Summary

While there is a wealth of research into the health impact of tobacco smoking, there is relatively little on the effects of cannabis smoking.
Research investigating whether the inhalation of cannabis smoke causes damage to the lungs and airways focuses on whether this effect is independent of the effects of tobacco smoke or not.
In general, the studies indicate that there is an increased negative health impact on those who smoke cannabis compared to those who do not smoke at all. When cannabis is smoked together with tobacco then the effects are additive.
However, what is not clear is whether it is the addition of the cannabis or the tobacco which is more harmful or whether this is the result of the combined effects of equally harmful substances.
Some key findings emerge from the research:
• The cannabis smoked today is much more potent that that smoked in the 1960s. The average cannabis cigarette smoked in the 1960s contained
about 10mg of tetrahydrocanabinol (THC), the ingredient which accounts for the psychoactive properties of cannabis, compared to 150mg of THC today. This means that longitudinal studies carried out in the 1960s and 1970s may not be
indicative of the effects of cannabis cigarettes
smoked today.
• Studies comparing the clinical effects of habitual cannabis smokers versus nonsmokers demonstrate a significantly higher prevalence of chronic and acute respiratory symptoms such as chronic cough and sputum production, wheeze and acute bronchitis episodes.
• 3-4 Cannabis cigarettes a day are associated with the same evidence of acute and chronic bronchitis and the same degree of damage to the
bronchial mucosa as 20 or more tobacco cigarettes a day.
• Cannabis tends to be smoked in a way which increases the puff volume by two-thirds and depth of inhalation by one-third. There is an
average fourfold longer breath-holding time with cannabis than with tobacco. This means that there is a greater respiratory burden of carbon
monoxide and smoke particulates such as tar than when smoking a similar quantity of tobacco.
• Cannabis smoking is likely to weaken the immune system. Infections of the lung are due to a combination of smoking-related damage to the
cells lining the bronchial passage (the fine hair-like projection on these cells filter out inhaled microorganisms)and impairment of the principal immune cells in the small air sacs caused by cannabis.
• The evidence concerning a possible link between cannabis smoking and Chronic Obstructive Pulmonary Disease (COPD) has not
yet been conclusively established. A number of studies indicate a causal relationship between the two whereas others contradict these findings.
• Research linking cannabis smoking to the development of respiratory cancer exists although there have also been conflicting findings. Not
only does the tar in a cannabis cigarette contain many of the same known carcinogens as tobacco smoke but the concentrations of these are up to 50% higher in the smoke of a cannabis cigarette. It also deposits four times as much tar on the respiratory tract as an unfiltered cigarette of the same weight. Smokers of cannabis and tobacco have shown a greater increase in cellular abnormalities indicating a cumulative effect of smoking both.
• The THC in cannabis has been shown to have a short term bronchodilator effect. This has lead to suggestions that THC may have therapeutic benefits in asthma. However, the noxious gases, chronic airway irritation or malignancy after long term use associated with smoking would seem likely to negate these benefits.

Recommendations
From a clinical perspective the main effects of smoking cannabis on the lungs are increased risk of pulmonary infections and respiratory cancers. Benzpyrene, a known constituent of the tar of cannabis cigarettes has been shown to promote alterations in one of the most common tumour suppressor genes, p53, hence facilitating the development of respiratory cancer. Gene p53 is
thought to play a role in 75% of all lung cancers. The British Lung Foundation recommends a public health education campaign aimed at young people to ensure that they are fully aware of the increased risk of pulmonary infections and respiratory cancers associated with cannabis smoking. The increased potency of the cannabis smoked today compared to the cannabis smoked twentythirty years ago suggests that earlier studies may underestimate the effects of cannabis smoking. In addition the lack of conclusive evidence concerning
the link between cannabis smoking and Chronic Obstructive Pulmonary Disease (COPD) underlines the need for further research.
The British Lung Foundation recommends that further research is undertaken to take into account the increased potency of today’s cannabis and to establish what link (if any) there is between COPD and cannabissmoking.

Source: British Lung Foundation Report ‘A Smoking Gun’ 2007