Can You Fail a Marijuana Drug Test from Second-Hand Smoke?

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Lee Johnson

Lee Johnson is the senior editor at CBD Oracle, and has been covering science, vaping and cannabis for over 10 years. He has a MS in Theoretical Physics from Uppsala...

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Medically reviewed byAbraham Benavides, MD

Medically reviewed by

Abraham Benavides, MD

Dr. Abraham Benavides is an international cannabis science advisor, health coach, and full-tuition merit scholar of the GW School of Medicine. Abe pioneered and published first-author research with the Cannabis...

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Smoking marijuana before a drug test
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So the worst has happened: you had a drug test at work and your employer flagged you for marijuana use. But, the problem is, you haven’t actually smoked marijuana. Racking your brain for a possible cause, you realize that you were in the room when people were smoking weed recently.

The question then presents itself: can you fail a drug test from second-hand marijuana smoke? While you’re hopefully asking because you genuinely haven’t smoked yourself, it’s also possible you’re wondering if second-hand exposure could cover the fact that you really have smoked. We’ve spoken to experts and looked through the data to find out.

Is It Possible to Fail a Drug Test Because of Second-Hand Smoke?

It’s very unlikely you would fail a urine drug test for marijuana because you’ve been exposed to the smoke second-hand. However, it is technically possible.

Based on the evidence collected and summarized in the references, it’s clear that THC metabolites do reach the urine of people passively exposed. However, despite being detectable, it’s unlikely that anybody would fail a drug screening at the federally-recommended 50 ng/ml cutoff from passive exposure.

This can happen, though, as shown by a National Institute on Drug Abuse (NIDA) study [1] in which non-smokers were exposed to the smoke of 16 joints per day, for six days, in a small, unventilated room. Every single participant tested positive at 20 ng/ml at some point during the study, and some would have failed at 50 ng/ml. Similarly, a series of Johns Hopkins University studies [2, 3, 4] using stronger cannabis in an unventilated (but larger) room had one participant fail a urine test at the federal cutoff.

However, in every other case, participants were rarely positive even at the lower 20 ng/ml cutoff. It is no coincidence that one case where people could have actually failed was in an unventilated room and involved a lot of consumption over a long period of time. 

We spoke to Dr. Edward Cone, who was lead author on two out of the three Johns Hopkins University studies, and he agreed that the exposure in them was extreme. He commented that, “The level of exposure in my study was completely unrealistic,” explaining:

“The study was deliberately designed to push the envelope to the extreme. Just consider that two of the sessions were conducted in a sealed unventilated room with 10 people sitting around a table. Each smoker had a non-smoker on either side nearly touching each other. The smokers smoked as many marijuana cigarettes as they wanted (up to 10) over the course of the hour-long session. The smoke got so thick that most of the non-smokers wore goggles during each session.”

For the broader question of whether you could fail a drug test from second-hand exposure, Dr. Cone clarified that many factors are relevant to the result:

“The degree of passive exposure is highly dependent upon numerous factors such as the nature of the environment where smoke is encountered. Key factors include degree of air movement, degree of air exchange, size of enclosure, distance to source of smoke, number of smokers, quality of marijuana (THC content), manner of smoking and exhaling, and so on.”

However, under the assumption of the standard, federally-recommended urine test, he summarized that:

“Casual exposure to marijuana smoke would not cause a positive marijuana drug test by [Urine Mandatory Guidelines] standards. However, as soon as you recognize that marijuana is being smoked nearby, I would caution anyone to either remove themselves from the area or change the environment to lessen the exposure.”

Will You Fail a Blood or Saliva THC Test Because of Second-Hand Smoke?

If you’re tested soon after you’re exposed, you could fail a blood or saliva test because of second-hand smoke. However, you are unlikely to fail if it’s been a couple of hours since you were exposed.

Research covering saliva testing [4, 5] found that while it’s definitely possible to fail immediately after being exposed to smoke, it’s very unlikely you’d fail the federally-recommended 4 ng/ml screening test after a few hours.

For blood tests, there is no federal limit, but based on the evidence [3, 4, 6, 7, 8] the picture is much the same as with saliva. While you may fail initially, it’s very unlikely that blood levels would remain high enough to fail for longer than a couple of hours.  

RELATED: How Long Does Weed Stay in Your System?

You Can Fail, But Is It a Good Excuse?

While it is possible that second-hand smoke made you fail a marijuana test, if you did fail you were probably exposed quite a lot and possibly even felt slightly high as a result of it.

First off, the fact that only fairly extreme conditions (exposed in a small, unventilated room) produced failed drug tests won’t really bode well for you if you’re using it as an excuse. Dr. Cone drew a distinction that really emphasizes the main point:

“I’d like to distinguish between “knowing” and “unknowing” exposure circumstances. Marijuana smoke has a distinct odor, and is especially different from tobacco smoke. Most people have become familiar with both odors and can readily distinguish the two. Knowing exposure is, in my opinion, just another way of administering [the] drug. If you deliberately expose yourself then you are responsible for your actions. If unknowing exposure occurs, then exposure is generally much lighter than knowing exposure.”

It’s possible that cumulative exposure would make things worse [1], but it’s hard to imagine you would regularly breathe in that much smoke without realizing the risk. Dr. Cone also made this point when we asked if someone exposed daily (by a roommate, say) could fail a test:

“It would primarily depend upon the degree of smoke exposure and environmental factors. And it would have to be considered knowing exposure. Given daily exposure to extreme smoke conditions for extended periods of time, accumulation could occur resulting in a positive urine test.”

The bigger issue is a finding in the Dr. Cone series of studies [3], where non-smoking participants reported some subjective drug effects. Participants were more likely to say they felt relaxed or had pleasant drug effects for around an hour and a half after exposure, and they performed worse on a cognitive test for impairment. 

We asked Dr. Cone if this means you would feel “high” if you’d inhaled enough to fail a drug test: 

“In the scenario in which knowing exposure occurred resulting in a positive drug test, it is likely the person would also notice behavior effects resulting from inhaling marijuana smoke.”

Putting this all together, “I was in the room when someone was smoking!” is a pretty crappy excuse for a failed test. If this is true, you must have been in a small room while someone smoked, likely more than just a joint or two, and it’s possible you were actually high yourself. So to anyone familiar with the evidence, even a very generous interpretation of your claim doesn’t look great.  

Limitations of the Evidence

Although there are several studies addressing this issue, it’s worth noting that the data is still quite limited. Many studies urine test participants who were exposed to marijuana smoke, for example, but many of these use low-THC weed that falls far short of current dispensary-level products. Additionally, several studies use very small rooms and a lot of smoke, to the point where it’s unlikely anyone would really be exposed this way.

That said, there is enough data for urine testing that we can be relatively confident we understand the situation. There is less data for other testing methods, though, so these conclusions should be taken with a pinch of salt.  


Ultimately, saying you failed a drug test because of marijuana isn’t a good idea. It’s questionable whether people would believe you, and even if they did you’d have to have been around a lot of smoke to really fail. If this is really the case, you may have a bit of a debate on your hands. 

View All References (9)

[1] Cone, E. J., Johnson, R. E., Darwin, W. D., Yousefnejad, D., Mell, L. D., Paul, B. D., & Mitchell, J. (1987). Passive inhalation of marijuana smoke: Urinalysis and room air levels of delta-9-tetrahydrocannabinol. Journal of Analytical Toxicology, 11(3), 89–96.

About the Study:

  • Exposure conditions: 12.2 m3 unventilated room. Multiple tests were conducted, but one involved the smoke from 16 joints for one hour per day, for 6 days.
  • Amount smoked: 16 joints per day for six consecutive days. 2.8% THC
  • Test and cutoff: Urine tests, 20 ng/mL
  • Results: No participants failed the urine test, with a max of 20 ng/mL. Only one participant was confirmed positive in the blood test.

[2] Cone, E. J., Bigelow, G. E., Herrmann, E. S., Mitchell, J. M., LoDico, C., Flegel, R., & Vandrey, R. (2014). Non-smoker exposure to secondhand cannabis smoke. I. Urine Screening and confirmation results. Journal of Analytical Toxicology, 39(1), 1–12. 

About the Study:

  • Exposure conditions: 25.7 m3 chamber, with six smokers and six non-smokers in each of three experiments, lasting one hour. Experiment 3 had ventilation.
  • Amount smoked: Three sessions total, with 545.9, 1627.2 and 1864.5 mg of THC consumed in each, total.
  • Test and cutoff: Urine tests (20, 50, 75 and 100 ng/mL cutoffs).
  • Results: No participant was positive at 100 or 75 ng/mL, but one was positive at 50 ng/mL. Many people were positive at the lower 20 ng/mL cutoff.

[3] Herrmann, E. S., Cone, E. J., Mitchell, J. M., Bigelow, G. E., LoDico, C., Flegel, R., & Vandrey, R. (2015). Non-smoker exposure to secondhand cannabis smoke II: Effect of room ventilation on the physiological, subjective, and behavioral/cognitive effects. Drug and Alcohol Dependence, 151, 194–202.

About the Study:

  • Exposure conditions: As in Cone et. al. (2015)
  • Amount smoked: Volunteers smoked 14.4 grams of 11.3% THC cannabis in the unventilated and 16.5 g in the ventilated.
  • Test and cutoff: Blood (0.5 ng/mL limit of quantification) and urine (50 ng/mL or 20 ng/mL)
  • Results: Unventilated: One urine specimen was positive by federal standards, 4 of 6 positive at 20 ng/mL. Blood THC peaked at 3.2 ng/mL and was detectable for up to 3 hours. Ventilated: No urine specimens positive, even at 20 ng/mL. Blood THC peaked at 0.7 ng/mL but was undetectable shortly after.

[4] Cone, E. J., Bigelow, G. E., Herrmann, E. S., Mitchell, J. M., LoDico, C., Flegel, R., & Vandrey, R. (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.

About the Study:

  • Exposure conditions: 25.7 m3 chamber, with six smokers and six non-smokers in each of three experiments, lasting one hour. Experiment 3 had ventilation.
  • Amount smoked: Three sessions total, with 545.9, 1627.2 and 1864.5 mg of THC consumed in each, total.
  • Test and cutoff: Saliva (4 ng/mL) and blood (0.5 ng/mL).
  • Results: All participants tested positive by saliva at some point in sessions 1 and 2, and half were positive in session 3. Blood tests peaked at mean values of 1.4 ng/ml, 3.1 ng/ml and 0.5 ng/ml in tests 1, 2 and 3, respectively.

[5] Niedbala, R. S., Kardos, K. W., Fritch, D. F., Kunsman, K. P., Blum, K. A., Newland, G. A., Waga, J., Kurtz, L., Bronsgeest, M., & Cone, E. J. (2005). Passive cannabis smoke exposure and oral fluid testing. II. Two studies of extreme cannabis smoke exposure in a motor vehicle. Journal of Analytical Toxicology, 29(7), 607–615.

About the Study:

  • Exposure conditions: 15.3 m3 van, with no ventilation. Four volunteers were exposed for 20 minutes. In the first study, they stayed in the van for 45 minutes. In the second the left immediately after smoking concluded.
  • Amount smoked: Four joints in each study. The first study involved spliffs, mixed with tobacco and containing 39.5 mg of THC each. The second study used pure cannabis and had 83.2 mg of THC each.
  • Test and cutoff: Saliva (3 ng/mL) and urine (50 ng/mL).
  • Results: In study 1, all saliva samples tested positive immediately after exposure, ranging from 3.6 to 7.5 ng/ml. In study 2, all saliva samples were negative, with a maximum of 1.2 ng/ml. While metabolites were measurable in urine, nobody was positive in either study.

[6] Law, B., Mason, P. A., Moffat, A. C., King, L. J., & Marks, V. (1984). Passive inhalation of cannabis smoke. Journal of Pharmacy and Pharmacology, 36(9), 578–581.

About the Study:

  • Exposure conditions: 28 m3 unventilated room. Six joints were smoked and non-smoking four volunteers remained in the room for three hours.
  • Amount smoked: 102.6 mg total, 71 mg actually smoked.
  • Test and cutoff: Urine (2 ng/mL) and blood (unknown cutoff)
  • Results: All participants had THC-COOH in their urine, but the maximum level in the whole study was 6.8 ng/ml. Nothing was detected in the blood.

[7] Mørland, J., Bugge, A., & Skuterud, B. (1985). Cannabinoids in blood and urine after passive inhalation of cannabis smoke. Journal of Ethnopharmacology, 17(3), 298.

About the Study:

  • Exposure conditions: 1.65 m3 car, five volunteers exposed for 30 minutes.
  • Amount smoked: Unknown
  • Test and cutoff: Urine (20 ng/mL) and blood (unknown). 
  • Results: Blood THC levels were between 1.3 and 6.3 ng/ml after exposure. Urine screens also detected cannabinoids above the cutoff.

[8] Rohrich, J., Schimmel, I., Zorntlein, S., Becker, J., Drobnik, S., Kaufmann, T., Kuntz, V., & Urban, R. (2010). Concentrations of  9-tetrahydrocannabinol and 11-nor-9-carboxytetrahydrocannabinol in blood and urine after passive exposure to cannabis smoke in a coffee shop. Journal of Analytical Toxicology, 34(4), 196–203.

About the Study:

  • Exposure conditions: 200 m3 room with good ventilation, for three hours. In a coffee shop where numerous customers smoked marijuana. 
  • Amount smoked: Unknown.
  • Test and cutoff: Urine (25 ng/mL) and blood (3 ng/mL)
  • Results: No participants failed the urine test, with a max of 20 ng/mL. Only one participant was confirmed positive in the blood test.

Perez-Reyes, M., Di Guiseppi, S., Mason, A. P., & Davis, K. H. (1983). Passive inhalation of marihuana smoke and urinary excretion of cannabinoids. Clinical Pharmacology and Therapeutics, 34(1), 36–41.

About the Study:

  • Exposure conditions: Three tests: 1) Two joints smoked in a closed, 15.5 m3 room. 2) Two joints smoked in a car, 3.5 m3. 3) Four joints smoked per day, for three days
  • Amount smoked: 2.8% THC marijuana. Two joints in studies 1 and 2, four joints per day in study 3.
  • Test and cutoff: Urine (20 ng/mL). 
  • Results: 1) No participant tested positive. 2) One of two subjects was positive. 3) One participant tested positive after the third day.