What is secondhand cannabis smoke?

Secondhand cannabis smoke is a complex chemical mixture of smoke emitted from burned or combusted cannabis . In addition to cannabinoids like THC and CBD, secondhand cannabis smoke contains fine particulate matter that can be breathed deeply into the lungs.(1) 

Cannabis smoke can also contain leftover pesticides, solvents, mold, heavy metals, and other chemicals from burning or combustion.(2,3)

How does secondhand cannabis smoke compare to secondhand tobacco smoke?

Cannabis smoke contains many of the same constituents and toxins as tobacco smoke,(2) and some toxins have been found in higher quantities in cannabis smoke than in tobacco smoke, including ammonia, hydrogen cyanide, nitric oxide, and some aromatic amines.(2)

What are the potential health risks from exposure to secondhand cannabis smoke?

Although knowledge about the harms of secondhand tobacco smoke is widespread, (4) many people still assume exposure to secondhand cannabis smoke is harmless.(3,5) Burning any plant material releases toxic chemicals and fine particles that can go deep into the lungs and can cause cardiovascular disease, or make existing diseases of the heart and lungs worse. (6–8) Inhaling smoke, in general, is not safe for your lungs and cardiovascular system.(6–8)

We don’t have good studies on the effects of secondhand cannabis smoke in humans. However, a recent study exposed rats to secondhand cannabis smoke in a controlled lab setting and found that one-minute of exposure to secondhand cannabis smoke impacted arteries in a comparable way to tobacco smoke exposure, making it harder for arteries to expand and allow for healthy blood flow.(3) Furthermore, arteries in the rats exposed to cannabis smoke recovered at a slower rate than after exposure to tobacco smoke.(3) These effects occurred even when the cannabis being consumed did not contain cannabinoids (THC or CBD), and even when the cannabis rolling paper was not used. The cannabis used in these studies was free from pesticides and did not contain stems or seeds.(3) Short-term exposure to secondhand cannabis smoke can be harmful to health, and repeated exposure to cannabis smoke may permanently damage the artery walls, which in turn can cause blood clots, heart attack, and even stroke.(9–11)

Secondhand cannabis smoke may also have other health harms. The World Health Organization review of the health effects of cannabis did not find evidence that THC is a cancer-causing agent, but concluded that evidence suggests that cannabis smoke is carcinogenic.(12) The impacts of secondhand exposure to cannabis smoke were not reviewed by the WHO. In 2009, the California Office of Environmental Health Hazard Assessment added cannabis smoke to its Proposition 65 list of carcinogens, noting at least 33 individual constituents present in cannabis smoke that have been classified as carcinogenic.(13)

Can ventilation prevent health harms from secondhand cannabis smoke?

Many of the harmful constituents found in cannabis smoke cannot be eliminated through air ventilation systems or air cleaning technologies. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) states in their standards for ventilation for acceptable indoor air quality that there is no safe level of exposure to secondhand smoke, that cannabis smoke should not be allowed indoors, and that ventilation and other air filtration technologies cannot eliminate all of the health risks caused by cannabis and other smoke.(14)

Can exposure to secondhand cannabis smoke result in a positive drug test?

Evidence indicates that adults exposed to secondhand cannabis smoke under usual conditions would not test above standard cutoffs for cannabis use on a workplace urine test or driving impairment blood test.(15) However, three recent studies found that, in extreme cases of heavy exposure, (e.g., long duration in a confined, unventilated space) nonsmokers had positive tests for THC in oral fluid and blood up to three hours following the exposure.(16–18)

What is thirdhand cannabis smoke?

Thirdhand smoke is the residue from smoke that remains on walls, carpets, furniture, and other surfaces long after the smoker has left the room.  Chemicals in this residue can continue to “off gas” and expose non-users to potentially harmful toxicants, including additional harmful particulate matter that can be inhaled deeply into the lungs, and volatile organic compounds (VOCs). While studies have documented the constituents of thirdhand tobacco smoke,(19) no studies have been conducted assessing the constituents of thirdhand cannabis smoke.  

What do we know about secondhand exposure to vaped cannabis?

Electronic smoking or vaping devices release secondhand aerosol. There have not been any studies on the constituents in or effects of secondhand cannabis aerosol. Studies on vaped nicotine have found that aerosol from electronic nicotine products contains fine particulate matter and other potential toxins that decrease the surrounding air quality and can be harmful to bystanders.(20,21) More research is needed to understand the potential impacts from secondhand cannabis aerosol. (See also the factsheet on Vaping Cannabis .)

What policies are being put in place related to secondhand cannabis smoke exposure in public and in workplaces?

In most of the states that have regulated adult cannabis use (including Washington State), statewide smokefree policies have been amended to include cannabis smoke, or cannabis policies have been written to mirror smokefree tobacco policies that protect bars, restaurants, worksites, and other public places.  However, all states have had regulatory and/or legislative discussions at the state or local level about providing exceptions public consumption of marijuana in certain places, such as cannabis cafes, lounges, or outdoor spaces.

Helpful Resources 


  1. Hiller F, Wilson Jr. F, Mazumder M, Wilson J, Bone R. Concentration and particle size distribution in smoke from marijuana cigarettes with different delta-9-tetrahydrocannabinol content. Fundam Appl Toxicol. 1984;4(3):451-454.
  2. Moir D, Rickert WS, Levasseur G, et al. A comparison of mainstream and sidestream marijuana and tobacco cigarette smoke produced under two machine smoking conditions. Chem Res Toxicol. 2008;21(2):494-502. doi:10.1021/tx700275p.
  3. Wang X, Derakhshandeh R, Liu J, et al. One Minute of Marijuana Secondhand Smoke Exposure Substantially Impairs Vascular Endothelial Function. J Am Hear Assoc. 2016;5(8). doi:10.1161/JAHA.116.003858.
  4. U.S. Department of Health and Human Services. The Health Consequences of Smoking — 50 Years of Progress. A Report of the Surgeon General. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health; 2014. http://www.surgeongeneral.gov/library/reports/50-years-of-progress/full-report.pdf.
  5. Johnston LD, O’Malley P M, Bachman JG, Schulberg HC. Monitoring the Future National Survey Results on Drug Use, 1975-2004: Volume 1, Secondary School Students. Bethesda, MD: National Institute on Drug Abuse; 2005.
  6. Brook R, Rajagopalan S, Pope C, et al. Particulate matter air pollution and cardiovascular disease: an update to the scientific statement from the American Heart Association. Circulation. 2010;121:2331-2378.
  7. Pope C, Burnett R, Krewski D, et al. Cardiovascular mortality and exposure to airborne fine particulate matter and cigarette smoke: shape of the exposure‐response relationship. Circulation. 2009;120:941-948.
  8. O’Toole T, Hellmann J, Wheat L, et al. Episodic exposure to fine particulate air pollution decreases circulating levels of endothelial progenitor cells. Circ Res. 2010;107:200-203.
  9. Yeboah J, Folsom A, Burke G, et al. Predictive value of brachial flow‐mediated dilation for incident cardiovascular events in a population‐based study: the Multi‐Ethnic Study of Atherosclerosis. Circulation. 2009;120:502-509.
  10. Yeboah J, Crouse J, Hsu F, Burke G, Herrington D. Brachial flow‐mediated dilation predicts incident cardiovascular events in older adults: the Cardiovascular Health Study. Circulation. 2007;115:2390-2397.
  11. Yeboah J, Sutton-Tyrrell K, McBurnie M, Burke G, Herrington D, Crouse J. Association between brachial artery reactivity and cardiovascular disease status in an elderly cohort: the Cardiovascular Health Study. Atherosclerosis. 2008;197:768-776.
  12. World Health Organization. The Health and Social Effects of Nonmedical Cannabis Use. Geneva; 2016.
  13. California Enviornmental Protection Agency – Reproductive and Cancer Hazard Assessment Branch Office of Environmental Health Hazard Assessment. Evidence on the Carcinogenicity of Marijuana Smoke.
  14. ASHRAE Standard 62.1-2016: Ventiliation for Acceptable Indoor Air Quality.
  15. Colorado Department of Public Health & Environment. Monitoring Health Concerns Related to Marijuana in Colorado: 2016. Denver; 2016.
  16. Cone E, Bigelow G, Herrmann E, et al. Nonsmoker Exposure to Secondhand Cannabis Smoke. III. Oral Fluid and Blood Drug Concentrations and Corresponding Subjective Effects. J Anal Toxicol. 2015;39(7):497-509.
  17. Herrmann E, Cone E, Mitchell J, et al. Non-smoker exposure to secondhand cannabis smoke II: Effect of room ventilation on the physiological, subjective, and behavioral/cognitive effects. Drug Alcohol Depend. 2015;151:194-202.
  18. Cone E, Bigelow G, Herrmann E, et al. Non-smoker exposure to secondhand cannabis smoke. I. Urine screening and confirmation results. J Anal Toxicol. 2015;39(1):1-12.
  19. Jacob P, Benowitz NL, Destaillats H, et al. Thirdhand Smoke: New Evidence, Challenges, and Future Directions. Chem Res Toxicol. 2017;30(1):270-294.
  20. Fuoco F, Buonanno G, Stabile L, Vigo P. Influential parameters on particle concentration and size distribution in the mainstream of e-cigarettes. Environ Pollut. 2014;184:523-529.
  21. Goniewicz M, Knysak J, Gawron M, et al. Levels of selected carcinogens and toxicants in vapour from electronic cigarettes. Tob Control. 2014;23:133-139.

Citation: Schauer G. Secondhand Exposure to Cannabis Smoke.  Addictions, Drug & Alcohol Institute (ADAI), University of Washington, June 2018. 

This report was produced with support from the Washington State Dedicated Marijuana Fund for research at the University of Washington.