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ARCHIVED CONTENT: As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date each article was posted or last reviewed. No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.
Moderate consumption of marijuana doesn't adversely affect lung function, according to a study.
A study published in 2012 in The Journal of the American Medical Association (JAMA) had some good news for people who smoke marijuana: smoking at a rate of one joint a day for as long as seven years doesn't seem to affect lung function adversely.
And then there was the really good news for pot smokers: smoking marijuana in moderation may actually improve lung function, perhaps because inhaling gives the lungs and the chest muscles a bit of a workout.
Comparisons to tobacco
The JAMA report came from researchers affiliated with the Coronary Artery Risk Development in Young Adults (CARDIA) study, which has monitored risk factors for cardiovascular disease in more than 5,000 men and women for 20 years. In this study, the investigators focused on whether long-term marijuana use had the same effect on lung function as tobacco smoking and on the risk of developing chronic obstructive pulmonary disease (COPD).
Chronic bronchitis and emphysema, the two disorders that constitute COPD, often occur in longtime tobacco smokers. Symptoms — mainly coughing and shortness of breath — don't usually develop till people are in their 50s or 60s. In chronic bronchitis, inflammation damages the bronchi, or air passages, over time. In emphysema, the damage occurs in the walls of the alveoli (tiny sacs in which gases are exchanged), reducing the lung's capacity to exchange oxygen and carbon dioxide.
Because the smoke from marijuana and tobacco contains many of the same substances, including toxic gases and tar, health experts have suspected that the risk of developing COPD from smoking marijuana might be similar to that of smoking tobacco. Yet the results of earlier studies on pot smoking had yielded puzzling results. Many short-term studies demonstrated that while inhaling pot led to coughing and phlegm production, there was no evidence that it decreased lung function.
The CARDIA study design gave researchers a chance to observe the effects of marijuana use over two decades. During five different medical exams — at the start of the study and two, five, 10, and 20 years later — participants were asked about their lifestyles, including tobacco and marijuana use. They were also given tests that measured how well their lungs functioned.
Counting up ‘joint-years'
The researchers looked at the participants' recent and past smoking and toking histories. Current use was assessed as light, moderate, or heavy. Lifetime tobacco exposure was measured in the standard way, pack-years. A pack-year is equivalent to smoking 20 cigarettes a day for a year, which comes out to 7,300 cigarettes. Marijuana exposure was measured in "joint-years." A joint-year is equivalent to one joint — or filled pipe bowl — a day for a year, or, simply enough, 365 joints.
To assess their lung function, the participants were asked to inhale as deeply as possible and then exhale into a device that measured the volume of air breathed into it. Two measures were used — FVC (forced vital capacity), the amount of air exhaled until the lungs were empty, and FEV1 (forced expiratory volume), the amount of air forced out of the lungs during the first second of an exhale. In broad terms, FVC measures how much air the lungs can hold, and FEV1, whether the bronchi are narrowed or blocked by phlegm.
The findings for cigarette smokers were in line with those from decades of earlier studies. Scores on both lung function tests declined as the daily habit rose and their pack-years increased.
The results for the marijuana-only users told a very different story. In general, scores on the lung function tests improved until use, both current and long-term, got very heavy. For example, FEV1, the measure of how strong the lungs are, improved with greater exposure up to 10 joint-years. After 10 joint-years, though, the pot smoking seemed to take a toll, and the FEV1 measures of those smokers declined. But FVC scores — the measure of lung capacity — increased with greater lifetime exposure at 10 joint-years and beyond.
The researchers suggested that the way pot is smoked may be the reason marijuana could have some pulmonary benefits. The deep, sucking inhalation may stretch lung tissue to expand lung volume. It may also strengthen the muscles of the chest wall, enabling pot users to inhale and exhale air more forcefully.
Reassuring for medical users
It would be a real exaggeration, though, to say this study proves that smoking marijuana is good for your lungs. The study's authors put their findings in a more tentative light: a lack of adverse effects on lung function from occasional use of marijuana and indications of possible harm to the lungs from heavy use. Moreover, the estimates of the exposure to marijuana were inexact. Joints vary substantially in size and potency. The data from CARDIA didn't distinguish between smoking marijuana in joints, by pipe, or with a bong (water pipe). A more precise comparison of marijuana and tobacco use will require controlled clinical trials. But, in the meantime, this study offers some assurance to people with chronic medical conditions who use pot in moderation for pain relief or appetite stimulation. If marijuana is the only thing they are smoking, they aren't likely to add COPD to their health woes.
As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date of last review or update on all articles.
No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.
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