Breathe uneasy: Ozone in our air is literally choking the life out of us.

RODALE NEWS, EMMAUS, PA—In the upper atmosphere, ozone (a form of oxygen) acts as a shield, helping to protect us from the sun’s ultraviolet (UV) radiation. But when it mixes with certain pollutants on the ground level, ozone not only makes us sick, it creates dangerous conditions that can actually shorten people’s lives, according to a new study published in The New England Journal of Medicine. “Ozone is more dangerous than we thought, at lower levels than we thought,” says Janice Nolen, assistant vice president of national policy and advocacy, and air quality national spokeswoman of the American Lung Association in Washington, D.C. “We knew it was dangerous on [Air Quality Index] Code Red, Code Orange kinds of days, but this study shows levels of ozone day-in and day-out can also be very dangerous, and can in fact shorten lives.”

THE DETAILS: Researchers looked at air pollution data from 96 U.S. cities and compared them to mortality rates among nearly 450,000 people who lived there during the 18-year study. They found that for each 10 part per billion increase in ground ozone concentration, there was a 4% increase in respiratory-illness-related deaths. In cities with the highest concentrations, like Riverside, CA, people are three times more likely to die from respiratory causes than people living in areas with low air pollution levels.

WHAT IT MEANS: Ozone is a more potent threat than we thought. Ground-level ozone doesn’t come directly from a tailpipe or smokestack, but occurs when volatile organic compounds from sources such as vehicle exhaust, drying paint, and the burning of coal or diesel fuel, are exposed to sunlight. Higher ground ozone levels have been linked with premature death, shortness of breath and chest pain, wheezing and coughing, inflammation in the lining of the lungs, increased susceptibility to respiratory infections and asthma attack, and increased hospitalization for people with lung diseases like asthma or chronic obstructive pulmonary disease. This study suggest that its impact can be felt at lower levels than anyone realized.

And it’s not just folks in urban environments who are threatened. Ozone usually occurs in cities during warmer months, but can also migrate to mountaintops, including wild places like Great Smoky Mountains National Park. There, it not only affects visitors’ health, but obstructs views and damages plants, and contaminates streams and the soil.

Here’s how to protect yourself and help clean up this emission mess: