The study, which was published in the Journal of the American Medical Association, or JAMA, found that long-term exposure to ground level ozone, the main component of smog, is like smoking a pack of cigarettes a day, as CNN reported. Just a slightly elevated level of air pollution can lead to lung damage, even for people who have never smoked.

"We found that an increase of about three parts per billion [of ground-level ozone] outside your home was equivalent to smoking a pack of cigarettes a day for 29 years," said Joel Kaufman, a physician and epidemiologist at the University of Washington who contributed to the study, as NPR reported.

The study tracked 7,071 adults aged 45 to 84 living in six U.S. cities: Chicago; Los Angeles; Baltimore; St. Paul, Minnesota; New York City and Winston-Salem, North Carolina for up to 18 years.

The researchers created an exposure assessment method that looked at air pollution levels outside participants' homes and carried out CT scans and breathing tests, according to U.S. News and World Report. They assessed environments for levels of fine particulate matter, nitrogen oxide, black carbon and ozone.

All major air pollutants were linked to an increase of emphysema, a debilitating, chronic and irreversible lung disease that causes shortness of breath and shrinks the amount of oxygen that reaches the bloodstream. It's almost always associated with smoking or long-term exposure to second hand smoke.

However, exposure to ground level ozone pollution showed the strongest link to an increased prevalence of emphysema. It was also the only pollutant to show an additional decrease in lung function, as CNN reported.

"Rates of chronic lung disease in this country are going up and increasingly it is recognized that this disease occurs in nonsmokers," said Kaufman, as U.S. News and World Report reported. "We really need to understand what's causing chronic lung disease, and it appears that air pollution exposures that are common and hard to avoid might be a major contributor."

This is particularly troubling since the climate crisis is accelerating ground level ozone. While most air pollutants have declined thanks in large part to the Clean Air Act, ground level ozone has actually increased. Ozone is colorless and forms when pollutants from fossil fuels interact with sunlight. Pollution from cars, power plants, refineries and chemical plants all contribute to smog, and it is on track to get worse, as U.S. News and World Report reported.

"These findings matter since ground-level ozone levels are rising, and the amount of emphysema on CT scans predicts hospitalization from and deaths due to chronic lower respiratory disease," said Dr. R. Graham Barr, a professor of medicine and epidemiology at Columbia University Irving Medical Center and a senior author of the paper, to CNN.

He added that smog "is accelerated by heatwaves, so ground-level ozone will likely continue to increase unless additional steps are taken to reduce fossil fuel emissions and curb climate change. But it's not clear what level of ozone, if any, is safe for human health."

"And so as climate change progresses, we expect that vulnerable populations and — even healthy populations — are going to see increased effects," said Emily Brigham, a pulmonologist and assistant professor of medicine at Johns Hopkins University who was not involved in this study, to NPR.

U.S. #AirPollution Is 'Completely Outrageous' https://t.co/w4l72nkcGo @NRDC @GCAS2018

— EcoWatch (@EcoWatch) September 21, 2018