Answer in short
Secondhand cannabis smoke is the smoke a nonsmoker breathes from another person's joint, bong, pipe, or vape. It carries THC, tar, carbon monoxide, ammonia, and fine particles. A nonsmoker in a closed room can absorb THC and later test positive on a urine screen. Short exposure can cause eye and throat irritation, coughing, headache, and in some cases a mild high. Children, pregnant people, and people with asthma or heart disease carry the most risk. Ventilation lowers the dose. It does not remove it.
exposure to secondhand cannabis smoke
What is in the smoke
- THC. The dose from secondhand smoke is lower than a direct hit, but it is measurable in blood and urine.
- Tar and fine particles, measured as PM2.5. Burning cannabis in a room can push particle levels past outdoor air quality limits.
- Carbon monoxide, ammonia, benzene, hydrogen cyanide, and polycyclic aromatic hydrocarbons.
- Most of the same combustion chemicals found in tobacco smoke, minus nicotine.
What research shows
A 2015 study in Drug and Alcohol Dependence put nonsmoking adults in a ventilated room with cannabis smokers for one hour. THC showed up in blood and urine. Some participants reported feeling high and showed small changes in heart rate and mood. Studies of children who live with cannabis smokers have found THC metabolites in their urine. The American Lung Association states that secondhand cannabis smoke irritates the lungs and can trigger asthma attacks.
Groups at higher risk
- Infants and children. Smaller airways, a faster breathing rate, and a developing brain.
- Pregnant people. THC crosses the placenta.
- People with asthma, COPD, or chronic allergies.
- People with heart disease. Smoke exposure raises heart rate and can narrow blood vessels.
- Anyone facing a drug test. Exposure can produce a positive result.
Thirdhand smoke
Residue from cannabis smoke settles on walls, carpets, furniture, and clothing. This residue is called thirdhand smoke. It can stay for days or weeks and re-release into the air. Wiping hard surfaces and washing fabrics cut the amount. They do not remove all of it.
Smoke from hemp and CBD flower
Hemp flower and CBD flower hold less THC than marijuana, often under 0.3 percent by dry weight. Burning them still releases fine particles, tar, and carbon monoxide. A nonsmoker in the room breathes those. THC exposure from hemp smoke is low. Lung irritation is not zero.
How to lower exposure
- Smoke outdoors, away from doors and windows.
- Use a room with open windows and a fan that pushes air outside. This lowers exposure but does not end it.
- Do not smoke in a car with children or nonsmokers.
- Choose edibles or tinctures. They produce no smoke.
- Change clothes and wash hands after smoking near children.
What is not known
Long-term research on secondhand cannabis smoke is thin. Most lung studies track tobacco. The THC dose from secondhand exposure depends on room size, air flow, amount smoked, and time in the room. No US federal agency sets a safe exposure limit for cannabis smoke.