Marijuana joints produce 3.5 times as much secondhand smoke as a Marlboro

Tobacco cigarettes are such strong sources of indoor air pollution that the only way to get acceptable indoor air quality is to make spaces smokefree.

Now, Wayne Ott and colleagues have done a real-world study of the levels of pollution caused by smoking a joint (or a bong or vaping cannabis) and found that marijuana products are as or even more polluting than tobacco cigarettes.

In their paper, “Measuring indoor fine particle concentrations, emission rates, and decay
rates from cannabis use in a residence,” Ott and colleagues had an experienced user smoke a joint or other cannabis products as well as a Marlboro tobacco cigarette using the same protocol in an apartment and carefully measured the pollution levels (as very fine particulate matter in the air, PM2.5). In all cases the smoker took one puff a minute for three minutes, then put the product out. Ott and colleagues measured how high the pollution levels got and how long it persisted.

The joints were the most polluting — 3.5 times that of a Marlboro (figure above), with the other cannabis products producing as much or more pollution than the cigarette.

Their finding that vaping cannabis was as polluting as a cigarette (but hung around longer) is particularly striking because, unlike combusted products, e-cigarettes do not product sidestream smoke (the smoke that comes off the lit end of the cigarette when it is idling). All the pollution from the cannabis vapes was exhaled e-cig areosol.

These results have important implications for including cannabis products in clean indoor air laws, including smokefree multiunit housing, because they are such strong pollution sources.

Here is the abstract:

Fifteen states have legalized the sales of recreational marijuana, and California has the largest sales of any state. Cannabis is most often smoked indoors, but few measurements have been made of fine particle mass concentrations produced by secondhand cannabis smoke in indoor settings. We conducted 60 controlled experiments in a 43 m³ room of a residence, measuring PM_2.5 concentrations, emission rates, and decay rates using real-time monitors designed to measure PM_2.5 mass concentrations. We also measured the room’s air exchange rate. During each experiment, an experienced smoker followed an identical puffing protocol on one of four different methods of consuming marijuana: the pre-rolled marijuana joint (24 experiments), the bong with its bowl containing marijuana buds (9 experiments), the glass pipe containing marijuana buds (9 experiments), and the commercially available electronic vaping pen with a cartridge attached containing cannabis vape liquid (9 experiments). For comparison, we used the same puffing protocol to measure the PM_2.5 emissions from Marlboro cigarettes (9 experiments). The results indicated that cannabis joints produced the highest indoor PM_2.5 concentrations and had the largest emission rates, compared with the other cannabis sources. The average PM_2.5 emission rate of the 24 cannabis joints (7.8 mg/min) was 3.5 times the average emission rate of the Marlboro cigarettes (2.2 mg/min). The average emission rate of the cannabis bong was 67% that of the joint; the glass pipe’s emission rate was 54% that of the joint, and the vaping pen’s emission rate was 44% that of the joint. The differences compared to the joint were statistically significant.

I do have one criticism: They did pairwise statistical tests without controlling for multiple comparisons, so their reported p values are too small. But, looking at the reported values, the key finding that joints (and the bong) are significantly more polluting than the Marlboro should hold even after fixing this error. There are probably not significant differences between the glass pipe, e-cig, and Marlboro are probably not significantly different.

The full citation is Wayne R. Ott, Tongke Zhao, Kai-Chung Cheng, Lance A. Wallace, Lynn M. Hildemann, Measuring indoor fine particle concentrations, emission rates, and decay rates from cannabis use in a residence. Atmospheric Environment: X 2021; 10: 100106. https://doi.org/10.1016/j.aeaoa.2021.100106. It is available here.