Secondhand Cannabis Smoke Produced Detectable THC in Blood and Urine of Non-Smokers | Research

The takeaway

Five people who never used cannabis tested positive for THC in blood and cannabinoids in urine after sitting in a car where others smoked marijuana for 30 minutes.

Read this if you are concerned about drug testing and want to know whether secondhand cannabis smoke can cause a positive test result.

Blood THC reached 1.3 to 6.3 ng/mL from passive exposure alone

What the researchers found

Five healthy volunteers who had never used cannabis sat in a small closed car (approximately 1,650 liters of air) while other people smoked marijuana or hashish for 30 minutes.

Immediately after exposure, THC was detectable in the blood of all five passive smokers at concentrations ranging from 1.3 to 6.3 nanograms per milliliter. Total cannabinoid blood levels exceeded 13 ng/mL in four of the five volunteers. Both THC and total cannabinoid concentrations dropped to near the detection limits of the assays within two hours.

Cannabinoids were also detectable in urine using both radioimmunoassay and EMIT testing methods. The researchers concluded that detecting cannabinoids in blood or urine is not unequivocal proof that someone actively smoked cannabis.

Why it matters

This study had direct implications for drug testing in workplaces, law enforcement, and forensic settings. It demonstrated that positive drug tests could result from passive exposure rather than active use, raising questions about the validity of drug test results as proof of cannabis consumption.

The numbers in context

Five volunteers. 30-minute exposure. Car air volume: approximately 1,650 L. Blood THC range: 1.3 to 6.3 ng/mL immediately after exposure. Blood levels fell near detection limits within 2 hours. Urine positive by both RIA (above 13 ng/mL) and EMIT (above 20 ng/mL).

How the study worked

Prospective study with five cannabis-naive volunteers passively exposed to cannabis smoke in a closed car (approximately 1,650 L air volume) for 30 minutes. Blood and urine were collected at multiple time points and tested using GC-MS for THC and RIA/EMIT for total cannabinoids.

What this study cannot tell us

The exposure conditions were extreme: a small closed car with active smoking for 30 minutes. This does not represent typical secondhand exposure in normal indoor or outdoor environments. Only five participants. The study did not assess whether the absorbed THC levels produced any psychoactive effects.

How to read the evidence

A controlled prospective study with objective measurements, but only five participants and extreme exposure conditions limit generalizability to real-world scenarios.

When this study was published

Published in 1985. Drug testing cutoff thresholds and assay sensitivities have changed since this study was conducted.

The bigger picture

This study launched decades of debate about passive cannabis exposure and drug testing. While modern studies have generally shown that realistic passive exposure (in ventilated spaces) rarely produces positive results at current testing thresholds, extreme exposure conditions can still trigger positive tests, an issue that remains relevant for workplace and legal drug testing.

Questions still open

At what exposure levels does passive inhalation fail to produce detectable cannabinoids? Do modern drug testing cutoffs adequately account for passive exposure? Did the participants experience any psychoactive effects at the detected blood levels?

Common questions

Can you fail a drug test from secondhand cannabis smoke?

In this study, 30 minutes in a small closed car with active smokers produced positive blood and urine tests. However, the exposure conditions were extreme and may not reflect typical real-world secondhand exposure.

How long did the THC stay in their blood?

Blood THC and cannabinoid levels fell to near detection limits within two hours after the 30-minute exposure ended.

Read the original research

Cannabinoids in blood and urine after passive inhalation of Cannabis smoke.

Journal of forensic sciences, 30(4), 997-1002

View on PubMed ↗

Citation

Mørland, J; Bugge, A; Skuterud, B; Steen, A; Wethe, G H; Kjeldsen, T. (1985). Cannabinoids in blood and urine after passive inhalation of Cannabis smoke.. Journal of forensic sciences, 30(4), 997-1002.