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    Second hand marijuana smoke: Is it harmful?

    Most people are familiar with second-hand smoke as a result of tobacco use, particularly from cigarettes, cigars or nicotine vaping. However, though tobacco and marijuana are different in their active ingredients (nicotine and THC, respectively), the harmful chemicals that remain in the surrounding environment and that can be inhaled by others are similar.   

    Is there a safe level of second-hand marijuana smoke?

    There is no “safe” level of exposure to second-hand smoke for children.1 Exposure to second-hand smoke is especially dangerous in environments where marijuana use is common. This can include areas of the home that are not ventilated with strong airflow or in cars. In fact, teens are known to “hot box” where they sit in a car with the windows up while smoking marijuana to maximize the effects of second-hand smoke. 

    According to one study, marijuana pollutes the air with about the same concentration of particles as tobacco.2 Smoked marijuana (i.e., through joints and blunts) appears to produce the most particles, almost twice as much as other methods, like vaping.3

    What are the side effects of second-hand marijuana smoke?

    Short-term side effects of second-hand smoke produced by marijuana include problems with feeling sedated and minor increases in heart rate. It may also cause problems with working memory which allows people to keep track of what they are doing.

    THC, the psychoactive ingredient in marijuana that causes a “high”, can be detected in blood and urine. This means a child or adult could test positive for marijuana if given a drug screen.

    Long-term effects include increasing the chances of catching a cold, the flu or other viral infections of the lungs. People struggling with allergies, asthma or heart disease may find that second-hand smoke makes their health conditions worse.

    What is thirdhand smoke?

    There is another level to smoke exposure, called thirdhand smoke. It is defined as the particles that settle on household items as a result of smoking. Ammonia, hydrogen cyanide, nitrogen oxides and other chemicals found in cigarette smoke are also found in marijuana smoke. Though one may smoke outside, particles from these chemicals can be brought into the household through clothes, skin, and dust and may react with the air within the home.  

    Research shows that for indoor thirdhand smoke, the primary way for particles to be ingested by nonsmokers is through the mouth. This is especially troublesome as thirdhand smoke settles on nearby objects, and babies who are teething will place objects in their mouths and may be exposed to the toxins within thirdhand smoke. 

    Marijuana-produced second-hand and thirdhand smoke may be reduced through better ventilation systems. In the case of shared living space, efforts to abstain from marijuana use, especially with children in the home, are best.   


    1. Posis, A., Bellettiere, J., Liles, S., Alcaraz, J., Nguyen, B., Berardi, V., Klepeis, N. E., Hughes, S. C., Wu, T., & Hovell, M. F. (2019). Indoor cannabis smoke and children’s health. Preventive Medicine Reports, 14.
    2. American Lung Association. Marijuana smoke. Available at:
    3. Zhao, T., Cheng, K., Ott, W.R., Wallace, L., & Hildemann, L.M. (2020). Characteristics of secondhand cannabis smoke from common smoking methods: Calibration factor, emission rate, and particle removal rate. Atmospheric Environment, 242, 117731.
    4. Herrmann, E. S., Cone, E. J., Mitchell, J. M., Bigelow, G. E., LoDico, C., Flegel, R., & Vandrey, R. (2015). Non-smoker exposure to secondhand cannabis smoke II: Effect of room ventilation on the physiological, subjective, and behavioral/cognitive effects. Drug and Alcohol Dependence, 151, 194–202.
    5. Johnson, A. B., Wang, G. S., Wilson, K., Cline, D. M., Craven, T. E., Slaven, S., Raghavan, V., & Mistry, R. D. (2021). Association between secondhand marijuana smoke and respiratory infections in children. Pediatric Research,
    6. Centers for Disease Control and Prevention. (2007). Best practices for comprehensive tobacco control programs. US Department of Health and Human Services, CDC. Available at:
    7. Burton, A. (2011). Does the Smoke Ever Really Clear? Thirdhand smoke exposure raises new concerns. Environmental Health Perspectives, 119(2), A70–A74.
    8. Matt G. E., Quintana P. J. E., Hovell M. F., Bernert J. T., Song S., Novianti N., Juarez T., Floro J., Gehrman C., Garcia M., & Larson S. (2004). Households contaminated by environmental tobacco smoke: sources of infant exposures. Tobacco Control, 13(1), 29–37.
    9. Yeh K.,d Li L., Wania F., & Abbatt P.D. J. (2022). Thirdhand smoke from tobacco, e-cigarettes, cannabis, methamphetamine and cocaine: Partitioning, reactive fate, and human exposure in indoor environments. Environment International, 160(107063).