Long-term exposures to air pollution.

and tobacco smokers and 92 tobacco smokers with 81 non-smokers. They found a dose-response relationship between cannabis consumption and the degree of airways obstruction and hyperinflation. They estimated that one cannabis joint was equivalent to 2.5 cigarettes for the effect on forced expiratory volume in 1 s/forced vital capacity and to 6 cigarettes for the effect on specific airways conductance. In contrast, there was no association between cannabis use and the prevalence of HRCT-defined emphysema. This study supports the view that cannabis affects airway function and causes obstruction. It is likely that the present results differ from the previous negative studies due to the inclusion of subjects with a relatively high cumulated cannabis consumption (substantial number of joint-years) and because the cannabis cigarettes of today contain more than 10 times as much THC than cigarettes from the 1960s, as has been put forward by the British Lung Foundation (BLF). The BLF also points out that there is a need for further research focusing on the link between cannabis and chronic obstructive pulmonary disease. Yet, the study by Aldington et al also shows the difficulties in conducting such studies. In spite of the fact that the investigators invited a general population sample of 3500 individuals, there were only 19 eligible persons smoking cannabis. Another approach using specific advertising for cannabis smokers therefore had to be employed. However, as the authors point out, this approach is not without problems because many heavy cannabis consumers also smoke other substances, which makes it difficult to isolate the effects of cannabis. In summary, although we know far less about the effect of cannabis on the lung than the effects of tobacco smoking, the study by Aldington et al confirms that cannabis smoking constitutes a substantial hazard to the lung.