Secondhand Cannabis Smoke

Secondhand cannabis smoke is the smoke from burning cannabis that is inhaled by people who are not smoking. Cannabis smoke contains a mix of chemicals. In addition to cannabinoids like THC and CBD, secondhand cannabis smoke contains fine matter that can be breathed deeply into the lungs.¹ It can also contain leftover pesticides, solvents, mold, heavy metals, and other chemicals.^2-3

People can get high from secondhand cannabis smoke

People have reported feeling the effects of cannabis after being exposed to secondhand smoke. Researchers tested this by creating conditions that are like what people might encounter in the real world. They then recorded how people felt after being exposed to secondhand smoke. They also measured physical responses associated with smoking cannabis.

Researchers found that in an unventilated room, THC and other cannabinoids could be found in people’s blood and urine after secondhand exposure, although at lower levels than seen after directly smoking cannabis. People exposed to secondhand smoke in this study reported feeling the effects of cannabis and were somewhat less coordinated when completing certain tasks.⁴ Room ventilation made it much less likely that people would experience the effects of secondhand smoke. In addition to ventilation, the size of the space and how long a person was exposed affected whether they felt the effects of cannabis.

Potential health risks from exposure to secondhand cannabis smoke

Although knowledge about the harm of secondhand tobacco smoke is widespread,⁵ many people still assume secondhand cannabis smoke is harmless.^3,6 Burning any plant material releases toxic chemicals and fine particles that can go deep into the lungs and cause cardiovascular disease or make existing diseases of the heart and lungs worse.^6–8 In general, inhaling smoke is not safe for your lungs and cardiovascular system.^7–9

One study exposed rats to secondhand cannabis smoke in a controlled lab setting. Researchers found that one minute of exposure to secondhand cannabis smoke affected the rats’ arteries in a similar way to tobacco smoke exposure, making it harder for arteries to expand and allow for healthy blood flow.⁴ They also found that arteries exposed to cannabis smoke recovered at a slower rate than arteries exposed to tobacco smoke.

Overall, the study showed that limited exposure to secondhand cannabis smoke can still be harmful, and that repeated exposure to cannabis smoke may permanently damage the artery walls. This in turn can cause blood clots, heart attack, and even stroke.^10–12

Secondhand cannabis smoke may harm health in other ways. The World Health Organization (WHO) review of the health effects of cannabis did not find evidence that THC can cause cancer. They did however find evidence that cannabis smoke itself may cause cancer.¹³

Cannabis smoke is comparable to secondhand tobacco smoke

Cannabis smoke contains many of the same chemicals and toxins as tobacco smoke.² Some toxins have been found in higher quantities in cannabis smoke than in tobacco smoke, including ammonia, hydrogen cyanide, and nitric oxide.²

Ventilation does not fully prevent harm from secondhand cannabis smoke

Many of the harmful chemicals found in cannabis smoke cannot be entirely removed through air ventilation systems or air cleaning technologies. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) states that:

1. There is no safe level of exposure to secondhand smoke.
2. Cannabis smoke should not be allowed indoors.
3. Ventilation and other air filtration technologies cannot eliminate all health risks caused by cannabis and other smoke.¹⁴

However, some studies indicate the ventilation can help to reduce the level of exposure,⁴ making severe harm less likely. It’s best to avoid smoking indoors when possible, and to open windows when you must be inside.

Exposure to secondhand cannabis smoke and drug tests

Studies indicate that adults exposed to secondhand cannabis smoke under usual conditions would not test positive for cannabis use on a workplace urine test or driving impairment blood test.¹⁵ However, some studies found that in extreme cases of heavy exposure (long duration in an unventilated space) nonsmokers had positive tests for THC in oral fluid and blood up to three hours following the exposure.^16–17

Exposure to secondhand cannabis aerosol can be absorbed into the body

Electronic smoking or vaping devices release secondhand aerosol (a fine mist or spray). One study used a computer-simulated person to model secondhand exposure to cannabis vaped indoors.¹⁹ They found THC aerosol was inhaled and absorbed through the skin, although at low levels. Studies on vaped nicotine have found that aerosol from electronic nicotine products contains fine matter and other potential toxins that lower the surrounding air quality and can be harmful to people nearby.^20,21 (Also see factsheet on Vaping Cannabis.)

Thirdhand cannabis smoke may contain harmful toxins

Thirdhand smoke is the residue from smoke that is left on walls, carpets, furniture, and other surfaces long after the smoker is gone. Chemicals in this residue can continue to “off gas” and expose non-users to potentially harmful toxins, including additional harmful traces of matter that can be inhaled deeply into the lungs. Studies have shown that THC from cannabis smoking is most likely to be found in carpet and flooring, increasing risk for toddlers.¹⁸

It is not legal to smoke cannabis in public or in workplaces

In most states that have legalized adult cannabis use (including Washington State), smokefree tobacco policies have been updated to include cannabis smoke.  However, all states have had discussions at the state or local level about allowing public consumption of cannabis in certain places, such as cannabis cafes, lounges, or outdoor spaces.

Helpful Resources

• Secondhand Marijuana Smoke and Kids. Harvard Health Blog, Harvard Medical School
• Protecting Nonsmokers from Secondhand Marijuana Smoke – 2018. American Nonsmokers’ Rights Foundation