PM 2.5

High levels of air pollution in Beijing contain a large quantity of PM 2.5. © iStockPhoto.com/beijingstory.

We know that air pollution is bad for your lungs, but some 20 years ago researchers made the surprising and consequential finding that fine-particle pollution can also affect your heart, causing heart attacks and worsening heart disease.

Fine-particulate matter with a diameter of 2.5 microns or less (known as PM 2.5), penetrates deeply into the alveolar region of the lung and may even be able to cross into the blood. Over the last two decades, researchers have worked to determine the physiological and molecular basis for these effects beyond the lung. Evidence suggests that PM 2.5 triggers an inflammatory response and causes oxidative damage, among other things. PM 2.5 may increase plaque formation in the blood vessels and could affect the autonomous nervous system, which controls the heartbeat. The complete picture is an object of ongoing research.

The first connection between fine particles and cardiovascular risk came from two landmark epidemiological studies (1, 2) that uncovered a robust relationship between levels of fine-particulate matter in the air of American cities and the risk to their inhabitants of death from both lung cancer and heart attack, after correcting for smoking and other factors. Another key study in 2009 of 51 United States metropolitan areas showed that the counties with the greatest improvement in PM 2.5 levels between roughly 1980 and 2000 had the largest increases in life expectancy, correcting for a number of other variables (3).

The US Environmental Protection Agency (EPA) began regulating small particles separately from the larger PM 10 (with a diameter of 10 microns) in 1997. PM 10 cannot penetrate as deeply into the lungs and its health effects are not as strong as PM 2.5. The agency has tightened PM 2.5 standards as additional evidence has emerged on the extent of the health effects from PM 2.5 exposure. In December 2012, the EPA decreased the long-term exposure standard for PM 2.5.

Indeed, researchers now know that both short-term and long-term exposures to PM 2.5 are problematic. Tens of thousands of people die prematurely in the United States each year because of short-term elevated PM 2.5 levels (4), which especially affect the elderly or those with heart problems. PM 2.5 poses an even greater long-term health risk. The latest assessment of the global burden of disease, published late 2012 in Lancet, attributes 3.2 million deaths each year to outdoor particulate matter pollution, ranking it the sixth largest contributor to the global burden of disease (5).

“What you end up with is almost unbelievably large effects of particle air pollution, and, conversely, almost unbelievably large estimates of benefits from the Clean Air Act,” says C. Arden Pope III of Brigham Young University, an author of several epidemiological studies (1–3). Indeed, according to the EPA, every dollar spent on pollution control in implementing its new standard will yield $12–$171 in health benefits (6).

These mortality statistics have huge implications for countries like China and India, where air pollution control is lagging and populations are high. In addition, in many developing countries, indoor air pollution—largely small particles generated from indoor stoves burning coal, wood, or other biomass—adds to the burden. Indoor air pollution from solid fuel burning takes an even greater toll on global mortality. The recent global burden of disease assessment attributed 3.5 million deaths annually to such indoor air pollution, ranking it the third leading contributor to the global disease burden (5).

One challenge in this research area is that PM 2.5 is a complex chemical mix, in contrast to other air pollutants regulated by the EPA, which are single components. PM 2.5 can come from a variety of sources—traffic, coal-fired power plants, industry, and more—and although some is emitted directly from these sources, much of it forms in the air when chemicals, such as sulfates, nitrates, and volatile organics react or condense to form fine particles. Researchers are trying to uncover which sources of PM 2.5 are most toxic. Knowing this information would allow regulators to focus on controlling particular sources of PM 2.5 to have the greatest health effects.