Researchers Investigate Harmful Effects of Exhaust
Every day heavy trucks, buses, and trains emit pollutants that result from burning the diesel fuel in their tanks. The emissions are known to be unhealthy, but are they also potentially life threatening?
A number of health studies of truck drivers, railroad employees, and others indicate increased risk for lung cancer, said Thomas Smith, professor of industrial hygiene in the Department of Environmental Health, but the related question is how much exposure is too much.
To
better answer that question, HSPH and HMS researchers will soon complete
a feasibility study to determine if the incidence of lung cancer in trucking
company employees can be linked to their personal exposures to diesel
exhaust. Estimation of past exposures to diesel exhaust is difficult but
appears to be feasible, said Smith. The study was funded by the Health
Effects Institute in Cambridge, which typically receives half of its funds
from the US Environmental Protection Agency and the other half from industry
groups.
"There is a saying by the 16th-century Swiss physician Paracelsus, 'Dose alone makes a poison,'" said Smith. "Our question is, 'What dose makes a relevant increase in risk?'" Smith and his colleagues are interested in measuring the environmental dose, which is an exposure received over a period of time.
The other investigators of the study are Douglas Dockery, professor of environmental epidemiology and associate professor of medicine (epidemiology) in the Department of Environmental Health; Eric Garshick, assistant professor of medicine at HMS; and Frank Speizer, professor of environmental science in the Department of Environmental Health and Edward H. Kass professor of medicine at HMS.
When diesel fuel burns inefficiently, it releases microscopic particles of carbon that contain polycyclic aromatic hydrocarbons (PAHs), which are chemicals known to cause cancer at high exposures. The most visible aspect of diesel fuel burning is black-carbon smoke. Researchers do not know exactly how black-carbon smoke damages the lungs, Smith said, but cells may be damaged when they ingest carbon particles coated with unburned fuels and when they metabolize the PAHs.
Experts have discussed the effects of diesel fuel on health for years, but recent rulings and studies have reignited national interest. In January, the Harvard Center for Risk Analysis released some results of an ongoing study comparing the pros and cons of diesel and natural gas fuels when used in heavy trucks and buses. The study found that both fuels offer advantages and disadvantages. Diesel fuel produces more particulate pollution than natural gas, but it also creates fewer greenhouse gases.
In the United States, California has forged ahead in diesel fuel legislation. In 1998, state regulators formally identified diesel particulate emissions as a toxic air contaminant, which required the California Air Resources Board to take steps to reduce emissions. Transit authorities will need to choose between using low-sulfur diesel fuel to cut particulate emissions or switching to an alternative fuel.
The US Environmental Protection Agency has not classified diesel fuel emissions as toxins to be regulated, but the agency is investigating the issue.
"Some substances in diesel exhaust cause cancer if the doses are
big enough," said Thomas, "but diesel exhaust contains small
amounts of hundreds of chemicals. Why don't we figure out how big the
dose has to be to cause an unacceptable risk? It is important to deal
with even small risks that cover large segments of people, but we need
to think rationally how we are going to deal with this problem, and good
environment dose-response data are critical for that evaluation."
