(CNN) Smoking weed while being a tobacco smoker may increase damage to the respiratory system, a new study found.
"There's a public perception that marijuana is safer than tobacco, and this study raises concern this may not be true," said lead study author Dr. Giselle Revah, an assistant professor in the department of radiology at the University of Ottawa in Ontario.
"The American Lung Association says the only thing that should go into your lungs is clean air, so if you're inhaling anything, it could potentially be toxic to your lungs," she said.
The preliminary study, published Tuesday in Radiology, a journal of the Radiological Society of North America, compared computed tomography (CT) chest scans from 56 people who smoked marijuana and tobacco with lung scans of 33 people who had been heavy cigarette smokers for over 25 years.
Scans from an additional 57 nonsmokers with no preexisting lung disease, chemotherapy or other history of lung damage were used as controls.
Some 75% of the people in the study who smoked marijuana and tobacco had emphysema, a disease of the small airways that causes damage to the air sacs in the lungs. About 67% of the tobacco-only smokers had emphysema, while only 5% of the nonsmokers had the disease, she said.
A difference of 8 percentage points between weed plus tobacco and tobacco-only smokers may not seem like a huge difference, but it was significant, Revah said.
"It suggests that marijuana has additional effects on the lungs than tobacco alone," Revah said. "Is it the combination of the marijuana and tobacco that makes more holes in the lungs and airway inflammation or just the marijuana itself?"
Another concern was the age of the marijuana smokers — many were much younger than 50, she said.
"These patients presumably had less lifetime exposure to smoke, except they're even sicker than those who are heavy tobacco smokers and have been doing it longer," Revah said. "We just don't know if it's a synergistic effect between the marijuana and the tobacco versus the marijuana alone."
Airway damage from smoking can quickly become permanent, she said.
"Airway inflammation early on is reversible," she said. "When I see mucus and thickening of the airways, if you stop the exposure that should improve. But sometimes that can lead to dilatation of the airways and when it's dilated, then it's irreversible."
The study had some limitations, Revah noted. It was small. There was little information on how much marijuana was smoked or how it was inhaled — bong, blunt or joint.
However, there are several differences in how weed and tobacco are consumed that could provide clues for further investigation, Revah said. For example, tobacco is typically smoked with a filter, while weed is not.
"If you're smoking an unfiltered joint, let's say, more particulates will reach the airways, get deposited and become irritants, which is why you see the mucus and the inflammation," she said.
In addition, tobacco smokers quickly exhale, while marijuana smokers often inhale and hold their breath to maximize the high, she said.
"People usually have a longer breath hold and a higher puff volume, so they are holding in the larger volume of smoke for a longer period of time," she said. "That could lead to micro-trauma of those airspaces. These are all questions for future research."
This isn't the first study to find lung damage from inhaling marijuana. A June study found cannabis users were 22% more likely than nonusers to visit an emergency department or be hospitalized. The main reason was physical injury, but respiratory concerns were a close second.
A 2021 study found teens are about twice as likely to report "wheezing or whistling" in the chest after vaping marijuana than after smoking cigarettes or using e-cigarettes.
"I often am approached by both parents and teens who believe vaping cannabis is 'OK' and better than smoking (a joint, blunt, doobie etc.)," Carol Boyd, founding director of the Center for the Study of Drugs, Alcohol, Smoking & Health at the University of Michigan in Ann Arbor told CNN in a prior interview.
"And so, they ask, 'Vaping is safe — right?' My reaction: 'You are fooling yourself. We know that inhaling hot tobacco/cannabis smoke into your lungs is unhealthy and can cause bronchitis or life-threatening breathing problems," said Boyd, who is also professor emerita in the department of health behavior and biological sciences at the University of Michigan's School of Nursing.
"And yet, you seem to believe that heating chemicals (including carcinogens) into a vapor and inhaling them is healthy? My answer is, 'No, it is not a healthy behavior,' " she said.