What Happens When You Breathe in Someone Else's Marijuana Smoke | Research

The takeaway

Secondhand marijuana smoke exposure leads to detectable cannabinoid metabolites in bodily fluids and can produce psychoactive effects, with ventilation and THC content as key factors.

Read this if you are concerned about the effects of being around marijuana smoke, whether at home, at work, or in public.

Secondhand marijuana smoke produced detectable THC metabolites in non-smokers' blood and urine

What the researchers found

This systematic review examined 15 studies on the health effects of secondhand marijuana smoke exposure, finding three consistent outcomes.

First, exposure to secondhand marijuana smoke produces detectable levels of cannabinoid metabolites (THC breakdown products) in the blood and urine of non-smokers. Second, non-smokers reported psychoactive effects after exposure. Third, eye irritation and discomfort were common complaints.

The strength of these effects was directly related to the THC content of the marijuana being smoked and was modulated by environmental factors: the amount of smoke produced, room ventilation, air volume, number of joints lit simultaneously, and number of smokers present.

No research was found on third-hand marijuana smoke (residue deposited on surfaces) or the long-term health effects of chronic secondhand exposure.

Why it matters

As marijuana legalization expands, secondhand smoke becomes a public health policy question analogous to tobacco. This review provides the evidence base for policies around where marijuana can be smoked, with implications for shared housing, public spaces, and workplace regulations.

The numbers in context

1,701 abstracts screened; 60 full-text reviewed; 15 studies included. Studies ranged from good to poor quality. THC content and ventilation were the primary environmental mediators of exposure effects.

How the study worked

Systematic review searching 6 databases from inception to October 2017. Abstract and full-text review were conducted in duplicate. Included studies were human, in vivo, or in vitro with more than one case, published in English or French, reporting original quantitative data. Study quality was assessed using the Downs and Black checklist. 15 studies met inclusion criteria from 1,701 abstracts.

What this study cannot tell us

All included studies were rated good to poor quality. No evidence was found on long-term exposure effects or third-hand smoke. The 15 included studies used varied methodologies and exposure conditions, making direct comparison difficult.

How to read the evidence

Systematic review with duplicate screening and quality assessment. Moderate because included studies varied in quality and no long-term data were available.

When this study was published

Published in 2017, searching databases through October 2017.

The bigger picture

The authors recommend aligning marijuana and tobacco smoking bylaws as the most effective policy approach. This is notable because it implies marijuana secondhand smoke should be treated with the same public health seriousness as tobacco secondhand smoke, even though the specific health risks may differ.

Questions still open

Does chronic secondhand marijuana smoke exposure carry cardiovascular or respiratory risks similar to secondhand tobacco? What THC exposure levels from secondhand smoke could cause a positive drug test? Are there safe ventilation standards for marijuana smoking spaces?

Common questions

Can you get high from secondhand marijuana smoke?

This review found that non-smokers can experience psychoactive effects from secondhand marijuana smoke exposure. The effects depend on THC content, ventilation, and how much smoke is present in the environment.

Can secondhand marijuana smoke cause a positive drug test?

The review confirmed that secondhand exposure leads to detectable cannabinoid metabolites in blood and urine. Whether levels would trigger a standard drug test depends on the extent and duration of exposure and the test's sensitivity threshold.

Read the original research

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

CMAJ open, 5(4), E814-E822

View on PubMed ↗Publisher’s paper ↗

Citation

Holitzki, Hannah; Dowsett, Laura E; Spackman, Eldon; Noseworthy, Tom; Clement, Fiona. (2017). Health effects of exposure to second- and third-hand marijuana smoke: a systematic review.. CMAJ open, 5(4), E814-E822. https://doi.org/10.9778/cmajo.20170112