A contact high—also known as a secondhand high—is the idea that secondhand exposure to cannabis smoke can get a person stoned or even leave them with THC in their system. For nonsmokers, this can lead to worries: Can you fail a drug test from a contact high? Can you wind up accidentally stoned?

These are reasonable questions to ask, and fortunately, we have a pretty good idea of how a contact high works.

THC in the air

When cannabis smoke or vapor is inhaled, cannabinoids like THC are absorbed into the lungs and enter the bloodstream. But how much THC do we actually absorb? Well, that depends.

Researchers from The British Journal of Anesthesia in 1999 reported, “Approximately 50% of the THC and other cannabinoids present in a cannabis cigarette enter the mainstream smoke and are inhaled. The amount absorbed through the lungs depends on smoking style. In experienced smokers, who inhaled deeply and hold the smoke in the lungs for some seconds before exhaling, virtually all of the cannabinoids present in the mainstream smoke enter the bloodstream.”

You may be thinking that 50% may not sound like much, but a more recent study suggests it could be even less. A 2005 study titled “Pharmacokinetics of cannabinoids” found that, “the variability of THC in plant material (0.3% to 30%) leads to variability in tissue THC levels from smoking, which is, in itself, a highly individual process. THC bioavailability averages 30%.”

In other words, studies show that inhaled cannabis smoke does not allow for 100% bioavailability, and while length and duration of the inhale may possibly play a role in absorption, exhaled cannabis smoke might still have THC present.

So we know secondhand cannabis smoke can maintain cannabinoids, but is it enough to get someone stoned? Or even affect a drug test?

THC in the bloodstream

There are a fair amount of older studies from the 1980s that investigate secondhand highs, but cannabis has changed since then. Cannabis is far more potent, and so the conclusions they draw are not as relevant today. Fortunately, some updated research does exist.

A 2015 study titled “Non-Smoker Exposure to Secondhand Cannabis Smoke II,” got right to the point when studying the effects of contact highs. Researchers recruited 12 participants, half of whom were smokers, and half of whom were nonsmokers. In the first experiment, they had them enter a small, unventilated chamber and proceeded to give each smoker 10 joints, each with a THC content of 11.3%. Over the course of an hour, the smokers consumed the joints while sitting around a table with the nonsmokers. After the hour was up, all participants exited the chamber, discarded protective clothing, washed their hands and faces, and proceeded to a separate room to complete the study assessment.

In the second experiment, researchers repeated the conditions, with one exception: the chamber was now ventilated.

Researchers found notable differences between the two experiments. In the first, where subjects were essentially hotboxing with no ventilation, the nonsmokers did in fact have detectable levels of THC in blood and urine tests. In addition, researchers wrote, “Exposure to secondhand cannabis smoke in an unventilated chamber the size of a small room produced minor increases in heart rate, mild to moderate subjective drug effects, and minor, but detectable, levels of performance impairment on some behavioral/cognitive assessments.”

In other words? Under very extreme conditions, the nonsmokers did in fact seem to get high, and even had traces of THC in their system.

Interestingly, in the second study, ventilation made all the difference. Researchers reported, “Nonsmokers in the ventilated session did not have detectable levels of cannabinoids in blood beyond the initial 30 minutes following the exposure period, did not screen positive on urine tests, and did not report significant increases in subjective drug effects.”

Be considerate of your smoke

All in all, research makes the case that contact highs are in fact real, but considering the circumstances, you may not have a lot to worry about. Yes, if you enter a small, unventilated space filled with cannabis smoke, you very well may feel the effects and have justified worries about cannabinoids showing up on a drug test. However, if you’re in a well ventilated space or room with someone smoking, or smell some weed walking down the street, you don’t need to worry.

That said, as a smoker, it’s important to be considerate of others. It’s never alright to expose others to secondhand smoke against their will. So keep children and pets away from smoke, and if you’re going to spark up, be polite and make sure you’re doing it somewhere ventilated and where nonsmokers have the option to step away if they like.

Rae Lland

Rae Lland is a freelance writer, journalist, and former editor for Weedist and The Leaf Online. With a focus on culture, music, health, and wellness, in addition to her work for Leafly, she has also been featured in numerous online cannabis publications as well as print editions of Cannabis Now Magazine. Follow her on Instagram @rae.lland