How to Tell If Someone Was Passively Exposed to Cannabis Versus Actually Smoked It | Research

The takeaway

A systematic review of 21 studies found that specific biomarker patterns in urine, blood, oral fluid, and hair can help distinguish passive cannabis smoke exposure from active use, though extreme passive exposure can sometimes produce positive test results.

Anyone concerned about testing positive for cannabis from secondhand exposure, and professionals involved in drug testing.

THC-COOH absence in hair, oral fluid, and sweat is a key marker of non-use

What the researchers found

This systematic review identified biomarkers that can distinguish passive cannabis smoke exposure from active use across multiple biological matrices.

In everyday conditions, urinary THC-COOH levels from passive exposure should fall below standard positivity thresholds, especially when normalized to creatinine levels. In blood, passive exposure produces very low THC and THC-COOH concentrations compared to active use.

In hair, oral fluid, and sweat, the absence of THC-COOH is a key indicator. Its presence in hair suggests regular consumption; in oral fluid or sweat, it indicates recent use. However, under extreme exposure conditions (heavily smoky, unventilated environments), positive tests can occur even from passive exposure across all matrices.

The authors recommended that people who must demonstrate cannabis abstinence should avoid heavily smoky, unventilated environments.

Why it matters

Distinguishing passive from active cannabis exposure has important legal, occupational, and forensic implications. A false positive from secondhand smoke could have consequences for employment, custody disputes, driving enforcement, and drug treatment compliance monitoring.

The numbers in context

958 papers identified, 21 selected for review. THC-COOH in urine should be below positivity thresholds after passive exposure (especially creatinine-normalized). Blood THC and THC-COOH concentrations from passive exposure are very low. THC-COOH absence in hair, oral fluid, and sweat indicates no active use.

How the study worked

A systematic review was conducted, identifying 958 papers on passive cannabis exposure and selecting 21 for detailed analysis. Studies examined biomarkers in urine, blood, oral fluid, hair, and sebum following controlled passive exposure conditions.

What this study cannot tell us

The review acknowledged that extreme passive exposure conditions can produce positive results. Real-world passive exposure varies widely based on ventilation, proximity, duration, and cannabis potency. The studies reviewed used controlled laboratory conditions that may not perfectly represent real-world scenarios.

How to read the evidence

This is a systematic review of 21 studies across multiple biological matrices, providing strong evidence for biomarker interpretation guidelines.

When this study was published

Published in 2016. Cannabis potency and testing technology have both evolved, though the fundamental biomarker principles remain applicable.

The bigger picture

As cannabis use becomes more common in public spaces, passive exposure is an increasing concern for people subject to drug testing. This review provides evidence-based guidance for forensic experts and regulatory bodies on how to interpret test results in the context of potential passive exposure.

Questions still open

How has the increasing potency of cannabis affected passive exposure biomarker levels? Should drug testing cutoff values be updated to account for passive exposure research?

Common questions

Can secondhand cannabis smoke make you fail a drug test?

Under normal conditions, passive exposure produces biomarker levels below standard testing thresholds. However, in extreme conditions (heavily smoky, unventilated rooms), positive tests are possible. The review recommends that people who need to demonstrate abstinence avoid such environments.

How can labs tell if someone smoked or was just nearby?

The key is THC-COOH (a metabolite). Its presence in hair suggests regular use. In blood and urine, the concentration patterns differ between active and passive exposure. Using creatinine-normalized urinary THC-COOH is particularly helpful for making this distinction.

Read the original research

A systematic review of passive exposure to cannabis.

Forensic science international, 269, 97-112

View on PubMed ↗Publisher’s paper ↗

Citation

Berthet, Aurélie; De Cesare, Mariangela; Favrat, Bernard; Sporkert, Frank; Augsburger, Marc; Thomas, Aurélien; Giroud, Christian. (2016). A systematic review of passive exposure to cannabis.. Forensic science international, 269, 97-112. https://doi.org/10.1016/j.forsciint.2016.11.017