Using a large national database, the research team compared the lung function of marijuana and tobacco smokers during a 20-year period. The data revealed that tobacco smoke had exactly the effect shown in all prior studies — increasing a person’s cumulative exposure to cigarettes results in loss of air flow and lung volumes; the opposite was true for marijuana smoke.

“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,” Kertesz says. “Those increases were not large, but they 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.”

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Kertesz cautions that smoking marijuana is not an avenue to better lung health.

“It’s not enough of an increase that would make you feel better,” he says “Healthy adults can blow out 3 to 4 liters of air in one second. The amount of gain, on average, from marijuana is small, 50 ccs or roughly a fifth of a can of coke. So it’s not something that would be noticeable.”

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Also, Kertesz says, the increase does not hold steadfast over time.

“The relationship changes for people who get to high levels of lifetime exposure,” he says. “At that point, the data suggests there is a decline in lung air-flow rate. There also may be other damaging effects that don’t manifest until extremely high levels of exposure; we did not have enough very heavy marijuana smokers in this study to determine this.”