Health effects of exposure to second- and third-hand marijuana smoke: a systematic review

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Recreational marijuana has been legalized in 11 jurisdictions; Canada will legalize marijuana by July 2018. With this changing landscape, there is a need to understand the public health risks associated with marijuana to support patient-care provider conversations, harm-reduction measures and evidence-informed policy. The objective of this work was to summarize the health effects of exposure to second- and third-hand marijuana smoke. In this systematic review, we searched 6 databases from inception to October 2017. Abstract and full-text review was conducted in duplicate. Studies were included if they were human, in vivo or in vitro studies with more than 1 case reported in English or French, and reported original, quantitative data. Three outcomes were extracted: 1) cannabinoids and cannabinoid metabolites in bodily fluids, 2) self-reported psychoactive effects and 3) eye irritation and discomfort. Of the 1701 abstracts identified, 60 proceeded to full-text review; the final data set contained 15 articles. All of the included studies were of good to poor quality as assessed with the Downs and Black checklist. There is evidence of a direct relation between the tetrahydrocannabinol content of marijuana and effects on those passively exposed. This relation is mediated by several environmental factors including the amount of smoke, ventilation, air volume, number of marijuana cigarettes lit and number of smokers present. No evidence was identified assessing exposure to third-hand marijuana smoke or the health effects of long-term exposure. Exposure to second-hand marijuana smoke leads to cannabinoid metabolites in bodily fluids, and people experience psychoactive effects after such exposure. Alignment of tobacco and marijuana smoking bylaws may result in the most effective public policies. More research is required to understand the impact of exposure to third-hand smoke and the health effects of long-term exposure to second-hand smoke.