A variety of sources produce fine particles, and they include diesel engines, automobile tires, coal-fired power plants and oil refineries.
Comparing exposure within the New York and the Los Angeles metropolitan areas, the study found that the risks were evenly distributed in the vicinity of New York while some areas around Los Angeles, including neighborhoods near the Ports of Los Angeles and Long Beach, had elevated health risks.
The extended epidemiological analysis, which draws on data gathered from 350,000 people over 18 years, and an additional 150,000 people in more recent years, was conducted for the Health Effects Institute by scientists at the University of Ottawa.
The institute was created by the Environmental Protection Agency and the industries that it regulates with the goal of obtaining unbiased studies.
The link between fine particles, the diameter of which is smaller than a 30th of a human hair, and cardiopulmonary disease has been established for two decades, and the E.P.A. has regulated such emissions since 1997. In 2006, despite mounting evidence that the particles were deadlier than first thought, the agency declined to lower chronic exposure limits.
The United States Court of Appeals for the District of Columbia Circuit declared that decision inadequate, and the Obama administration is now considering what level is appropriate.