WELLINGTON, New Zealand - A single joint of marijuana obstructs the flow of air as much as smoking up to five tobacco cigarettes, but long-term pot use does not increase the risk of developing emphysema, new research suggests.
The study by New Zealand's Medical Research Institute found that longtime pot smokers can develop symptoms of asthma and bronchitis, along with obstruction of the large airways and excessive lung inflation. The paper was released Tuesday ahead of its publication in the journal Thorax.
'The study shows that one cannabis joint causes a similar degree of lung damage as between 2.5 and five tobacco cigarettes,' said lead author Sarah Aldington.
However, the researchers found that the progressive chronic lung disease emphysema, often associated with cigarette smoking, was uncommon among marijuana smokers. Only 1.3 percent of the long-term pot smokers were found to have signs of the disease compared to 16.3 percent of those who combined marijuana and tobacco, and 18.9 percent of those who only smoked tobacco.
Marijuana smokers had symptoms that included wheezing, coughing, chest tightness and phlegm - all of which were associated with tobacco smokers, except chest tightness.
The study, which used lung function tests, high-resolution X-rays and questionnaires, also revealed that among marijuana smokers damage occurred to the small, fine airways which are important for taking in oxygen and removing waste gases. The extent of damage rose in proportion to the number of joints smoked.
Last week, another study published in The Lancet medical journal suggested that using marijuana may increase the likelihood of becoming psychotic, with even infrequent use potentially raising the overall small risk by up to 40 percent.
The three-year Thorax study involved 339 people in New Zealand, where pot smoking is fairly common. An estimated 160 million people use marijuana worldwide.
Participants were recruited into four groups based on smoking habits - nonsmokers, tobacco-only smokers, tobacco and marijuana smokers, and marijuana-only smokers.
To qualify as a long-term marijuana user, participants had to have smoked a minimum of one joint a day for five years, said institute director Richard Beasley, who also participated in the study. Tobacco users had to have smoked a pack a day for one year.
Earlier studies have shown that smoking one joint results in three to five times more carbon monoxide and tar inhaled than smoking a cigarette of the same size. The New Zealand research also showed that the 'products of combustion' in marijuana are very similar to tobacco, Beasley said.
Part of the reason for this is the way joints are smoked, with users often inhaling and holding the smoke in longer for a better hit. Marijuana joints typically do not have filters and they have shorter butts than cigarettes with a higher smoke temperature. Pot also is commonly smoked through various types of pipes.