 2005; Weisel et al., 2005). In contrast, PDCB is commonly detected in indoor air, often at moderate to high concentrations. Exposures occur in both residential and occupational settings, including the manufacture of polyphenylene sulfide resins, deodorants, mothballs, dyes, pharmaceuticals, and agricultural products (ATSDR, 2006). PDCB also has been found in finished drinking water, surface water, groundwater, soil, sediments, in meats owing to its use in deodorant blocks in animal stalls (ATSDR, 2006; Environment Canada, 1993), and in honey and royal jelly due to its use as an insecticide for empty beehives and bee houses stored indoors (ATSDR, 2006; Environment Canada, 1993; Tananaki et al., 2009).
The widespread use of products containing PDCB suggests the importance of understanding the emissions, concentrations, exposures, and health risks associated with this chemical, especially in indoor environments that have the highest potential for exposure. Exposure can be assessed by measuring airborne concentrations and PDCB metabolites in blood, urine, adipose tissue, and breast milk (Aronson et al., 2007; ATSDR, 2006). Exposure has been associated with several adverse effects. Inhalation exposure has produced malignant tumors in the livers of mice and hyperplasia in the kidneys of rats (Aiso et al., 2005). Parenteral exposure (subcutaneous and intraperitoneal) reduced sperm production and had anabolic–androgenic effects in rats and mice (Takahashi et al., 2011). In adult humans, elevated inhalation exposure has been linked to increased white blood cell counts (Hsiao et al., 2011) and decreased pulmonary function (Elliott et al., 2006). PDCB has been classified as possibly carcinogenic to humans (Group 2B) (IARC, 1999b). California lists PDCB as a possible human carcinogen and in 1994 assigned an inhalation cancer unit risk estimate (URE) of 1.1 × 10−5 per μg/m3 (California OEHHA, 2009). The US Environmental Protection Agency considers PDCB to be a low-risk pesticide because of the lack of evidence for carcinogenicity, and it lists a non-cancer chronic inhalation reference concentration (RfC) of 800 μg/m3 (USEPA, 2010). The US Occupational Safety and Health Administration (OSHA) permissible exposure limit is 450 mg/m3 as an 8-hour time-weighted average concentration (OSHA, 2012). For general indoor air, Japan has specified a guideline value for lifetime exposure of 240 μg/m3 (Japanese Ministry of Health Labour and Welfare, 2001).
The uses and exposure patterns of PDCB in buildings are very similar to those of naphthalene, which is employed for many of the same uses, and also sold in a solid form that sublimates rapidly (Batterman et al., 2012a; Jia and Batterman, 2010). Both chemicals have been identified as priority pollutants in residences (Logue et al., 2011). In addition to its use as a deodorizer and repellent, naphthalene is also a product of incomplete combustion and a component of gasoline and gasoline vapor. Consequently, naphthalene tends to be ubiquitous, for example, detected wherever combustion occurs or gasoline is stored. In contrast, PDCB levels can be very low or undetectable levels in buildings where this chemical is not used.
This paper provides current information on PDCB concentrations and risks in indoor and outdoor settings in four Michigan cities. We characterize indoor levels in nearly 300 homes, estimate sources of variability using variance proportions, examine high-end concentration distributions, discuss spatial trends of ambient concentrations, derive composite emission rates from houses, and estimate risks.