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.

A review of six year's worth of medical research also showed strong evidence that pollution can help clog arteries, and a small yet consistent association between short-term exposure to air pollution and premature death.

The foremost message for these high-risk groups remains that they should work to control their modifiable traditional risk factors -- blood pressure, cholesterol, diabetes, smoking, Brook said.

The American Heart Association said fine particulates could work in several ways, including by causing inflammation.

It's possible that certain very small particles, or chemicals that travel with them, may reach the circulation and cause direct harm, Brook said.

These responses can increase blood clotting and thrombosis, impair vascular function and blood flow, elevate blood pressure, and disrupt proper cardiac electrical activity which may ultimately provoke heart attacks, strokes, or even death.

The group recommends that the elderly or anyone with heart disease, high blood pressure, high cholesterol, or diabetes should pay attention to air pollution and air quality index warnings.

People can limit their exposure as much as possible by decreasing their time outside when particle levels are high and reducing time spent in traffic -- a common source of exposure in today's world, Brook said.

Michigan