Guidance for Benzene in Residential Indoor Air
Cat : H144-14/1-2-2013E-PDF
ISBN : 978-1-100-23131-0
Benzene is a volatile organic compound (VOC) with a relatively high vapour pressure, moderate-to-high water solubility, and low octanol/water partition coefficient, that is released primarily to air. It has been identified by Health Canada as a priority indoor air contaminant through consultations with provincial and territorial health departments, as well as with key stakeholders in industrial and environmental organizations.
Exposure of the general Canadian population to benzene is attributed predominantly to indoor air by inhalation because indoor levels generally exceed those outside, and time spent indoors is typically greater than time spent outdoors. Other routes of exposure (ingestion and dermal absorption) contribute minimally to total benzene intake (Health Canada 2009). In studies by Health Canada, the median concentrations of benzene measured in Canadian residences ranges from 0.5 to 2.2 µg/m3 (Health Canada 2012; Health Canada 2010a; Health Canada 2010b; Héroux et al. 2008). Indoor levels are approximately three-fold higher in homes with attached garages compared to those with detached garages, or no garages. Median outdoor concentrations are usually less than 1 µg/m3. The ratio of indoor/outdoor levels (I/O) ranges from 1.5 to 2.4 (median) and remains above 1 at the 25% percentile, suggesting a significant contribution of indoor sources in almost all homes.
More than half of Canadian single-family homes have an attached garage. Attached garages, when present, are the major indoor source of benzene in homes because vehicle exhaust and evaporative emissions from gas-powered equipment and stored solvents in garages can enter a home (Héroux et al. 2010; Héroux et al. 2008; Jia, Batterman and Godwin 2008; Batterman, Jia and Hatzivasilis 2007). Smoking is also a significant contributor to indoor benzene levels (Héroux et al. 2010; Héroux et al. 2008).
The known sources could not account for all benzene measured in studies and there may be other unidentified sources in homes. Some non-smoking homes without attached garages may have indoor benzene levels similar to homes with attached garages. While benzene has been detected in building materials, emission rates have generally been low (<4 µg m-2 h-1) with few exceptions (e.g., adhesives and caulking) (Choi et al. 2010; Won et al. 2005; Yu and Crump 1998; US EPA 1992; Wallace, Pellizzari and Leaderer 1987). Few household or consumer products have reported benzene content (Kwon et al. 2008; Kwon et al. 2007; Sack et al. 1992; US EPA 1992; Wallace, Pellizzari and Leaderer 1987) and, in the majority of studies, benzene was not associated with household products or activities (Missia et al. 2010; Héroux et al. 2008; Jia, Batterman and Godwin 2008; Jia, D'Souza and Batterman 2008; Park and Ikeda 2006; Brown 2002; Ilgen et al. 2001; Kim, Harrad and Harrison 2001). Therefore, a more systematic approach to identifying other indoor sources of benzene is warranted.
Another potential indoor source of benzene would be vapour intrusion if the groundwater or soil underlying the house is contaminated. In addition, if benzene is present in the domestic water supply, volatilization from water during bathing, or showering, or while water is running from faucets may occur. The Health Canada drinking water guideline has been developed to account for all exposure pathways (ingestion, inhalation, dermal absorption) and thus, if a water supply complies with the drinking water guideline regarding benzene content, the health risk of such exposure would be negligible.
Human exposure to benzene has been linked to dizziness, tremors, nausea, vomiting, headache, and drowsiness after minutes of exposure to high levels in the range of 700 to 3000 ppm (2240 to 9600 mg/m3). Effects after subchronic to chronic exposures as low as <1 ppm (<3.2 mg/m3) include progressive deterioration in hematopoietic function including bone marrow damage, changes in circulating blood cells and altered immune response (ATSDR 2007).
Benzene is classified as a carcinogen (US EPA 1998; Environment Canada and Health and Welfare Canada 1993; IARC 1987). Benzene affects the blood-forming system at low levels of occupational exposure ≤ 1 ppm (3.2 mg/m3), and there is no evidence of a threshold. Chronic exposure to benzene has been shown to cause leukemia, a cancer of the blood or bone marrow, in occupationally exposed workers; and leukemia and lymphomas in laboratory rats and mice (Health Canada 2009; OEHHA 2001; Hayes et al. 1997; Rinsky, Young and Smith 1981). The carcinogenic mode of action for benzene is not well understood; however, a series of important biological events progressing from metabolism to development of leukemia has been proposed (Meek and Klaunig, 2010).
Health Canada and other organizations have characterized the cancer risk associated with exposure to benzene in air (Health Canada 1996; OEHHA 2001; US EPA 2000). All of these values can be expressed as an increased risk of leukemia over a lifetime. The basis for these values is epidemiological studies of occupationally exposed individuals. The derivation of any reference value requires careful consideration of many factors and the adoption of several assumptions. Differences in these key inputs are reflected in the range of published toxicological reference values for benzene. However, considering the uncertainties identified with extrapolation of risks associated with exposures at occupational levels to lower, environmentally relevant concentrations, the shape of the dose response curve and the mode of action of benzene-associated leukemia, these reference concentration values are within a similar band of uncertainty.
From the cancer risk analyses by Health Canada (1996), the United States Environmental Protection Agency (US EPA 2000) and the California Environmental Protection Agency's Office of Environmental Health Hazard Assessment (OEHHA 2001), the concentrations associated with a 1x10-6 (one in one million) risk of leukemia range from 0.06 µg/m3 (the most justifiable unit risk identified by OEHHA (2001)) to 0.45 µg/m3 (the upper bound of the range presented by US EPA (2000)). Guidance on benzene levels indoors has been developed by the World Health Organization (WHO 2010) and European Commission (2005). Both of these organizations suggest that benzene levels indoors should be minimized as much as possible, and neither organization has developed a numerical guideline value for benzene levels indoors.
The real cancer risk from benzene in most Canadian homes, while not always negligible, is very low. Environmental levels are at least three orders of magnitude lower (in µg/m3 range) than the occupational exposure levels in the key studies (in mg/m3 range), and conservative assumptions were used when extrapolating from the risks based on high exposures. However, the uncertainty associated with the estimation of cancer risk at environmental exposure levels may be reduced when a clear mode of action of benzene toxicity in humans is established and applied to better estimate the dose-response relationship at low exposures.
The range of estimates of carcinogenic risks of benzene indicates that there may be a low but non-negligible risk at indoor exposure levels. On this basis, from a practical perspective, Health Canada has opted to use a qualitative approach, recommending that individuals take actions to reduce exposure to benzene indoors as much as possible. Measures to control known indoor sources may reduce benzene concentrations such that the risk to residents is very low. As further sources are identified and effective control measures developed, Health Canada will incorporate additional recommendations on reducing benzene levels in its communications to health and building professionals and the public.
While all indoor sources of benzene in Canadian homes could not be characterized, addressing the strongest predictors identified is likely to have the most significant impact on indoor levels. Exposure reduction strategies should be targeted towards these primary sources of benzene indoors over which homeowners have control, namely attached garages and indoor smoking. Indoor benzene levels may be minimized by:
- Preventing leaks from an attached garage to the house;
- Making sure that there is an appropriate seal between the home and the garage, particularly for any door that connects the two. This can be achieved by providing an appropriate air barrier and a sealed door between the garage and house and drywalling shared walls between the garage and house. These actions will also reduce the air exchange between the home and the garage;
- Installing an exhaust fan in an attached garage;
- Not idling vehicles in an attached garage;
- Not starting gas-powered equipment in an attached garage; and,
- Not smoking inside the home or the garage.
Where possible, removing solvents and gas-powered equipment, tools or engines from attached garages, since most small engines do not have emission controls on evaporative releases, may also be considered.
While there is some evidence (CMHC 2004; Ilgen et al. 2001) to support a reduction in indoor benzene levels originating from attached garages following these exposure reduction strategies, further research is required to understand which mitigation measures are the most technically feasible and cost-effective at reducing the migration of benzene from attached garages indoors. As well, research is needed into the predictors of elevated benzene in homes with detached garages or without garages.
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