You might think that only high levels of air pollution above the EPA standard are dangerous, but new research shows that even low-level air pollution poses a risk to the human population, and is linked to death.

A team at Harvard T.H. Chan School of Public Health found that death rates among people over age 65 are higher in zip codes with more fine particulate air pollution (PM2.5) than in those with lower levels of PM2.5. The harmful effects from the particles were observed even in areas where concentrations were less than a third of the current standard set by the Environmental Protection Agency (EPA).

"Most of the country is either meeting the EPA standards now, or is expected to meet them in a few years as new power plant controls kick in," senior author Joel Schwartz, professor of environmental epidemiology, said in a statement. "This study shows that it is not enough. We need to go after coal plants that still aren't using scrubbers to clean their emissions, as well as other sources of particles like traffic and wood smoke."

Previous studies have linked both short- and long-term exposure to PM2.5 with conditions such as heart disorders, increased blood pressure, and reduced lung function, all which can lead to mortality. It has also been associated with stroke and anxiety, as well as reduced cognitive function.

For the first time, researchers studied the effect of soot particles in the air in the entire population of a region, including rural areas. They used satellite data to determine particle levels and temperatures in every zip code in New England. This way, they could examine the effects of PM2.5 on locations far from monitoring stations, and look at the effects of short-term exposures and annual average exposures simultaneously.

After following 2.4 million people in this region from 2003-2008, they realized that both short- and long-term PM2.5 exposure was significantly associated with higher death rates. Astonishingly, that was even true in zip codes with annual exposures below EPA standards.

Specifically, short-term (two-day) exposure led to a 2.14 percent increase in mortality per 10 μg/m3 increase in PM2.5 concentration. Meanwhile, long-term (one-year) exposure led to a 7.52 percent increase in mortality for each 10 µg/m3 increase.

"Particulate air pollution is like lead pollution, there is no evidence of a safe threshold even at levels far below current standards, including in the rural areas we investigated," concluded Schwartz. "We need to focus on strategies that lower exposure everywhere and all the time, and not just in locations or on days with high particulate levels."

The results were published in the journal Environmental Health Perspectives.

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