You’ve probably heard the term “secondhand high” before. Also known as a contact high, the concept has become a popularized plot point in films and TV shows. You may have even been to a smoky concert hall yourself and walked away feeling a little lightheaded, even if you never took a single puff.

This could be concerning for those who fear that exposure to secondhand weed smoke could get them involuntarily stoned or cause them to fail a drug test. Therefore, it’s important to know whether second-hand cannabis smoke can get you high or enter your system.

So, do you have to inhale weed to get high? Or can you really get a secondhand high from being around other people smoking cannabis? Is it really a thing?

According to a 2015 Johns Hopkins University study — the answer is both yes and no.

Researchers started with a dozen people — six cannabis smokers and six non-smokers. In the first experiment, all 12 subjects spent an hour together in a small unventilated room, during which time each smoker went through 10 “high-potency” joints (with 11.3% THC content). Afterward, the non-smokers reported feeling “pleasant,” more tired, and less alert. And sure enough, their blood and urine tests came up positive for THC.

The second experiment repeated the scenario, but this time in a room with ventilation. The non-smokers in this experiment later said they felt “hungry” — the study did also finish up around lunchtime — but none of them tested positive for any noticeable amount of THC.

Researchers concluded that being exposed to marijuana smoke under “extreme conditions” can indeed give non-smokers a contact buzz. Outside of that very limited scope, though, any secondhand effects you might feel around cannabis smoke are likely to be the result of the power of suggestion. You can’t get high from catching a whiff of someone’s joint while walking down the street, but you will feel some effects if you are sitting in an unventilated enclosure filled with smoke, also known as hotboxing.

In other words, if you spend a lot of time in a small room with the windows sealed shut while your friends are smoking, your blood and urine might test positive for THC and you might feel its effects. But outside of that, it is more likely that your “contact high” is all in your head, so to speak.

Is THC active after cannabis smoke is exhaled?

If we pull a page from the 1999 British Journal of Anesthesia, we learn that the lungs absorb most of the THC when cannabis smoke is inhaled. Researchers have discovered that approximately fifty percent (50%) of THC and other cannabinoids present in cannabis cigarettes, or joints, make it into the smoke and are inhaled.

“Experienced smokers, who inhale deeply and hold the smoke in the lungs … virtually all of the cannabinoids present in the mainstream smoke enter the bloodstream,” leaving very little THC in the surrounding air to be inhaled and absorbed by a passive inhaler.

When considered together, this and the 2015 Johns Hopkins study show you would need to be in an unventilated room for some time to feel anything. More than likely, though, the cannabinoids will have disappeared into the air before even reaching you.

Are there any other studies on secondhand highs?

Studies performed during the mid- to late-1980s investigating the mystery of the secondhand high determined that the acute toxicity of cannabis was extremely low, therefore making it difficult to feel the effects without direct inhalation. While their conclusions may still apply, cannabis has changed over the years and the studies may need reexamination.

The THC potency of cannabis has increased as cultivation techniques and technologies have advanced. In the early 1970s, the average joint contained roughly 10 mg of THC, whereas a modern joint may contain 60-150 mg of THC or more. The THC potency in today’s marijuana flowers is far greater than the weed from the 1960s and 1970s, therefore much of the early research produced from studying secondhand highs may be outdated.

While there’s an abundance of research demonstrating the adverse health effects of secondhand exposure to cigarette smoke, there is little evidence to suggest that secondhand marijuana smoke carries the same detrimental health risks as tobacco smoke. Considering that past research has found marijuana smoke to be less carcinogenic than cigarette smoke, there’s doesn’t seem to be an immediate public health concern regarding secondhand weed smoke. That being said, as long as you’re not stuck in a poorly ventilated room during a heavy smoke session, you shouldn’t be concerned about feeling stoned or having THC enter your system.

Can you get high from smelling weed in the air or walking through the remnants of secondhand weed smoke? No, it’s highly unlikely you’ll experience a secondhand high or have cannabis byproducts show up on a drug screening. As the 2015 Johns Hopkins University research study shows, in order to catch a secondhand high, you’d have to be under extreme conditions and lack proper ventilation.

Nonetheless, weed smokers should still be respectful of people who don’t consume cannabis. The next time you spark one up, try to be aware of your surroundings and make an attempt to keep the smoke and strong odor away from non-smokers. To enjoy a smooth smoking session without affecting your non-partaking neighbors, cannabis users should spark up in well-ventilated areas to ensure passive inhalers will not feel the effects of the smoke or test positive for weed.