SUN VALLEY, CA - DECEMBER 11:  The Department of Water and Power (DWP) San Fernando Valley Generating Station is seen December 11, 2008 in Sun Valley, California. Under a new climate plan before state regulators, California would take major steps toward cutting greenhouse gas emissions. If adopted by the California Air Resources Board, it would be the most ambitious global warming prevention plan in the nation, outlining for the first time how businesses and the public would meet the 2006 law that made the state a leader on global climate change. The action would lead to the creation of a carbon-credit market to make it cheaper for the biggest polluters to cut emissions, and change the ways utilities generate power, businesses use electricity, and personal transportation    (Photo by David McNew/Getty Images)
David McNew/Getty Images North America/Getty Images
SUN VALLEY, CA - DECEMBER 11: The Department of Water and Power (DWP) San Fernando Valley Generating Station is seen December 11, 2008 in Sun Valley, California. Under a new climate plan before state regulators, California would take major steps toward cutting greenhouse gas emissions. If adopted by the California Air Resources Board, it would be the most ambitious global warming prevention plan in the nation, outlining for the first time how businesses and the public would meet the 2006 law that made the state a leader on global climate change. The action would lead to the creation of a carbon-credit market to make it cheaper for the biggest polluters to cut emissions, and change the ways utilities generate power, businesses use electricity, and personal transportation (Photo by David McNew/Getty Images)
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(CNN) —  

Long-term exposure to air pollution, especially ground-level ozone, is like smoking about a pack of cigarettes a day for many years, a new study says, and like smoking, it can can lead to emphysema.

The study, published Tuesday in the medical journal JAMA, is the largest of its kind. It looked at exposure to air pollution – specifically to ground-level ozone, fine particulate matter, nitrogen oxide and black carbon. The study looked at more than 7,000 adults ages 45 to 84 for over a decade in six US metropolitan areas – Baltimore, Chicago, Los Angeles,  New York City, St. Paul, Minnesota, and Winston-Salem, North Carolina.

Using a CT scan, researchers were able to see that that exposure to each of the pollutants was associated with the development of emphysema, a lung condition that causes shortness of breath, and is usually associated with cigarette smoking. It’s a debilitating chronic disease that shrinks the amount of oxygen that reaches your bloodstream.

Scientists were able to show a decline in lung function with a spirometry, a simple test that measures how much air you can breathe out in one forced breath.

The patients were all healthy when they started the study, and researchers controlled for factors that could compromise lung health, including age and whether the person was a smoker or was regularly exposed to secondhand smoke.

The strongest association between a pollutant and emphysema was seen with exposure to ozone, which was the only pollutant associated with an additional decline in lung function.

Exposure to ozone irritates and inflames the lining of our lungs when we breathe it in. It can leave us winded, cause asthma attacks, make us more susceptible to infection.

Ground-level ozone is the part of smog that you can’t see. It’s colorless and it comes from the photochemical transformation that occurs when pollutants interact with sunlight.

“The increase in emphysema we observed was relatively large, similar to the lung damage caused by 29 pack-years of smoking and 3 years of aging,” said Dr. R. Graham Barr,  the Hamilton Southworth professor of medicine and epidemiology at Columbia University Irving Medical Center and a senior author of the paper. One pack-year means smoking a pack of cigarettes a day for a year.

Air pollution levels were estimated at the home addresses of the participants. They were taking part in the Multi-Ethnic Study of Atherosclerosis Air Pollution (MESA Air) and MESA Lung studies.

Ambient concentrations of fine particulates and nitrous oxide, but not ozone, decreased significantly over the study period, the researchers said.

“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 Barr.

With the climate crisis, there could be much higher levels of ground-level ozone in the future.

“Ground-level ozone is produced when UV light reacts with pollutants from fossil fuels,” added Barr. “This process 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.”