Secondhand Cannabis Smoke: Is It Dangerous?

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.

The legal status of cannabis in the UK creates specific considerations around secondhand smoke. Even in private residences, cannabis smoke that drifts into shared spaces or neighbouring flats can create legal complications if reported to police.

Tenancy agreements often prohibit smoking of any kind inside the property, and cannabis smoking specifically can be grounds for eviction. Social housing tenants face particularly strict rules — a single complaint about cannabis smoke can trigger a safeguarding investigation. For more on the legal framework, see our UK Cannabis Law Guide.

In vehicles, the driver and passengers can be charged with possession if cannabis is being smoked in the car, and the driver faces additional charges of driving while impaired. Even secondhand exposure in a vehicle could theoretically contribute to a drug driving charge. See our Driving Guide for more details.

Workplaces have the right to enforce smoke-free policies that include cannabis. Positive drug tests from secondhand exposure are rare but possible. If you work in safety-critical roles (transport, healthcare, construction), even trace exposure could have professional consequences. UK employers generally require zero-tolerance policies for safety-critical positions.