Can You Fail A Drug Test From Secondhand Marijuana Smoke?

Sophia Delphi May 12, 2022 - 7 min read
Fact Checked
image of a scientist testing for drug

“Man, the smoke was so thick I got a contact high!”

People have been joking for decades about getting stoned at a party or concert where just about everyone was smoking up.

But were those contact highs real, or just wishful thinking?

Studies done during the 1980s determined that the chance of getting high from secondhand weed smoke was extremely low. The only non-smoking participants whose urine showed significant amounts of cannabis metabolites were those who were exposed to what the researchers called “noxious smoke conditions” in a small, unventilated room.

But that was the 1980s, and three things have changed since then.

First, weed is much more potent than it was back then; there is a good chance that secondhand exposure to today’s weed smoke could have a greater impact. Second, ventilation techniques have greatly improved over the last forty years. Finally, few people had to worry about companies subjecting them to pre-employment or random testing in the ‘80s.

So old studies don’t really answer the question of whether you can fail a drug test from secondhand smoke.

The issue of contact highs and drug testing has been revisited in recent years, though. Here are the results.

How Much THC Is In Secondhand Smoke?

The “definitive” study on this question doesn’t actually provide definitive answers. It was published in the British Journal of Anesthesia in 1999, and it had three interesting findings.

  • Only about 50% of the cannabinoids in raw flowers actually make it into the smoke that people inhale from a bowl or joint. (Other estimates place that number as low as 30%.)
  • Almost all of the THC and other cannabinoids in weed smoke stay in the lungs, at least when experienced smokers are doing the toking,
  • As a result, very little THC remains in the smoke they exhale. Nearby non-smokers would be exposed to almost none of the psychoactive cannabinoids unless they were sitting with a smoker in an unventilated room for a long time.

However, those conclusions don’t provide specific numbers or amounts and don’t account for the significant increase in potency we’ve seen in the 21st century. Maybe the smoke from schwag weed wouldn’t have an effect on others sitting in the same ventilated room. But would that be different if someone was smoking one of today’s incredibly potent strains with more than 30% THC content?

Researchers have conducted new studies to update what we know over the last decade.

Secondhand Weed Smoke in the 2000s

The most important work on the subject was done at Johns Hopkins University in 2015.

It involved two groups of volunteers between the ages of 18 and 45. Half of them smoked at least twice per week, and the other half hadn’t used weed in at least six months (and came up negative on a pre-study urine test).

First, six smokers and six non-smokers were seated in an unventilated 10×13 room for an hour. Each smoker was given ten joints with a THC content of 5.3%, and they smoked for as long as they wanted to. Next, six smokers and six (different) non-smokers were in the same room, and the smokers were given joints with 11.3 THC content. The final session was similar to the second, but the room was well-ventilated for this one.

The non-smokers took two different types of urine tests after the sessions, one test with a more forgiving cutoff level than the other. They were tested regularly over the next 24 hours. All of the tests checked for the presence of THC-COOH, the THC metabolite that most urine tests look for.

Here’s what happened.

  • After both sessions in the unventilated room, all of the non-smokers immediately came up dirty on the less-sensitive urine test, but only one tested positive four hours later.
  • Three-quarters of non-smokers in the unventilated room initially tested positive on the more-sensitive test, but all tested negative within 24 hours.
  • In that second group, the more-sensitive urine test detected secondhand THC-COOH for longer periods of time when more-potent weed had been smoked.
  • But none of the non-smokers who sat in the ventilated room failed any of the drug tests. (The samples were evaluated by five labs; one found a single non-smoker who came up dirty on the sensitive test, but the other four labs all said she was clean.)

Here’s the CliffNotes version.

  1. Those exposed to secondhand cannabis smoke in a ventilated room tested negative.
  2. Those exposed in an unventilated room tested positive on the less-sensitive test most employers use, but only for a few hours.
  3. Most of those exposed in an unventilated room tested positive on the more-sensitive test, but for a maximum of 24 hours. The positive results lasted longer when stronger weed was smoked.

There was one other interesting finding. Non-smokers in the unventilated room reported that they felt more tired, less alert, and “pleasant” after the session. By contrast, non-smokers in the ventilated room reported no physical effects — except that they were hungry. Most of us can certainly understand those results.

A systematic review of the literature done several years later, and published in the Canadian Medical Association Journal, reported basically the same results. It concluded that people could test positive after secondhand smoke exposure in a poorly-ventilated area, and they may feel somewhat stoned.

There was one other interesting fact highlighted in that review: THC may be detected in the body after just 15 minutes of exposure to secondhand weed smoke. That could be an issue for the relatively few people who have to take a blood test to check for cannabis use. (Blood tests look for THC; urine tests look for the metabolite THC-COOH.)

That Means You Can Fail a Drug Test From Secondhand Smoke?

Yes, it’s possible — but it’s extremely unlikely. You’d have to be in a room where the smoke’s so thick that it’s somewhat difficult to see the smoker next to you. (If you don’t believe us, check out the picture accompanying the Johns Hopkins study.)

If you have a drug test coming up, however, it’s best to err on the side of safety and avoid any smoke-filled rooms for a couple of days beforehand.

Can You Fail a Drug Test from Secondhand Smoke: FAQ

Q: What if I’ve been at a party where secondhand smoke might affect a urine test, and I get hit with a drug test the next day?
A: First, see if there’s any way to put the test off for 24 hours. You’ll almost definitely test clean by then. Second, drink as much water and/or sports drinks as you can, and urinate as much as possible before the test. And as a last resort, take some vitamin B-2 and a creatine supplement a couple of hours before you have to give urine; it might make your urine look more natural and mask any residual traces of the THC-COOH.

Q: OK, I admit I took a couple of hits at that party.
A: If you’re not a regular smoker, you should test clean after 1-3 days—otherwise, best of luck.

Q: So contact highs are real?
A: Research indicates that they are — but unless the smoke is thick, the ventilation is poor, and you’re exposed for a while, the chances are that all you end up feeling are the munchies.