Lead author Dr. Mark Pletcher, associate professor at the University of California, San Francisco, and colleagues at the 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 -- and accounts for an estimated 443,000 deaths annually, or nearly one in every five U.S. deaths, the Centers for Disease Control and Prevention said.
"We found exactly what we thought we would find in relation to tobacco exposure: a consistent loss of lung function with increasing exposure," Pletcher said in a statement. "We were surprised that we found such a different pattern of association with marijuana exposure."
The study, published in the Journal of American Medical Association, found the more tobacco is used the worse air flow rate and lung volume measured. There's a straight-line relationship: the more you use, the more you lose.
"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 10 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."