New Scientist Marijuana Special Report - Introduction

Claim THREE: "Smoking marijuana can lead to abnormal functioning of lung tissue . . ."
Smoking a couple of joints is as bad for your lungs as consuming a whole packet of cigarettes, say the anti-dope brigade. Their opponents say smoking marijuana has never caused anyone to die from lung cancer. So, is marijuana smoke more -- or less -- dangerous than tobacco smoke?

The person to ask is Donald Tashkin, a lung expert at the University of California at Los Angeles. For the past 15 years, Tashkin's team has been keeping a close eye on the respiratory systems of more than 130 regular marijuana smokers, comparing them with groups of people who smoke either just tobacco, tobacco and marijuana, or nothing at all. It's the biggest study of its kind in the world. And the results so far suggest that in some respects, yes, marijuana is more dangerous than cigarettes. But in one important respect, joints may actually be better for you -- especially if you're an athlete.

First, the bad news. While the cigarette smokers in the study were ploughing through 20 or more a day, the marijuana smokers seldom consumed more than three or four joints. Despite this, the marijuana smokers coughed and wheezed as much as the cigarette smokers. In both groups, about one in five people complained of coughing up phlegm and suffering bouts of bronchitis.

And when it came to cellular damage to the lungs, there was also little to choose between them. Both groups had too many mucus-secreting cells lining their airways and too few hair cells, and both groups showed evidence of abnormalities in cell nuclei and changes in genes known to have an early role in the development of cancers.

The similarity may seem puzzling given that the marijuana smokers were consuming so much less plant material. But there are good reasons for it, says Tashkin. The first is that joints yield up to three times the tar of cigarettes because they are more loosely packed and don't have filters. The second reason is that marijuana smokers inhale more deeply and hold their breath longer.

"We actually quantified this and found that the breath-holding time was increased about fourfold," says Tashkin. "That resulted in about a 40 per cent greater deposition of tar." Tashkin's final factor -- contested by some researchers -- is that marijuana smoke is richer in benzopyrene and other polycyclic aromatics known to trigger cancerous changes in cells.

So smoking marijuana can cause lung cancer, after all? Well, maybe. Despite the gloomy cell biology, epidemiologists have so far failed to find a link between marijuana and serious lung diseases. That might be because there isn't one. Or it might be because "the marijuana epidemic" (as Tashkin calls it) is still young and the people who started smoking in the 1960s haven't reached an age when cancers become common.

Meanwhile, some researchers are worried about another aspect of marijuana smoke -- its ability to interfere with immune cells that help to fight off lung infections. Tashkin's team has just discovered that immune cells isolated from the lungs of marijuana users are unusually bad at killing bacteria, 35 per cent worse, in fact, than similar cells taken from cigarette smokers. The marijuana-exposed cells were also below par at producing molecules needed to mount inflammatory responses. In normal marijuana smokers, the effects may be too slight to make much difference. Tashkin fears, however, that the same might not be true in people with AIDS, many of whom use cannabis to stimulate their appetites.

Footballer's fancy
There's some good news, though, for dope-smoking cricketers and footballers: marijuana smoke won't lead to blocked airways or emphysema. Despite all the cellular changes noted by Tashkin's team, the researchers found that even heavy smoking of marijuana had no impact on any physical measure of lung function. In fact, among their subjects, smoking three joints per day caused no greater rate of decline in lung capacity and the ability to breathe than smoking no marijuana per day.

And the reason for this silver lining? It could well be back to those sluggish immune cells, speculates Tashkin: "If cannabis impairs the ability of immune cells to produce inflammatory cytokines, you might be spared mucosal damage in peripheral airways."

From New Scientist, 21 February 1998