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.
The study, published in of Journal of the American Medical Association, also offered up a nugget that likely will surprise many: Evidence points to slight increases in lung airflow rates and increases in lung volume from occasional marijuana use.
Air flow is the amount of air someone can blow out of their lungs one second after taking the deepest breath possible. The volume measure is the total amount of air blown out once someone has taken the deepest breath possible.
In the paper published 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.
Cumulative cannabis use is associated with higher forced vital capacity [the volume of air that can forcibly be blown out after full inspiration], total lung capacity, functional residual capacity [the volume of air present in the lungs at the end of passive expiration] and residual volume.
Cannabis was also associated with higher airways resistance but not with forced expiratory volume in one second [the maximum volume of air that can be forcibly blown out in the first second during the FVC test], forced expiratory ratio, or transfer factor. These findings were similar amongst those who did not smoke tobacco. By contrast, tobacco use was associated with lower forced expiratory volume in one second, lower forced expiratory ratio, lower transfer factor and higher static lung volumes, but not with airways resistance."
"With marijuana use increasing and large numbers of people who have been and continue to be exposed, knowing whether it causes lasting damage to lung function is important for public-health messaging and medical use of marijuana," according to one of the study's co-authors, Stefan Kertesz. "At levels of marijuana exposure commonly seen in Americans, occasional marijuana use was associated with increases in lung air flow rates and increases in lung capacity."
He added that those increases, though not large, nonetheless were statistically significant. "And the data showed that even up to moderately high-use levels -- one joint a day for seven years -- there is no evidence of decreased air-flow rates or lung volumes," he said.
Cannabis appears to have different effects on lung function to those of tobacco.
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.
"We found exactly what we thought we would find in relation to tobacco exposure: a consistent loss of lung function with increasing exposure," said Mark Pletcher, MD, MPH, associate professor in the Division of Clinical Epidemiology at UCSF. "We were, however, surprised that weÂ found such a different pattern of association with marijuana exposure."
"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, smokeÂ 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...Maybe
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.
Researchers believe the results can supplement the growing body of knowledge about beneficial aspects of low to moderate marijuana use in controlling pain, stimulating appetite, elevating mood and managing other chronic symptoms.
"Our findings suggest that occasional use of marijuana for these or other purposes may not be associated with adverse consequences on pulmonary function," Pletcher said. "On the other hand, our findings do suggest an accelerated decline in pulmonary function with heavier use -- either very frequent use or frequent use over many years -- and a resulting need for caution and moderation when marijuana use is considered."
Like cigarette smokers, marijuana users can develop throat irritation and coughs, but the study didn't focus on those. It also didn't examine lung cancer, but other studies haven't found any definitive link between marijuana use and cancer.
Raw Cannabis is Best
Despite this findings, raw cannabis is considered by many experts as a dietary essential. As a powerful anti-inflammatory and antioxidant, some classify it as one of the most important plants on earth. The biggest benefits from the plant may come not by smoking it, but rather by consuming it in its raw and natural form.
CBD (Cannabidiol), one of the main constituents of the cannabis plant has been proven medically to relieve many diseases including the inhibition of cancer cell growth. Recent studies have shown it to be an effective atypical anti-psychotic in treating schizophrenia. CBD also interferes with the amount of THC your brain processes, balancing the psychotropic effect of marijuana.
A British company, GW Pharma, is in advanced clinical trials for the world's first pharmaceutical developed from raw marijuana instead of synthetic equivalents and they say they'll have a mouth spray to treat cancer pain on pharmacy shelves by late 2013.
Why raw? Heat destroys certain enzymes and nutrients in plants. Incorporating raw cannabis allows for a greater availability of those elements. Those who require large amounts of cannabinoids without the psychoactive effects need to look no further than raw cannabis. In this capacity, it can be used at 60 times more tolerance than if it were heated.
Marco Torres is a research specialist, writer and consumer advocate for healthy lifestyles. He holds degrees in Public Health and Environmental Science and is a professional speaker on topics such as disease prevention, environmental toxins and health policy.