The evidence is stronger than ever that pollution from industry, traffic and power generation causes strokes and heart attacks, and people should avoid breathing in smog, the American Heart Association said on Monday.

Fine particulate matter from burning fossil fuels such as gasoline, coal and oil is the clearest offender, the group said.

“Particulate matter appears to directly increase risk by triggering events in susceptible individuals within hours to days of an increased level of exposure, even among those who otherwise may have been healthy for years,” said Dr. Robert Brook of the University of Michigan in Ann Arbor, who headed the group writing the report.