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More people in apartment buildings and condominiums reported exposure to cannabis smoke than tobacco one year before legalization, a new University of Toronto study says. 

The ��˾��ֱ�� public-health researchers based their findings on a telephone survey of Ontario adults by the Centre for Addiction and Mental Health in 2017. Extrapolating from the CAMH results, the researchers suggest about 827,000 people in multi-unit dwellings were exposed to cannabis smoke from hallways, other units or elsewhere in their building – as opposed to about 728,000 who smelled tobacco smoke.

“I was surprised there were more reports of cannabis than tobacco,” says co-author Michael Chaiton (pictured left), an associate professor in the Dalla Lana School of Public Health. “We’ve known anecdotally there had been increasing calls and complaints about cannabis smoke, but we hadn’t been able to quantify it until now.”
 

Chaiton hopes the results will contribute to the debate over whether to regulate cannabis like alcohol, by forbidding its consumption in public spaces, or like tobacco, by prohibiting its use in indoor public spaces or near buildings.

He and his co-authors Alanna Chu and Pamela Kaufman, an assistant professor at Dalla Lana, in the International Journal of Environmental Research and Public Health. 

The team has been tracking exposure to tobacco smoke in residential buildings for many years, and recently they have become increasingly concerned about cannabis second-hand smoke.

Despite the surprising results, Chaiton says it’s very possible that the study underreported cannabis exposure, since the authors found people who consume cannabis themselves were more likely to report smelling it – suggesting that not everybody can identify cannabis by its pungent scent.  

The study, the first to examine the prevalence of involuntary cannabis exposure in residential buildings, also showed that people of lower income, who more often live in multi-unit buildings, are at higher risk of exposure to second-hand smoke and any related health effects from other people’s cannabis use. 

But those health effects are not yet fully clear.

Third-hand smoke, the residue from smoke that clings to walls and furniture or settles in dust, is a particular concern for tobacco because nicotine is sticky and accumulates easily on surfaces like drapery, clothing and furniture.

But Chaiton says researchers don’t yet know whether cannabis travels the same way through apartment buildings, including ventilation systems, or how it contributes to third-hand smoke contamination. 

The next step is to determine whether exposure reports have increased since the legalization of cannabis in October 2018.

“If there are more people smoking cannabis or smoking more freely, indoor use in buildings may increase,” Chaiton says. “But, on the other hand, people may be more willing to go outside, lessening second-hand exposure indoors. As well, people may be less likely to report it to surveyors because it’s legal – or they may be more likely to complain if they have less fear of repercussions.”

Funding for the study was provided by the Canadian Institutes for Health Research and the Ontario Ministry of Health and Long-Term Care.