Closing the research loop: a risk-based approach for communicating results of air pollution exposure studies - Research

Environmental Health Perspectives, Jan, 2004 by Devon C. Payne-Sturges, Margo Schwab, Timothy J. Buckley

As part of our discussion with the CAC, we reviewed the components and assumptions often employed in risk assessment, such as adult body weight and inhalation rates used to estimate a dose and the application of data from animal studies where extrapolation from higher dose to lower dose is done. The CAC felt we should pursue the health risk interpretation as long as we were clear about the caveats when we distributed results to participants. We met regularly with the CAC to go over communication strategy (timing, format, and content) and discuss progress in the study.

Risk Assessment Approach

In response to the challenge set before us by the community, we reviewed the literature regarding risk communication and community-based research in search of a suitable model. Akland et al. (1997) reported personal exposure measurements for a variety of contaminants in Lower Rio Grande Valley by comparing individual measurements with values from other exposure assessment studies, as well as with Texas Effects Screening Levels--concentrations equivalent to a 1 in 10,000 cancer risk level. We drew on this approach, comparing individual and community exposure measurements to the U.S. EPA's National Human Exposure Assessment Survey (NHEXAS), which was conducted in 1995 across six midwestern states (Illinois, Indiana, Ohio, Michigan, Minnesota, and Wisconsin; Clayton et al. 1999). We then developed risk-based estimates for the target population based on the South Baltimore exposure measurements and toxicity values from the database developed by the U.S. EPA for their Cumulative Exposure Project (Caldwell et al. 1998; Morello-Frosch et al. 2000; Woodruff et al. 1998, 2000). To meet the CAC goals, the approach was extended to address cumulative risk.

Risk assessment is a way of organizing information that bridges science and policy. It is an accepted way to consider the potential toxicity of a given pollutant to estimate the likelihood that a population exposed to that level of pollution will experience a given health effect (National Research Council 1983; U.S. EPA 1986, 1999). The potential toxicity values that we used were the unit risk estimates (UREs) associated with carcinogens and the reference concentrations (RfCs) associated with noncancer effects for each of the pollutants monitored in the South Baltimore study. The URE represents the excess cancer risk over background associated with continuous lifetime exposure to a pollutant and is typically expressed as risk or probabiliity of cancer for a 70-year exposure (U.S. EPA 1986). UREs are derived either from occupational studies in humans, typically adult males, when available, or from toxicologic studies in animals (Woodruff et al. 2000). The URE values in this database follow the U.S. EPA default assumption: In the absence of model-specific information, the slope is based on a no-threshold model. UREs based on animal data are the upper 95% confidence bound of the estimated cancer potency or slope of the dose-response curve. The URE is a plausible upper-bound estimate of the risk (i.e., the risk is likely to be lower, but may be greater; U.S. EPA 1999). For air pollutants, the URE is typically expressed as excess cancer risk per microgram of pollutant per cubic meter of air.