"The pollutants of most concern are carbon monoxide and carbon-containing particles in the atmosphere," Ron Wyzga, an EPRI executive, told a news conference.

The EPRI is an independent, non-profit center set up to study health effects associated with the power industry. Some environmental groups say it is biased because it receives industry funding.

The EPRI has been studying the health effects of air pollution over Atlanta, as a typical Eastern U.S. city, since 1998. The study is epidemiological, meaning it looks at the population as a whole and not at individual effects and so far it has only looked at short-term effects.

Details have been given to the U.S. Environmental Protection Agency, the group said.

The more pollution, the higher the rate of heart-related deaths, emergency room admissions, visits to doctors and "events" forcing activation of defibrillators implanted in the chests of heart patients, the study found.

"When you go from a relatively low-pollution "clean" day ... to an average day, you see an increase in heart deaths of about 7 percent," Wyzga said.

In one study backed by EPRI, Kristi Metzger of Emory University in Atlanta and colleagues collected information on 4.4 million emergency room visits at 31 hospitals from 1993 to 2000.

They found cardiovascular disease incidents in general went up in winter and were associated specifically with higher levels of ozone, nitrogen dioxide and carbon monoxide and of tiny particulates.

The study, published in the journal Epidemiology, did not break down the visits by specific type of heart emergency.

Particulates -- bits of airborne metal, silica, soot, and other compounds, have longed been linked with health problems. In particular the sulfur-based compounds emitted by fossil fuel-burning power plants have been targeted.

"In Atlanta we did not see any consistent results with sulfates," Wyzga said. This immediately attracted criticism from environmental groups.