Although the author practices in Colorado, the information is likely generalizable. This review clearly reflects the author’s biases, yet its composition was motivated by alarming experience in everyday practice. Discussions of cannabis’ effects are relevant not only to the healthcare system, but to legal, business, environmental, legislative, and other branches within a public health framework. This article does not address those other facets.Many of the previous research studies have focused on cannabis with a much lower THC level limiting applicability to cannabis sold at dispensaries today. Finally, the words “marijuana” and “cannabis” were used interchangeably throughout the article. This was done to maintain the wording from the studies cited consistent with their original language. No difference should be implied with the alternating use of these terms. While the harmful health effects of secondhand tobacco smoke exposure and the benefits of smoke free policies are well known1, there is little known about the pollutants that arise from cannabis use. Cannabis smoke is chemically similar to tobacco smoke2, 3 and PM2.5 exposure is a known cause of cardiopulmonary and metabolic disease4, 5. However, some communities allow cannabis smoking as an exception to existing smoke free laws and many more communities are considering similar legislation. To assess the air pollution associated with cannabis use, we measured airborne PM2.5, cannabinoid and nicotine concentrations in a cannabis store in California that permitted smoking, vaporizing, and dabbing in an on-site consumption area 6. We chose a dispensary that was noisy enough to mask the sound of our air sampling instruments. Nine visits were made; four in 2018 and five in 2019. All experiments were conducted between 15:00 – 19:00 . Airborne particles 2.5 µm in diameter and smaller were measured in the marijuana grow system consumption lounge using laser photometers. PM2.5 samples were collected on filters to quantify nicotine and cannabinoids.

Background samples were collected within 100 meters of the dispensary; either outdoors, in a pedestrian plaza, or indoors, in a coffee shop. The instruments were carried in backpacks and the experiments were conducted without permission from the businesses. The dispensary enforced a 30-minute limit in the lounge during most of our visits. When possible, the backpacks were relayed between researchers to collect multiple readings in a single experiment. The instruments were turned off each time one researcher left the lounge and turned back on after the next researcher entered the lounge. Experiment durations ranged from 32 to 152 minutes.PM2.5 concentrations were measured in real time using laser photometers operated according to the manufacturer’s instructions . The instruments were turned off prior to entering the dispensary, because bags were checked at the door. Photometers were paired with air pumps in each backpack, to collect samples on filters to measure cannabinoids and nicotine. After entering the smoking lounge, the researchers chose a seat that was not near the entrance or the emergency exit. The instruments were then turned on and the backpacks were placed on the tables with the sampling inlets located at the shoulder level of seated patrons. The instruments were switched off before the researchers left the lounge. For more details on the air pumps, filters, filter cassettes and flow calibrations, please refer to the Supplementary Materials. We used impactors and cyclones to exclude particles over 2.5 μm in diameter from all samples and measurements. Cannabinoid content of the particulate material on the front filters was quantified at the Organic Analytical Laboratory of the Desert Research Institute , as described in the Supplementary Materials. The limits of detection were 1.85, 0.67, and 1.90 ng per filter for THC, CBD, and CBN, respectively. The limits of quantitation per sample varied, depending on the volume of air sampled: 0. 995 – 11.6 ng/m3 for THC, 0.36-4.2 ng/m3 for CBD and 1.0 – 12 ng/m3 for CBN. Nicotine was quantified by gas chromatography as described previously 7, modified by using a capillary column and using 5-methylnicotine as the internal standard. The LOQ ranged from 16 ng/m3 to 29 ng/m3. Cannabis consumption behavior was recorded to identify and count emitting sources. Researchers also observed and counted the occupants in the lounge. On separate counts, the perceived gender , and role of the people in the lounge were tallied. People were counted as employees only if they were wearing dispensary ID and clearly working. Employees on break were counted as customers. Researchers and employees were included in the occupancy counts.

Data from routine cigarette smoke generation experiments8 were used to derive calibration factors for the photometers. Gravimetric samples were collected and weighed before and after each cigarette smoke experiment. We plotted the unadjusted average photometer data against the gravimetric data and forced the line through zero. The slope was the calibration factor. The field photometric data were multiplied by the calibration factors to yield the final particle concentration values. Over nine visits and 10 hours of measurements, the average PM2.5 concentration in the dispensary smoking lounge was 840 µg/m3, with a standard deviation of 674 µg/m3 . During the four visits conducted before the new ventilation system was installed, the average PM2.5 was 905 µg/m3, with a standard deviation of 728 µg/m3. During the five visits conducted after the ventilation system was installed, the average PM2.5 concentration was 795 µg/m3, with a standard deviation of 636 µg/m3. To determine whether the PM2.5 concentration was significantly lower after the installation of the new ventilation system, we performed an overall test for coincidence of two regression lines. The p value was 0.16, indicating that the 12.2 % decrease in PM2.5 concentration was not statistically significant. The number of cannabis articles that were actively emitting smoke or other aerosols was counted at least twice per visit, except during the second relay on 3/8/2019 when source counts were not recorded. To assess the relationship between PM2.5 concentration and average number of sources per count, we performed a regression. The R2 was 0.100, with a p-value of 0.219, indicating that there was not a statistically significant relationship between PM2.5 concentrations and the average number of cannabis articles emitting aerosols . The relationship between PM2.5 concentrations and the number of occupants was also tested and found not significant . The occupant-normalized PM2.5 concentration ranged from 18-52 µg/m3 * persons.In descending order of prevalence, the following cannabis products and modes of use were observed at the dispensary: cannabis in rolling paper , cannabis in water pipes , cannabis concentrates consumed by dabbing, cannabis in hand pipes, cannabis vape pens and blunts. Overall, 91% of the cannabis consumed in the lounge was smoked, 5% was consumed by dabbing and 4% was consumed by using a vape pen. 71% of patrons smoked joints . We did not observe any tobacco use. At any given time, approximately 43% of all people within the lounge were actively using a cannabis product. The patrons were 69% male and 31% female . Employees were observed working in the lounge at every visit ; supervising, loaning out equipment, emptying ash trays, cleaning the tables and interacting with customers. The PM2.5 concentrations we observed are similar to the highest published concentrations measured in public places where people were smoking tobacco 9.

Because the nicotine concentrations in the dispensary were below 0.10 µg/m3, the cannabinoid concentrations were high and the background particle concentrations outdoors and in a nearby business were low, we believe that nearly all the PM2.5 measured in the dispensary derived from cannabis consumption. Our prior study of a lounge in a dispensary where only non-combustible methods of consumption were permitted, found median PM2.5 concentrations during peak business hours almost 10- fold lower than the medians observed in this dispensary 10. The average concentrations in this study are similar to the maximum PM2.5 concentrations that Ott et al. observed after smoking a single joint in a small, unventilated bedroom11. Unlike tobacco cigarettes, cannabis vertical farming does not come in a single, standardized portion, and people do not always consume the same amount per session. This variation may explain why we did not find a correlation between PM2.5 concentrations and the average number of aerosol-emitting sources or the number of occupants. Our finding that the installation of a new ventilation system did not cause a large or statistically significant decrease in PM2.5 concentrations suggests that it was not effective in reducing pollutant concentrations. Prior research has shown that ventilation alone is not sufficient to control PM2.5 from tobacco cigarettes 12. The psychoactive effects of THC are typically felt at a dose of 2-10 mg for an adult of average size and tolerance. This means that psychoactive effects were unlikely at the concentrations in the dispensary. These findings agree with Herrmann et al. who found subjective psychoactive effects in nonsmokers after one hour of secondhand exposure in a sealed and unventilated 12 m3 chamber during which 14.4 grams of cannabis with 11% THC were smoked, but not after exposure to a similar amount of cannabis smoke when the chamber was ventilated14. Cannabinol is an oxidation product of THC that can form during combustion or storage15, so CBN concentrations one tenth those of THC are plausible. The low concentrations of CBD suggest that THC-dominant products were in use. Average nicotine concentrations in businesses with active smoking indoors range from 0.6 to 76 µg/m39. The fact that nicotine samples from the dispensary were all below the limits of quantitation validates our observation that no one was smoking tobacco in the dispensary. Because the concentrations of PM2.5 were so high, it is likely that the employees of this dispensary were at risk of health effects from secondhand cannabis smoke exposure. Although cannabis has a number of scientifically-validated and positive medicinal effects16, cannabis smoke contains carcinogens and PM2.52, 3. If an employee was exposed for two hours at 840 µg/m3 and for 22 hours at 4 µg/m3, their 24-hour average exposure would be 72 µg/m3. The US Environmental Protection Agency air quality index for this amount of air pollution is “Unhealthy”17 and the anticipated health effects are “Increased aggravation of heart or lung disease and premature mortality in persons with cardiopulmonary disease and the elderly; increased respiratory effects in general population.”

Research on bar workers, comparing their respiratory health before and after tobacco smoking bans, found that bans improved respiratory symptoms18, 19 and lung function19 in both smokers and nonsmokers. This suggests that dispensary employees may incur health risks from their exposure to secondhand cannabis smoke at work, even if they are smokers of cannabis. It is also possible that the secondhand exposure may increase nasal congestion and diminish cardiovascular function in the dispensary customers. Research from our laboratory has shown that a 30-minute exposure to secondhand cigarette smoke, at 1,000 µg/m3 PM2.5, can increase nasal congestion in healthy, young nonsmokers20. Endothelial dysfunction, the loss of the ability of the cells lining the arteries to respond to normal increases in blood flow by dilating, is a risk factor for myocardial infarction21. Multiple studies have shown that short exposures to secondhand tobacco smoke at concentrations well below those seen in this dispensary cause endothelial dysfunction in healthy, young nonsmokers 22-24 and in healthy young smokers25, 26. One study has shown that exposures to cannabis smoke cause endothelial dysfunction in animals 27.We studied a business that was well-patronized and the PM2.5 and cannabinoid concentrations we measured may be higher than the concentrations in other dispensaries that allow smoking. We performed these experiments during peak business hours. Four of the experiments were conducted on Fridays, three on Thursdays, one on a Wednesday and one on a Tuesday. At other times of day or days of the week, there may have been less activity and lower concentrations of PM2.5. However, our experiments were conducted over 11 months, were not scheduled to coincide with special events and showed consistently high PM2.5 concentrations at all visits , so we believe our findings represent conditions at this business accurately. The Sidepak laser photometer calibration factor of 0.28-0.32 for secondhand cigarette smoke is well established 28-30. The calibration factors we derived are slightly lower and may reflect individual variations in our instruments. After we did this research, Zhao et al. published Sidepak calibration factors of 0.39 for joints, 0.40 for cannabis smoked in a bong and 0.31 for cannabis smoked in a small, glass pipe.