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Exposure to Marijuana Smoking: the Effect of Proximity

Institution: Stanford University
Investigator(s): Lynn Hildemann, Ph.D.
Award Cycle: 2019 (Cycle 28) Grant #: 28IR-0062 Award: $1,137,361
Subject Area: Environmental Exposure/Toxicology
Award Type: High Impact Research Project Award

Initial Award Abstract

With the recent legalization of marijuana in California, marijuana smoke is now an emerging air pollution source of concern, both indoors and outdoors.  Previous studies have shown that in close proximity to a cigarette smoker, exposure to secondhand smoke can be five to ten times as high as farther away – this “proximity effect” will also be an issue for individuals who are in proximity to marijuana smoke. If smoking takes place outdoors, or in indoor locations with open windows, others nearby could be inadvertently exposed.  If exposure is significant, then important policy issues will need to be addressed.   

There are different methods of smoking marijuana (joint smoking, bong smoking, and vaping), as well as marijuana/tobacco mixtures. Typical smoking behavior (e.g., the frequency and volume of puffs, the duration of breath holding) varies with both the method of smoking and what is being smoked. These variations will lead to differences in the concentrations and persistence of the individual components present in exhaled marijuana smoke, affecting the levels of secondhand exposure nearby.  

This study aims to characterize the levels of secondhand exposure in close proximity to marijuana smoke, focusing on two emission components with significant health implications: fine airborne particles (PM2.5), which can cause measurable respiratory problems in sensitive individuals, and tetrahydrocannabinol (THC), the main psychoactive component in marijuana.  PM2.5 will be measured with a high time resolution monitor (multiple measurements every minute), at different distances from the smoker, for different smoking conditions, smoking materials, and locations.  For THC, besides measuring the airborne levels at different proximities, this study will also analyze cheek swabs for THC, both for smokers and for non-smokers located nearby, as an independent measure of cumulative exposure.  The proximity effect will be assessed over a range of legally-allowed smoking locations, such as living rooms, garages, backyards, and parked cars.

The results of this research should provide critically needed exposure data for assessing the health risks of marijuana smoking to non-smoking neighbors and bystanders, in real-world situations.