When Good Buses Go Bad Diesel Exhaust Linked to Neighborhood Air Pollution

Mass transit can help reduce air pollution if it replaces enough cars. But not everyone reaps the rewards, as a Harvard-led team discovered after studying diesel emissions in Boston’s Roxbury neighborhood [EHP 109:341–347]. Jonathan Levy and colleagues from the Harvard School of Public Health collaborated with a local advocacy organization called Alternatives for Community and Environment to begin characterizing pollution patterns in the Dudley Square area using a novel combination of methods and resources. This study lays the groundwork for future epidemiologic assessments of traffic-related respiratory symptoms. The Dudley Square terminal is one of the busiest Massachusetts Bay Transportation Authority bus stops in Boston. The neighborhood surrounding this bus terminal sees numerous diesel-powered vehicles, with 1,150 buses and trucks garaged within a 1.5 mile radius of Dudley Square (including half of the Massachusetts Bay Transportation Authority bus fleet). Some neighborhood residents suspect the horde of diesel-powered vehicles may be contributing to the community’s high asthma hospitalization rate, which is nearly five times the state average (9.8 hospitalizations per 1,000 people versus 2.1 per 1,000 people). The current study was intended as a community-based pilot project to gain insight into whether the vehicles may be contributing significantly to air pollution exposures experienced by Roxbury residents. The team used three portable monitors to test a 1-mile-radius area around the terminal for two important diesel combustion byproducts—fine particulates and particle-bound polycyclic aromatic hydrocarbons (PAHs). To cover the neighborhood effectively, the researchers sent trained high school students out on sorties with the portable monitors to measure particulates and PAHs at 1minute intervals during the morning hours of July and August 1999. The readings varied widely, from 0.3 to 340.0 nanograms per cubic meter for PAHs and from 2.0 to 347.0 micrograms per cubic meter for particulates less than 2.5 micrometers in diameter. While taking their readings, the students documented their position every five minutes so the data could be plugged into maps developed through a geographic information system. They also tracked temperature and humidity, which might be expected to affect pollutant concentrations. The team then used statistical methods and data from fixed-site PAH monitors in the neighborhood to evaluate spatial and temporal patterns of air pollution in the neighborhood data. The team found temporal patterns in air pollution (higher levels during rush hour and on weekdays) that suggested a link with transportation sources. Through statistical models, the team found a significant link between higher PAH concentrations and proximity to the bus terminal. They also found slightly—though not significantly—higher concentrations of PAHs and particulates along bus routes and high-traffic roads in the neighborhood. Higher humidity and temperature each coincided with higher particulate concentrations throughout the neighborhood, and higher humidity coincided with higher PAH concentrations. Levy cautions that the study was not designed to investigate links with specific health problems in the neighborhood, and the researchers can’t develop any conclusions from this study about specific health risks, including the high asthma rates in the neighborhood, because they did not collect pertinent health data. It also was not a comprehensive study, given budget constraints, and the lack of a comprehensive monitoring network made it difficult to disentangle spatial and temporal effects. Nevertheless, Levy says, the study was able to gather key exposure information on Roxbury residents and provides a template for more comprehensive analysis. –Bob Weinhold