Comparative Assessment of Air Pollution-Related Health Risks in Houston
Sexton, Ken, Linder, Stephen H., Marko, Dritana, Bethel, Heidi, Lupo, Philip J., Environmental Health Perspectives
Background: Airborne emissions from numerous point, area, and mobile sources, along with stagnant meteorologic conditions, contribute to frequent episodes of elevated air pollution in Houston, Texas. To address this problem, decision makers must set priorities among thousands of individual air pollutants as they formulate effective and efficient mitigation strategies.
Objectives: Our aim was to compare and rank relative health risks of 179 air pollutants in Houston using an evidence-based approach supplemented by the expert judgment of a panel of academic scientists.
Methods: Annual-average ambient concentrations by census tract were estimated from the U.S. Environmental Protection Agency's National-scale Air Toxics Assessment and augmented with measured levels from the Houston monitoring network. Each substance was assigned to one of five risk categories (definite, probable, possible, unlikely, uncertain) based on how measured or monitored concentrations translated into comparative risk estimates. We used established unit risk estimates for carcinogens and/or chronic reference values for noncarcinogens to set thresholds for each category. Assignment to an initial risk category was adjusted, as necessary, based on expert judgment about the quality and quantity of information available.
Results: Of the 179 substances examined, 12 (6.7%) were deemed definite risks, 9 (5.0%) probable risks, 24 (13.4%) possible risks, 16 (8.9%) unlikely risks, and 118 (65.9%) uncertain risks.
Conclusions: Risk-based priority setting is an important step in the development of cost-effective solutions to Houston's air pollution problem.
Key words: air pollution, air toxics, comparative risk, diesel particles, hazardous pollutants, particulate matter, risk assessment. Environ Health Perspect 115:1388-1393 (2007). doi:10.1289/ehp.10043 available via http://dx.doi.org/ [Online 5 July 2007]
Despite three decades of progressively more extensive and stringent regulatory controls, there is ongoing concern about unhealthful ambient levels of air pollution in Houston, Texas (Mayor's Task Force 2006; Rice University 2006). Houston, with a population of > 2 million, is the fourth largest city in the United States, and the 10-county Greater Houston region, with a population of > 5 million, is the nation's seventh largest metropolitan area. Because Houstonians drive an average of > 140,000,000 miles/day, emissions from cars, trucks, and buses are a major source of airborne pollutants. Moreover, Houston is home to > 400 chemical manufacturing facilities, including two of the biggest refineries in the United States. The petrochemical complex along the Houston ship channel is the largest in the country, and the Port of Houston is the largest in the United States in terms of foreign tonnage and second largest in total tonnage. Aggregate airborne emissions from many small sources spread across Greater Houston, such as dry cleaners, gas stations, surface coating processes, and gasoline-fueled lawn maintenance equipment, add to the complex mixture of ambient air pollutants typically present in Houston's air.
Meteorologic conditions and patterns also contribute to the air pollution problem in Houston. Between April and October there are usually a high number of warm sunny days with quiescent breezes, causing ground-level buildup of air pollutant concentrations. Most air pollution episodes in Houston occur as the wind direction rotates continuously over a 24-hr period, trapping a mass of unmoving air over the city. Elevated ambient levels of air pollution often occur along with high temperatures and humidity, creating hazy, malodorous, and oppressive conditions in the city.
Cost-effective mitigation of Houston's air pollution predicament requires policy makers to set priorities among literally thousands of airborne compounds, and to make difficult trade-offs between the benefits of reducing risks to human health, on one hand, and the costs of controlling airborne emissions, on the other. …