Cannabis secondhand smoke: the short answer
Secondhand cannabis smoke is the mix of smoke exhaled by the person smoking and the smoke that comes off the burning end of a joint, blunt, pipe, bong, or hookah. Burning cannabis flower releases fine particulate matter (PM2.5), carbon monoxide, tar, ammonia, hydrogen cyanide, benzene, formaldehyde, and polycyclic aromatic hydrocarbons. Tobacco smoke carries the same chemicals. THC and other cannabinoids travel in the smoke, but a bystander takes in far less than the person smoking.
The EPA 24-hour air quality limit for PM2.5 is 35 micrograms per cubic meter. Studies of rooms where cannabis is smoked report PM2.5 levels above that number, often by a wide margin, and above outdoor levels on a bad air day.
Short-term effects on bystanders
Common reports include eye and throat irritation, cough, headache, and nausea. THC shows up in the urine of nonsmokers who spend time in rooms with heavy cannabis smoke. Intoxication from secondhand smoke at room concentrations is not documented well in humans; the doses measured are low. One 2016 study in rats found that one minute of exposure to secondhand cannabis smoke impaired the lining of blood vessels, a result the authors compared to tobacco smoke. Human data on long-term cardiovascular and respiratory effects are limited.
Children, pregnancy, and pets
THC crosses the placenta. ACOG advises against cannabis use during pregnancy, including smoked forms. Children in homes where cannabis is smoked indoors have THC metabolites in their urine in several published studies. Edible cannabis and vape cartridges left out are a poisoning risk for children and pets because of the THC dose, not the smoke.
Thirdhand smoke
Tar and cannabinoids settle on walls, carpet, furniture, and clothing. The residue, called thirdhand smoke, can be stirred back into the air and can react with ozone to form new particles. Washing surfaces and fabric lowers the load. Paint and carpet pad hold more than hard surfaces.
Ventilation, filters, and the limits
An open window does not clear cannabis smoke. ASHRAE guidance for smoke removal calls for exhaust at the source plus dilution with outdoor air, and no dilution rate has been shown to remove all secondhand smoke. A HEPA filter with a carbon prefilter removes particles and some gases. Neither filtration nor ventilation brings exposure to zero. The CDC position on secondhand tobacco smoke, that no safe level exists, is cited by health agencies as a reason to keep cannabis smoke outdoors.
Rules in stores and lounges
Most U.S. cannabis stores do not allow smoking or vaping inside. A small number of states license on-site consumption lounges, among them Nevada, California, Colorado, Illinois, and New York. Lounges operate under ventilation and signage requirements set by the state and the local jurisdiction. At a store, expect display-only product and a designated outdoor area, if one exists.
Rules at home and in rentals
California lists cannabis (marijuana) smoke on the Proposition 65 list of chemicals known to cause cancer, effective January 3, 2020. Landlords in several states, including California and Washington, may ban smoking cannabis on the property the same way they ban tobacco. Lease language controls, not state cannabis law.
How to lower exposure
- Smoke outdoors, away from doors, windows, and air intakes.
- Do not smoke in a car with passengers.
- Ask guests to smoke outside. Offer a chair on the porch.
- Run a HEPA unit with a carbon prefilter in the room where smoke lingers.
- Wash hands, change outer clothing, and wipe hard surfaces after indoor smoking.
- Keep edibles, cartridges, and concentrates locked and out of reach of children and pets.
What is still unknown
Long-term disease risk from secondhand cannabis smoke is not established. Most research covers tobacco smoke or short exposures. No standard exists for a safe cannabis smoke exposure level, and home air testing methods vary.