Previously, the AHA had not drawn firm conclusions about the long-term effects of chronic exposure to different pollutants on heart disease and stroke, because of flaws in the design and methodology of many pollution studies.

"The major thing now is that the level of science is good enough to warrant this alert," Brook told heartwire.

A panel of experts conducted the comprehensive review of the literature on air pollution and cardiovascular disease, focusing on particulate matter with specific emphasis on secondhand tobacco smoke.

Although they note, "Our understanding of the relevant biological mechanisms involved is incomplete," they say that the existing body of evidence is now "adequately consistent, coherent, and plausible enough to draw several conclusions."

These are:

Short-term exposure to elevated levels of particle pollution significantly contributes to increased acute cardiovascular mortality, particularly in the aforementioned at-risk subsets of the population.

Hospital admissions for several cardiovascular and pulmonary diseases are increased in response to higher concentrations of particle pollution.

Prolonged exposure to elevated levels of particle pollution is a factor in reducing overall life expectancy by a few years.

For example, European data suggest that around 40000 deaths per year in Austria, France, and Switzerland combined have been attributed to particulate matter, the statement says, and around one in 50 MIs were thought to be triggered by outdoor air pollution in one study from London.

The panel also notes that although the US Environmental Protection Agency (EPA) updated its National Ambient Air Quality Standards for particulate matter in 1997, "19% of all US counties with air-quality-monitoring systems are [currently] not meeting these standards."

The most current estimates suggest that attainment by the EPA of these standards would reduce total mortality by 23000 deaths annually and cardiovascular hospital admissions by 42000 per year in the US, they add.

Another important issue is tobacco legislation. Secondhand cigarette smoke is the single largest contributor to indoor air pollution when a smoker is present, studies show.

"Protecting individuals from secondhand smoke is critical," says M Cass Wheeler (CEO, American Heart Association). "Comprehensive smoking bans are in place in eight US states already and in hundreds of communities around the country. Eliminating secondhand smoke in workplaces, restaurants, and bars should be a priority for every state and community."

Information on the levels of ozone and particulate matter in more than 150 US cities is published daily by the EPA, along with a health-alert system that reflects recommended changes in activity, the panel notes. This information is available on the EPA AIRNow website.2 "The AHA supports these recommendations as guidelines for activity restriction for those at risk," the panel states.

People should be advised to exercise indoors on a day with high air pollution... rather than being told to stop activity altogether.

They also note that there are wide variations in fine-particulate matter levels between North American cities, and that even within a given town, location is important. For example, those who lived near a major road were more likely to die of a cardiovascular event in one of the studies they looked at.

Brook emphasizes, however, that "the risks [of air pollution] are small for an individual compared with the impact of established cardiovascular risk factors." And he says that defenses such as activity restriction should be viewed in an overall context. "For example, people should be advised to exercise indoors on a day with high air pollution or wait until later in the evening, rather than being told to stop activity altogether," he told heartwire.

Finally, the AHA panel says several scientific issues remain, including: improving the overall understanding of the underlying biological mechanisms, identifying the differential toxicity of various constituents and sources of air pollution, and instigating epidemiological investigations into some of the limitations of previous reports.