Last week, a recently reconstituted panel of science advisers to the E.P.A., the Clean Air Scientific Advisory Committee, sharply questioned the agency’s longstanding position that particulate pollution is causally linked with premature death, and it called for a new assessment of the pollutant. In a letter to the E.P.A. administrator, Andrew Wheeler, the committee’s chairman, Louis Anthony Cox Jr., said the agency had not provided “a sufficiently comprehensive, systematic assessment of the available science.” This is despite the fact that epidemiological studies from around the world have shown a robust association between real-world exposure to PM 2.5 and premature mortality.

The committee’s action is clearly a step toward weakening the standard for fine particulates and other air pollutants and would erode a central objective of the Clean Air Act, the 1970 law that is a pillar of the agency’s mission of protecting public health.

That law requires the E.P.A. to set standards, or limits, on certain so-called priority air pollutants at levels “requisite to protect public health” with “an adequate margin of safety” based on a review of the scientific knowledge about that pollutant. The agency does this by evaluating peer-reviewed scientific evidence and making a decision based on the weight of that evidence. Once the agency determines the standards, cities and counties must reduce their pollution to meet those limits.

PM 2.5, or fine particulate matter, is a mix of microscopic particles and liquid droplets. Some of it is emitted from construction sites, unpaved roads and fires. Most of it results from the combustion of fossil fuels and is emitted from power plants, industrial smokestacks and motor vehicles. Its tiny particles (smaller than 2.5 microns) are less than one-thirtieth the diameter of a human hair and are easily inhaled. On days when PM 2.5 levels spike, more people with heart and lung disease die than on cleaner days. Living in a place with chronically high levels of PM 2.5, as people do in Los Angeles, Pittsburgh and the San Joaquin Valley in California, for instance, increases one’s risk of dying prematurely.

For years, anti-regulatory forces, including some members of Congress, have been trying to characterize epidemiology, the study of disease and health outcomes in populations, as “pseudoscience.” They have argued that epidemiological studies should not be used to set air quality standards because the health effects of air pollution are hopelessly confounded by other risk factors, like poverty, poor diet, smoking and diabetes. This view contradicts decades of public health and medical science accepted by the National Academies of Science and the Centers for Disease Control and Prevention, among others.