Secondhand Cannabis Smoke: Is It Dangerous?
6 min readBeginner Level
What the research says about secondhand cannabis smoke — risks, how it compares to tobacco smoke, whether you can get high from it, and how to protect others.
This guide is for educational purposes only. Cannabis is illegal in the UK without a medical prescription. Always consult a healthcare professional before making decisions about cannabis use.
Secondhand cannabis smoke contains many of the same toxic compounds as tobacco smoke, including carbon monoxide, ammonia, hydrogen cyanide, and polycyclic aromatic hydrocarbons (PAHs) — many of which are known carcinogens.
Burning cannabis generates similar levels of tar and particulates as burning tobacco, depending on how it's combusted. A joint without a filter produces more tar than a cigarette with a filter. However, cannabis smokers typically consume less volume than cigarette smokers.
The long-term effects of regular secondhand cannabis smoke exposure are not well studied because cannabis remains illegal in many jurisdictions, making research difficult. However, the existing evidence suggests it carries health risks, particularly for children and people with respiratory conditions.
Vaping cannabis produces significantly fewer toxic byproducts than smoking because there is no combustion. Switching from smoking to vaping reduces both firsthand and secondhand exposure to harmful compounds.
It is extremely unlikely to get high from secondhand cannabis smoke in real-world conditions. Studies have found that non-smokers in poorly ventilated rooms with heavy cannabis smokers can absorb trace amounts of THC, but not enough to produce psychoactive effects.
One study found that non-smokers exposed to heavy cannabis smoke in an unventilated room for one hour had trace THC levels in their blood, but none reported feeling high or showed impairment in cognitive tests.
Extreme conditions (enclosed space, heavily smoked cannabis, multiple hours of exposure) might produce detectable THC but still unlikely to cause intoxication. However, this can be enough to trigger a positive drug test in sensitive workplace screening.
Children and pets are more vulnerable to secondhand smoke effects due to their smaller body mass. Smoking cannabis around children or pets should be avoided entirely, regardless of whether they might 'get high' from it.
The safest approach: avoid smoking cannabis indoors. If you smoke, do it outside and away from doors, windows, and air intakes. Smoke dissipates quickly in open air.
If you must consume indoors, use a dry herb vaporiser instead of smoking. Vaping produces significantly less odour and fewer toxic byproducts. Even better, use edibles, tinctures, or topicals which produce no airborne compounds at all.
Never smoke cannabis in a car with others present, even with windows open. The confined space concentrates smoke and exposes passengers to high levels of particulates.
Be considerate of neighbours. Cannabis smoke can travel through walls, vents, and shared spaces in apartments and flats. If you live in shared accommodation, consider switching to edibles, tinctures, or vaping to avoid affecting others.
Children should never be exposed to cannabis smoke. Their developing lungs and brains are more vulnerable to the harmful compounds in smoke. Secondhand cannabis smoke exposure in children has been linked to increased respiratory infections and asthma symptoms.
Cannabis smoke residue settles on surfaces (thirdhand smoke) and can be ingested by young children who put objects in their mouths. Smoking in a different room or even at different times does not eliminate this risk.
If you use cannabis and have children: use edibles, tinctures, or topicals instead of smoking; change your clothes and wash your hands before handling children after smoking; never store cannabis products where children can reach them.
In the UK, exposing a child to cannabis smoke could be considered a form of neglect or child protection concern. Social services may become involved if there are repeated concerns about a child's exposure to drugs.
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About the Author
DM
Dave Mak
Dave founded The Budophile to create clear, honest cannabis education for UK beginners. With a background in health research and a network of specialist contributors, he ensures every guide is accurate, evidence-based, and practical. He also runs Baked & Rated for product reviews and The Green Prescription for medical cannabis access guidance.
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