Day-to-day increases in air pollution, even at levels generally considered acceptable, are associated with increased deaths among the elderly.

Previous studies have suggested an association, but most have been based on small populations in metropolitan areas. This new study, in JAMA, used Medicare files and nationwide air pollution data to estimate 24-hour exposure in people who died between 2000 and 2012.

The researchers found that for each day-to-day increase of 10 micrograms per cubic meter in fine pariculate matter (PM 2.5), the small particles of soot that easily enter the lungs and bloodstream, there was a 1.05 percent increase in deaths. For each 10 parts per billion increase in ozone, a main component of smog, there was a 0.51 percent increase.

The effect was greater for low-income people, African-Americans, women and those over 70, and the risk remained significant even at levels below what the Environmental Protection Agency considers safe.