Smoking a single joint of cannabis has the same impact on breathing capacity as up to five cigarettes, according to a New Zealand study published on Monday in a specialist British journal, Thorax.
Medical Research Institute of New Zealand doctors recruited 339 volunteers, who were divided into four categories -- those who smoked only cannabis, those who smoked only tobacco, those who smoked both, and those who smoked neither.
Cannabis-only smokers were defined as those who had smoked the equivalent of at least one joint a day for five years; tobacco-only smokers were those who had smoked the equivalent of a packet of cigarettes a day for at least a year.
All the participants were scanned by computed tomography (CT) to get a high-definition image of their lungs and were given tests to assess their airflow, the term for their respiratory efficiency.
The most serious damage was found in tobacco smokers -- both tobacco-only and combined users -- who were the only volunteers to have emphysema, a degenerative and crippling lung disease.
Cannabis smokers had lighter symptoms, such as wheeze, cough, chest tightness and phlegm, which tobacco smokers also had.
But the CT scan also revealed that, among cannabis smokers, fine damage had occurred to their lungs. This had happened in small fine airways that are important for bringing in oxygen and taking away waste gases. As a result, their lungs had to work harder.
The extent of the damage rose in proportion to the number of joints that had been smoked.
In statistical terms, each joint that was smoked was the equivalent of smoking between two and a half and five cigarettes in one go for impairing lung efficiency.
The researchers, led by Richard Beasley, said the discovery of this hazard is "of major public health significance" given that cannabis is the most widely-used illegal drug worldwide.
According to the 2006 report by the UN Office on Drugs and Crime, an estimated 160 million people use cannabis, also called marijuana.
Beasley suggests that the reason for cannabis' heavy toll on the lungs stems the fact that joints are usually smoked without a filter and are smoked down as far as possible, which means the smoke is hotter when it arrives in the lungs.
In addition, cannabis users tend to inhale smoke more deeply and then hold their breath in order to get a bigger "high".
On Saturday, a study published in the British journal The Lancet said there was now clear evidence of a link between cannabis smoking and mental ill-health.