Instead, the EPA under Bush administration set the limit at 15, a level that Lippmann said "is just too high. From a public health perspective, in my mind, that was inexcusable."
Looking at the new report, "there is no surprise here," Lippmann said. "They found a slightly smaller estimate than those that have been around before."
Some scientific questions remain to be answered, he said. "What is it about these fine particles that make them dangerous?" he said. "What has been settled is that they undoubtedly do great harm. But which chemical entities within the particles that do the damage is unknown."
The study does help settle one basic issue, said study co-author Majid Ezzati, an associate professor of community health at the Harvard School of Public Health.
"We knew that air pollution was bad, but has lowering it been good over the long run?" he said. "The political spectrum has been divided on it. This study indicates that, yes, having lower air pollution has been good for the health of people in these cities."
Learn how air pollution can affect health from the U.S. National Library of Medicine.
SOURCES: C. Arden Pope III, Ph.D., professor, economics, Brigham Young University, Provo, Utah; Morton Lippmann, Ph.D., professor, environmental medicine, and director, New York University Center for Particulate Matter Health Effects Research Center, Tuxedo Park, N.Y.; Majid Ezzati, Ph.D., associate professor, community health, Harvard School of Public Health,
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