A large-scale national study suggests low to moderate use of marijuana is less harmful to users’ lungs than exposure to tobacco, even though the two substances contain many of the same components.

This comprehensive study, led by UCSF and University of Alabama at Birmingham, collected data from more than 5,000 U.S. adults for more than 20 years.

Smoking cigarettes can cause significant lung damage, including respiratory symptoms, chronic obstructive pulmonary disease and lung cancer. It accounts for an estimated 443,000 deaths, or nearly one in every five deaths, each year in the United States, according to the U.S. Centers for Disease Control and Prevention (CDC). Data for the long-term effects of marijuana use on the pulmonary system has been scarce until now.

Mark Pletcher, MD, MPH Mark Pletcher, MD, MPH

“We found exactly what we thought we would find in relation to tobacco exposure: a consistent loss of lung function with increasing exposure,” said the paper’s lead author, Mark Pletcher, MD, MPH, associate professor in the Division of Clinical Epidemiology at UCSF. “We were, however, surprised that we found such a different pattern of association with marijuana exposure.”

In a paper published today in the Journal of American Medical Association (JAMA), researchers analyzed the relationship between current and lifetime exposure to marijuana and pulmonary function. The Coronary Artery Risk Development in Young Adults (CARDIA) study collected medical data from 5,115 men and women in four U.S. cities from 1985 to 2006.

They measured air flow rate – the speed in which a person can blow out air – and lung volume, which is the amount of air a person is capable of holding, typically about six liters of air for an adult male. Lung function was measured using a common medical device called a spirometer that measures air flow when the participant breathes in and out.

“Essentially with tobacco, the more you use, the more loss you have with both of the indicators, air flow rate and lung volume,” said the paper’s last author Stefan Kertesz, MD, MSc, associate professor in the Division of Preventive Medicine at the University of Alabama at Birmingham School of Medicine and the Birmingham VA Medical Center. “There’s a straight-line relationship: the more you use, the more you lose.”

The same was not true with marijuana use. Air flow rate increased rather than decreased with increased exposure to marijuana up to a certain level.

“An important factor that helps explain the difference in effects from these two substances is the amount of each that is typically smoked,” Pletcher said. “Tobacco users typically smoke ten to 20 cigarettes/day, and some smoke much more than that. Marijuana users, on average, smoke only two to three times a month, so the typical exposure to marijuana is much lower than for tobacco.”

“And marijuana is one where a lot of people dabble with it in their late teens and 20s, and some people continue with relatively low levels for a long period of time,” Kertesz added.

Heavy Marijuana Use May Take Toll

Although there was a suggestion that very heavy use of marijuana might be taking a toll on the lungs, the researchers could not get reliable estimates of the effects of very heavy marijuana exposure, as such smokers were relatively rare in the study population.

All participants in the study began as young, healthy adults 18 to 30 years old from four communities: Oakland, Chicago, Minneapolis and Birmingham. They volunteered to be part of this long-term medical research study, agreeing that their data could be used to explore questions, including about tobacco and marijuana use.