Non-smokers exposed to secondhand cannabis smoke tested positive for THC in blood and oral fluid for up to 3 hours | Research

The takeaway

In an unventilated room, non-smokers passively exposed to cannabis smoke showed positive THC tests in blood and oral fluid for up to 3 hours and reported subjective effects that increased with higher-potency cannabis.

Read this if you're concerned about secondhand cannabis smoke exposure in your home or workplace.

Positive THC tests in non-smokers for up to 3 hours after exposure

What the researchers found

Six non-smokers sat alternately with six cannabis smokers in a sealed chamber for one hour across three sessions. In the first session, smokers used 5.3% THC cannabis; in sessions 2 and 3, they used 11.3% THC cannabis (session 3 had ventilation).

Non-smokers tested positive for THC in oral fluid and blood for up to 3 hours after exposure. The amounts of THC absorbed by non-smokers were proportionally less than active smokers but still measurable. Subjective effects (feeling high, sedated) correlated with the degree of exposure and cannabis potency.

The results indicated that extreme secondhand cannabis smoke exposure in an enclosed, unventilated space can produce effects that mimic, though to a lesser extent, active cannabis smoking. Ventilation substantially reduced exposure levels.

Why it matters

As cannabis potency increases, concerns about secondhand exposure grow. This study demonstrates that passive exposure in enclosed spaces can produce detectable THC levels and subjective effects, which has implications for workplace testing, childcare settings, and non-smoker rights.

The numbers in context

6 smokers, 6 non-smokers. Cannabis potency: 5.3% and 11.3% THC. 1-hour exposure sessions. Positive blood and oral fluid tests for up to 3 hours post-exposure. Ventilation substantially reduced exposure. Subjective effects correlated with potency.

How the study worked

Controlled exposure study with 6 cannabis smokers and 6 non-smokers in a sealed chamber. Three sessions varied cannabis potency (5.3% and 11.3% THC) and ventilation conditions. Oral fluid (ELISA and LC-MS/MS) and blood (LC-MS/MS) were analyzed at multiple time points. Subjective effects were rated before and after each session.

What this study cannot tell us

Extreme exposure conditions (sealed chamber, no ventilation) do not represent typical real-world scenarios. Small sample (6 non-smokers). Only two potency levels tested. Short-term exposure effects studied; long-term consequences of repeated passive exposure are unknown.

How to read the evidence

Controlled exposure study with validated biological measures. Small sample but rigorous methodology under extreme conditions.

When this study was published

Published in 2015. Cannabis potency has continued increasing since this study.

The bigger picture

Secondhand smoke concerns have driven tobacco policy for decades. As cannabis use becomes more widespread, similar questions arise about passive exposure. This study provides concrete data that enclosed, unventilated exposure to high-potency cannabis can produce measurable effects in non-smokers.

Questions still open

At what ventilation levels does secondhand cannabis exposure become negligible? Could repeated passive exposure in a household affect children? Do secondhand CBD exposures produce any effects?

Common questions

Can you get high from secondhand cannabis smoke?

Under extreme conditions (sealed room, no ventilation, 1 hour), non-smokers reported mild subjective effects and tested positive for THC. In normal ventilated settings, significant exposure is much less likely.

Can secondhand cannabis smoke cause a positive drug test?

Under extreme enclosed exposure conditions, yes, for up to 3 hours after exposure. With ventilation, exposure levels dropped substantially. In typical real-world settings, a positive test from passive exposure alone would be rare.

Read the original research

Nonsmoker Exposure to Secondhand Cannabis Smoke. III. Oral Fluid and Blood Drug Concentrations and Corresponding Subjective Effects.

Journal of analytical toxicology, 39(7), 497-509

View on PubMed ↗Publisher’s paper ↗

Citation

Cone, Edward J; Bigelow, George E; Herrmann, Evan S; Mitchell, John M; LoDico, Charles; Flegel, Ronald; Vandrey, Ryan. (2015). Nonsmoker Exposure to Secondhand Cannabis Smoke. III. Oral Fluid and Blood Drug Concentrations and Corresponding Subjective Effects.. Journal of analytical toxicology, 39(7), 497-509. https://doi.org/10.1093/jat/bkv070