What is secondhand cannabis smoke?
Secondhand cannabis smoke is the smoke exhaled by a person who is smoking plus the smoke that rises off the burning joint, bowl, or blunt. It carries THC, fine particles, carbon monoxide, and many of the same combustion byproducts found in tobacco smoke. Anyone sharing the room breathes it.
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What the smoke contains
- THC and other cannabinoids. The amount in the air depends on the product, the amount smoked, and the size of the space.
- Fine particulate matter (PM2.5). Particles 2.5 micrometers and smaller reach the deep lung and pass into the bloodstream.
- Carbon monoxide. A product of incomplete combustion.
- Tar. The sticky residue left after combustion.
- Polycyclic aromatic hydrocarbons. Some of these are known carcinogens.
- Volatile organic compounds such as benzene and toluene, at lower levels than tobacco smoke.
- Aldehydes such as formaldehyde and acrolein, which irritate the airways.
How much THC reaches a non-smoker
A 2016 Johns Hopkins study measured urinary carboxy-THC in six non-smokers who sat in a room where cannabis was smoked. In the unventilated condition, levels rose above the 50 ng/mL cutoff that many labs use to confirm exposure. In the ventilated condition, some samples showed detectable metabolite but below that cutoff. The study was small. It still shows one session can move a non-smoker's urine THC above a test threshold when air does not move.
Short-term health effects
Reports list eye irritation, coughing, wheeze, headache, and nausea. People with asthma can have an attack after brief exposure. Children, pregnant people, and people with heart or lung disease face higher risk because their lungs or circulation handle particles and carbon monoxide with less margin. Long-term health effects of repeated secondhand cannabis smoke are not well studied. Most long-term secondhand data comes from tobacco research.
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Thirdhand residue
Combustion residue settles on walls, carpet, upholstery, car seats, and clothing. THC and combustion markers stay on surfaces for days. Children who crawl on floors and put objects in their mouths contact these surfaces. Wiping hard surfaces, washing hands, and laundering clothes cut the load. The health impact of thirdhand cannabis residue is not quantified.
can you get high from secondhand smoke
Does vaping create secondhand exposure?
Yes, with a different mix. Vaping releases an aerosol that holds THC, ultrafine particles, propylene glycol, vegetable glycerin, and flavor chemicals. It does not produce carbon monoxide or tar at the levels a burning joint does. The aerosol still deposits THC and particles into the room, and bystanders still inhale them.
How to reduce exposure
- Smoke outdoors or in a garage with the door open to the outside.
- Keep distance from children, pregnant people, and anyone with asthma or heart disease.
- Use a window fan that pushes air out of the room. A fan that only stirs the air does not help.
- Do not smoke in a car with other people. Car cabins are small and hold smoke.
- Change outer clothing and wash hands after smoking near kids.
- Choose an edible or tincture when smoke is the problem. These produce no secondhand smoke, though they carry their own dosing risks.
U.S. rules
Many states ban cannabis smoking anywhere tobacco smoking is banned. Some legal states permit consumption at licensed indoor lounges while banning it in apartments with shared ventilation. Federal property, national parks, and most public housing remain off limits. Rules differ by state, county, and landlord.