Can You Fail A Drug Test From Secondhand Marijuana Smoke?


Failing a drug test because you were around someone else who was smoking weed? It sounds almost ridiculous.

Better safe than sorry, though, so let’s look into the possibility.

What the “Experts” Say

If you ask drug testing companies they don’t use the word “ridiculous,” but they do claim that passive exposure to cannabis smoke isn’t enough to come up dirty. Well, at least they try to claim it.

For example, US Drug Test Centers (which conducts tests for individuals and companies across America), says “exposure to second-hand smoke is not enough for you to test positive for marijuana.” And Quest Diagnostics (one of the country’s giants in the testing industry) says “There are no published, peer-reviewed studies that indicate…that someone would test positive due to ‘passive’ or ‘incidental’ exposure.”

But they also hedge their bets. Later in their explanation, the first company also says “Is it possible to fail a drug test due to second-hand smoke exposure? Yes. Does that mean it’s likely or possible? Not at all.” And the second company reframes its position later in its statement, saying “You very likely (our emphasis, not theirs) won’t fail a drug test from secondhand smoke.”

All of a sudden, those strong statements don’t seem quite as strong. And the National Institute on Drug Abuse isn’t very helpful; it doesn’t address the issue of testing at all, only saying that a contact high from second-hand smoke is “unlikely.”

So what’s the real story? It turns out that actual research (and yes, it’s published research) sheds more light on the real possibility of being nailed, simply because you were in a room where other people were smoking up.

Actual Research on Second-Hand Smoke and Drug Tests

cannabis smoke floating in black background

Some of the statistics used to prove why drug tests won’t come up positive from second-hand exposure are indisputable. For example, the Journal of Analytical Toxicology published a Johns Hopkins study, showing that the amount of THC in the blood of “passive” smokers is about 100 times less potent than in the blood of a nearby smoker.

That’s a strong argument for a dirty blood test for weed being “unlikely.” However, let’s look at studies and research, not an argument.

University of Calgary

The Cummings School of Medicine at the University of Calgary did research into the question as Canada prepared for legalized cannabis. And their study (published in the Canadian Medical Association Journal) found, among other things, that THC is detectable in the body if someone has been exposed to second-hand marijuana smoke in a poorly-ventilated room with the windows closed, for just 15 minutes.

Even more importantly for our purposes, the research found that the THC can be detected by a blood or urine drug test, depending on the levels of THC the test is designed to search for. Even more surprisingly, those levels can be detected for as long as 48 hours. Not all employers use testing procedures that would be triggered by the levels caused by second-hand smoke, but ones who have a “zero-tolerance” policy could quite possibly detect the THC and take punitive action.

John Hopkins University

The Calgary study went one step further than the 2015 research done at Johns Hopkins that we mentioned earlier. Those scientists set up two scenarios: one well-ventilated, one in an unventilated room that they aptly called a “hot box.” In each room, six participants smoked high-potency pot for an hour while six others (half regular smokers, half who’d never smoked) sat in the same room but didn’t smoke.

After the scientific smoke sesh, everyone was tested both for cognitive impairment and for blood or urine THC levels; the test period lasted 34 hours (with some rest allowed, of course). The nonsmokers who were in the well-ventilated room didn’t test positive or show any noticeable effects.

The ones in the hot box, however, reported experiencing a contact high and showed signs of slight impairment – and THC was detectable in their blood for as long as 22 hours after their exposure.

Conclusions That Can Be Drawn

What does all of this mean? Basically, it says that comforting statements by drug testing companies saying you won’t test positive after passive exposure are overstating the case. That’s why they clarify their positions further down in their statements with words like “unlikely.” And those are the words to remember.

It is, in fact, unlikely that you can fail a drug test from secondhand smoke. But if you’re sitting in a smoke-filled room, where everyone else is toking and there’s no real ventilation – yes, you definitely are at risk for a blood or urine test that’s going to take place in the next day or two.

The drug testing companies and university researchers may find that to be an extremely unlikely scenario; in fact, they even describe the hot box experiment as “extreme.” But most of us have been there, at one time or another in our lives (even if we weren’t the non-smokers). It may not be the usual way that someone comes into contact with passive pot smoke – but it’s certainly not impossible to imagine, either.

So what do you do if you’re facing a drug test, the day after you’ve been partying in a room filled with smoke?

How to Clear Second-Hand THC from Your Body

We’ll assume that maybe you’d had a cocktail or two, or were trying to hook up with someone else who was in the smoky room, and didn’t take the common sense step of simply leaving. Or maybe you had no idea you were going to be hit with a surprise drug screening. In any event, you’re left asking “what now?”

If you can’t postpone the drug test (for some reason, employers frown on that), try to flush your system as quickly as possible. Drinking about a half-gallon of water, or liquids that are high in electrolytes like Gatorade, will speed things up and hopefully flush enough THC to let you pass the test. You can also try to mask the THC by taking 100 milligrams of vitamin B-2 or B-12 a couple of hours before the test. The best idea is probably to do both.

In short, if you started feeling a contact high when you were in a smoky room, you probably have enough THC in your system to be in danger of failing a drug test for marijuana – especially if your company has a zero-tolerance policy

And for future reference, if you find yourself in a room where everyone else is getting their party on and you’re not – at the very least, open a window if you plan on staying. As we said at the start, better safe than sorry.

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