SEATTLE — Scientists have known for years that long-term exposure to air pollution raises the risk of heart disease, but a highly anticipated study led by a University of Washington environmental health expert finally explains why. In a decadelong analysis involving more than 6,000 people in six states, Dr. Joel Kaufman found that people living in areas with more outdoor pollution built up calcium in the arteries of their hearts faster than those who lived elsewhere — increasing a known risk for heart attack and stroke. “On average we found a 20 percent acceleration in the rate of the calcium deposits,” said Kaufman, 54, director of the UW’s occupational and environmental medicine program. “I would say the results are a little more clear-cut and dramatic than I expected when I started this.” The $30 million study, funded by the U.S. Environmental Protection Agency and the National Institutes of Health, is the largest to measure both metrics of air pollution exposure and health markers over time. Results were published Tuesday in the journal The Lancet. It relied on data collected through the Multi-Ethnic Study of Atherosclerosis and Air Pollution (MESA Air), which studies the effects of pollution in six U.S. metropolitan areas: Baltimore, Chicago, Los Angeles, New York, St. Paul, Minn., and Winston-Salem, N.C. Through what authors of an accompanying Lancet editorial called “meticulous measurements,” Kaufman’s analysis looked at the exposure of participants to the fine particulate matter present in pollution, tiny bits less than 2.5 microns in diameter, too small to be seen with the naked eye. Those fine particles are referred to as PM 2.5. The study, which tracked air quality in participants’ communities and near their homes, also measured exposure to nitrogen oxide and nitrogen dioxide and black carbon or soot, pollutants typically associated with traffic. Participants also visited study clinics multiple times to collect health measurements. In a complicated conclusion, the researchers found that for every 5 micrograms per cubic meter higher concentration of PM 2.5, or for every 35 parts per billion higher concentration of oxides of nitrogen, participants saw an increase of 4 units per year of a marker for coronary-artery calcium, called the Agatston score.