Association between levels of fine particulate matter pollution and mortality risk

Investigators who extended the Harvard Six Cities fine particulate air pollution study by eight years found that reduced levels of tiny particle pollution during this period lowered mortality risk for participants.

The results appear in the second issue for March 2006 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.

The findings of the original Harvard Six Cities study (1979 to 1990) revealed an association between levels of fine particulate matter pollution and mortality risk. The new study, which was conducted from 1990 to 1998, reports on this later period of reduced air pollution concentration.

Francine Laden, Sc.D., of Channing Laboratory in Boston, Massachusetts, and three associates found that the largest drops in adjusted mortality rates were in cities with the greatest reduction in fine particulate air pollution (PM2.5). The investigators' findings remained valid even after setting controls for the general increase in adult life expectancy that occurred in the U.S. during both study periods (1979 to 1989 and 1990 to 1998).

"This reduction was observed specifically for deaths due to cardiovascular and respiratory disease and not from lung cancer, a disease with a longer latency period and less reversibility," said Dr. Laden.

The study population consisted of 8,096 white participants residing in Watertown, Massachusetts; Kingston and Harriman, Tennessee; St. Louis, Missouri; Steubenville, Ohio; Portage, Wyocena, and Pardeeville, Wisconsin; and Topeka, Kansas. The average age of participants at the start of the original study was 50, with women comprising 55 percent of the cohort.

"Current smoking on enrollment ranged from 33 percent in Topeka and 40 percent in Watertown, and former smoking ranged from 21 percent in Harriman to 25 percent in both Topeka and Watertown," said Dr. Laden.

The annual mean concentration of PM2.5 (fine particulate matter) declined during the eight-year study period by 7 micrograms per cubic meter of air per decade in Steubenville, 5 micrograms in St. Louis, 3 micrograms in Watertown, 2 micrograms in Harriman, 1 microgram in Portage and less than a microgram in Topeka.

The improved mortality relative risk due to decreased PM2.5 during the second study period, as compared to the first, was 0.73.

In an editorial on the article in the same issue of the journal, Bert Brunekreef, Ph.D., of the Institute for Risk Assessment Science at the Universiteit Utrecht and the University Medical Center in Utrecht, The Netherlands, wrote:

"The investigators show that the city-specific reduction of PM2.5 was associated with a reduction in mortality rates. The reason why this is so important is that, until now, it was not clear whether the cohort studies were showing effects that resulted from lifetime cumulative exposure. If so, late changes in exposure would have little, if any, effect on survival. These new findings suggest another dynamic--namely that recent exposures do matter. This would be consistent with pollution affecting primarily a dynamic "pool" of susceptible individuals whose susceptibility itself may to some extent have been increased by lifelong, cumulative pollution exposure. We do know that smoking cessation leads to reductions in respiratory, cardiovascular and lung cancer risks, with different lags. The findings in this study, which show no effect on pollution reduction on lung cancer and the strongest effects on respiratory and cardiovascular mortality reduction, seem to show a similar pattern. The practical implication is that pollution reduction, even beyond the relatively low levels that have been achieved in the past half-century, will lead to public health benefits."

Dr. Brunekreef also highlights the study's limitations: the size of the study population was relatively small; some effects of clear medical importance were not considered statistically significant; and the PM2.5 concentrations during the second phase of the study were estimated. Moreover, because participants in the last phase were not interviewed regularly, potential variables--such as a change in smoking habits--may not be reflected in the data. He concluded that additional studies are needed.