Can you test positive for THC from secondhand smoke?
In practice, almost never. Studies show: only under extreme secondhand exposure (hours in a closed room with heavy users), low THC traces are briefly measurable in urine. Excluded under normal everyday conditions.
Key facts
- Normal case: no positive
- Extreme scenarios: possible
- Urine cutoff: 50 ng/mL
- Blood test: even less sensitive
Detailed answer
Controlled studies (Cone et al., Cousineau) show: secondhand smoke can, in extreme scenarios (unventilated room, several active smokers, several hours), produce low metabolite levels — usually below standard urine test cutoffs (50 ng/mL). In well-ventilated rooms and normal social situations, values fall far below detection. Blood tests are even less sensitive to passive exposure.