Smoke-free policies in multi-unit housing “should protect residents from involuntary exposure to all types of second-hand smoke.”

A new study estimates about 17 per cent of Canadians living in detached single-family homes and multi-unit housing (MUH) in 2019 were exposed to second-hand cannabis smoke (SHCS).

That proved better and worse, respectively, than U.S. residents living in legal and illegal cannabis states, investigators note in the pre-proof study published online this week in the peer-reviewed journal, Preventive Medicine.

Overall, Canadian researchers found that 16.9 per cent of Canadian residents had been exposed to SHCS at least once in the previous month compared to 20.6 per cent of those in legal U.S. states and 15.5 per cent of those in U.S. states where cannabis remains illegal. The study includes smoke that originates in another unit or outdoors.

To reach those percentages, investigators from the University of Waterloo, the University of Toronto, the Ontario Tobacco Research Unit (OTRU) and the Centre for Addiction and Mental Health considered data from the 2019 International Cannabis Policy Study. Their estimates were based on the prevalence of self-reported exposure to SHCS at home (33,024) and self-reported SHCS incursions into MUH, which includes smoke from another unit or from outdoors (15,634).

Broken down by type of unit, 25.7 per cent of Canadian MUH residents reported at least monthly SHCS incursions, slightly less than the 26.6 per cent from U.S. legal states, but more than the 20.1 per cent from U.S. illegal states.

Research posted by OTRU in 2019 noted that 6.6 per cent and 7.5 per cent of respondents reported being exposed to environmental tobacco smoke and environmental cannabis smoke.

“Sociodemographic factors associated with incursions suggested MUH residents reporting incursions lived in qualitatively different MUH than those not reporting incursions,” authors of the latest study write.

The 2019 OTRU study reached a similar conclusion.

“Exposure correlates were primarily associated with characteristics of those who lived in MUH who tend to be members of more vulnerable populations,” authors noted.

Whether adult-use cannabis is legal or not, investigators involved in the latest study contend that “smoke-free policies in MUH should protect residents from involuntary exposure to all types of second-hand smoke.”

In March of 2021, U.S. researchers released study results indicating adults in legal states who also have children living at home more commonly use weed compared to states with no legal use. Legalization for recreational and medical use were both linked with a significantly higher prevalence of past-month and daily cannabis use, study authors reported.

With more U.S. states legalizing adult-use cannabis, “educational efforts to reduce children’s exposure to second-hand cannabis smoke may be essential to protect children in this era of new or rapidly shifting cannabis policy,” lead investigator Renee Goodwin, Ph.D. of the Columbia Mailman School of Public Health noted at the time.

Another study published in 2021 suggested children exposed to SHCS from parents may experience viral respiratory infections, such as the common cold, more frequently than children whose parents do not consume weed.

A study review in the Canadian Medical Association Journal five years ago concluded, “there is evidence of a direct relation between the THC content of marijuana and effects on those passively exposed.”

That said, the 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.”

Study authors made clear they did not identify any evidence “assessing exposure to third-hand marijuana smoke or the health effects of long-term exposure.”

The Centers for Disease Control and Prevention reports the known risks of second-hand tobacco smoke exposure raises questions about whether or not SHCS causes similar health risks.