New Zealand researchers conducted a study in which they investigated tobacco and cannabis impacts on lungs. They found that smoking a single joint of cannabis had the same impact on breathing capacity as up to five cigarettes.

The research involved 339 volunteers, who were divided into four groups – cannabis only smokers, tobacco only smokers, those who smoked both, and those who smoked neither.

The cannabis smokers were those volunteers who had smoked the equivalent of at least one joint a day for the previous five years and tobacco smokers who had smoked the equivalent of a packet of cigarettes a day for at least a year. The participants were then subjected to computed tomography (CT) to get images of their lungs and were also given tests to assess their airflow.

Tobacco smokers - both tobacco-only and combined users had the most severely damaged lungs - they were the only volunteers who had emphysema, a degenerative and crippling lung disease. Cannabis smokers on the other hand had lighter symptoms like wheezing, coughing, chest tightness and phlegm, which tobacco smokers also had.

However, cannabis smokers were also found fine damage that had occurred to their lungs. This had happened in small fine airways that are important for bringing in oxygen and taking away waste gases. The extent of the damage was relative to the number of joints smoked.

In statistical terms this means that each joint smoked is the equivalent of smoking between two and a half and five cigarettes in one go for impairing lung efficiency.

Some of the reasons for cannabis's heavy toll on the lungs is that fact that it is usually smoked without a filter and down as far as possible with smoke being hotter when it arrives in the lungs. Also, cannabis smoke is inhaled more deeply and longer in order to get a bigger "high".