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

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.  

Conclusion

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.