Short answer
Yes. Secondhand cannabis smoke contains many of the same harmful compounds as tobacco smoke. That list includes fine particulate matter, carbon monoxide, tar, and polycyclic aromatic hydrocarbons. It also carries THC. Nonsmokers in a closed room can absorb THC and test positive on a urine screen. The research base on long-term disease outcomes from cannabis smoke is smaller and less complete than the research base on tobacco smoke. Some risk estimates carry wide uncertainty because of that gap. The absence of proof is not proof of safety.
What is in secondhand cannabis smoke
- Fine particles (PM2.5) small enough to reach the deep lung and the bloodstream.
- Carbon monoxide, which takes the place of oxygen on red blood cells.
- Tar and irritants that damage airway tissue.
- Carcinogens found in tobacco smoke, including benzene, formaldehyde, and polycyclic aromatic hydrocarbons.
- THC, the psychoactive compound in cannabis.
The American Lung Association reports that cannabis smoke contains many of the same toxins, irritants, and carcinogens as tobacco smoke.
Human exposure evidence
A 2015 Johns Hopkins study put nonsmoking adults in a room with cannabis smokers for one hour. THC showed up in their blood and urine at low levels. Those levels stayed below common impairment thresholds. The room was ventilated, which means an unventilated room would likely produce higher readings.
Does Secondhand Smoke Cause a Positive Drug Test? What Actually Shows Up
A 2016 study from the University of California, San Francisco exposed rats to secondhand cannabis smoke. Blood vessel function dropped after one minute of exposure. The effect lasted about 90 minutes. Tobacco smoke produced a similar effect that lasted about 30 minutes.
effects of cannabis secondhand smoke
The 2018 National Academies report on cannabis and health found limited evidence on secondhand cannabis smoke and health harms in adults. It found insufficient evidence on cancer outcomes. Insufficient evidence means the studies have not been done, not that the smoke is harmless.
Children and secondhand exposure
Children breathe faster than adults and have smaller airways. Studies of children in homes where cannabis is smoked report more respiratory symptoms, including cough and wheeze. Those studies are small and rely on parent reports. Public health agencies advise keeping all smoke away from children, pregnant people, and anyone with asthma or heart disease.
How it differs from tobacco smoke
Cannabis is often smoked less often per day than cigarettes. Cannabis joints often lack filters, and the smoke is inhaled deeper and held longer. That pattern can raise tar exposure per session. The Centers for Disease Control and Prevention states that no risk-free level of secondhand smoke exposure exists.
How to lower exposure
- Smoke outdoors, away from doors, windows, and air intakes.
- Choose edibles, tinctures, or capsules. These produce no smoke.
- Use HEPA air purifiers and open windows. These cut particle counts but do not remove all of them.
- Do not smoke in a car with passengers.
- Set household rules before guests arrive.
US legal notes
Cannabis is illegal under federal law. Many leases and apartment rules ban smoking, and some state laws let landlords ban cannabis smoking on the property. Check your lease and local ordinances. This article is not medical advice.