(Reuters Health) - Older adults are more likely to die on days when air pollution rises, even when contaminant levels are below the limit considered safe by U.S. regulators, a new study suggests.

Researchers looked at 22 million deaths nationwide to see if there was any connection between fatalities and fluctuations in daily concentrations of ozone, an unstable form of oxygen produced when pollution reacts with sunlight, and so-called PM 2.5, tiny particles that include dust, dirt, soot and smoke.

Most of the deaths in the study occurred on days when ozone and PM 2.5 levels were below the limits set by the U.S. Environmental Protection Agency (EPA).

Previous studies have linked air pollution to an increased risk of chronic health problems and premature death, but those studies focused on cities, said study co-author Joel Schwartz of the Harvard T.H. Chan School of Public Health in Boston.

“So we did not know if the same association held in small cities, towns, or rural areas,” where pollution levels are lower, Schwartz said by email. “Now we have them all, so we know that it does apply everywhere, not just in big cities.”

The study focused on deaths from 2000 to 2012 for people in more than 39,000 ZIP codes nationwide who were insured by Medicare, the U.S. health program for the elderly and disabled.

Researchers compared satellite data on daily PM 2.5 and ozone levels on the days people died in specific ZIP codes to air quality levels on another day within a week or two of each fatality.

EPA standards cap 24-hour PM 2.5 at 35 micrograms per cubic meter of air (ug/m3) and 8-hour ozone at 70 parts per billion (ppb).

During all of the days examined in the study, 94 percent had PM 2.5 levels below 25 ug/m3, and 95 percent of the deaths occurred on these days, the study found.

At the same time, 91 percent of the days examined had ozone levels below 60 ppb, and 93 percent of the deaths occurred on these days.

Even when air quality still met EPA standards, each 10 ug/m3 daily increase in PM 2.5 levels was associated with an increase of 1.42 deaths per day for every million people, the researchers report in the Journal of the American Medical Association. Each 10 ppb increase in daily ozone levels was associated with 0.66 more deaths for every million people.

The increased risk of death associated with daily spikes in PM 2.5 and ozone levels persisted even when researchers restricted their analysis to days when the air quality complied with EPA standards.

Among other things, breathing polluted air can worsen existing respiratory conditions, such as asthma or chronic obstructive pulmonary diseases, said Griffith Bell, a researcher at the Eunice Kennedy Shriver National Institute for Child Health and Human Development in Bethesda, Maryland.

“In children or young people, these exacerbations might not have serious consequences, but in the elderly and the very ill, who may already have difficulty breathing, breathing very polluted air might push them over the edge,” Bell, who wasn’t involved in the study, said by email.

Beyond a lack of data on young people, another limitation of the study is the lack of data on the long-term air pollution exposure, the authors note.

Still, people should be aware of the health effects of air pollution even when they don’t live in urban areas where traffic and smog may be well-known health risks, said Junfeng Zhang, author of accompanying editorial and an environmental health researcher at Duke University in Durham, North Carolina.

“Even those who live in rural areas with relatively low air pollution levels can get higher exposures on days when the regional air pollution levels are higher due to bad weather conditions or during a fire,” Zhang said by email. “They can also get higher exposures by spending time near roadways or diesel-powered equipment, and they can get higher exposures while driving.”

SOURCE: bit.ly/2BYt3zQ JAMA, online December 26, 2017.