The cancer risk from exposure to volatile organic compounds (VOCs) is vastly underestimated by current models that rely solely on ambient emissions. Researchers from the Johns Hopkins Bloomberg School of Public Health showed that cancer risk figures based on actual measured exposure for communities in Baltimore, Md. were as much as three-fold greater than estimates given by models. Their study is the first of its kind to directly compare the Environmental Protection Agency's (EPA) Assessment System for Population Exposure Nationwide (ASPEN) model results to indoor, outdoor and personal exposure measurements. Scientists use pollutant exposure measurements to estimate public health risk. Such a comparison is important in evaluating the validity of ASPEN, which is being used nationally to assess the public health impact of ambient air toxins. The study, "Personal Exposure Meets Risk Assessment: A Comparison of Measured and Modeled Exposures and Risks in an Urban Community" is published in the April 2004 issue of the journal Environmental Health Perspectives.

Timothy Buckley, the senior author of the study and an associate professor in the School's Department of Environmental Health Sciences, said, "The public health implications of our findings loom large. ASPEN has already shown that for many US census tracts, risk from ambient air toxins exceeds acceptable levels. Now our data indicates that because of significant indoor source contributions, these risks are much worse."

Passive air sampling badges were used to compare personal, outdoor and indoor residential concentrations of VOCs for 33 non-smoking, adult study participants to ASPEN ambient estimates. The researchers wanted to learn how well ASPEN represented indoor and personal exposures, since such measures form the basis for the true public health risk.

For VOCs that have significant indoor sources, such as chloroform, the researchers found that ASPEN far underestimated the measured personal exposures. In contrast, VOCs from vehicles, such as benzene, or VOCs such as carbon tetrachloride that occur at global background levels, the ambient estimate given by ASPEN was in agreement with the measured personal exposures. When the researchers combined all the VOCs into an assessment of risk, they found that ASPEN underestimated risk based on actual measured exposure by a factor of three.

Devon Payne-Sturges, the study's lead author, said, "Our results indicated that South Baltimore residents were routinely exposed to a number of VOCs that are considered to be toxic air pollutants by the EPA and at levels above public health benchmarks. Benzene, carbon tetrachloride and chloroform accounted for most of the risk."

Dr Payne-Sturges explained that the indoor environment continues to represent an important source of personal exposures to VOCs. She said, "The good news is that indoor sources can be controlled. Environmental tobacco smoke is the greatest culprit but other more subtle contributors, such as cleaning solvents and air fresheners, add up to represent a sizeable fraction of the risk. We need to do a better job of getting the word out about the importance of indoor sources and the means for their control." However, she acknowledged that some populations might have more difficulty in changing these aspects of their life, especially for people who depend on jobs where limits on smoking indoors are not enforced or for people who have limited access to "green product alternatives" for home use.

Dr Buckley said, "Our findings demonstrate the significance of indoor exposure sources and the importance of indoor and personal monitoring for the accurate assessment of a community's risk. The EPA is clearly on the right track in its development of models that will take into account indoor sources providing a true representation of the health risk from VOCs air toxin exposure."

The study was supported by grants from the Maryland Cigarette Restitution Fund, US Environmental Protection Agency Region III, Mickey Leland National Center for Urban Air Toxics Research, Johns Hopkins Risk Science and Public Policy Institute and Johns Hopkins Center in Urban Environmental Health.

Co-authors from the Johns Hopkins Bloomberg School of Public Health include Devon Payne-Sturges, Thomas Burke, Patrick Breysse, Marie Diener-West and Timothy Buckley. The study was completed while Dr. Payne-Sturges was a doctoral candidate at the School.