Considering the many opportunities we now have to inhale passive marijuana smoke, from walking along the sidewalk to sitting in a park to attending a concert, it’s natural that people are asking whether second-hand smoke can get them high. Those who need to test clean on drug screens for work, and for other reasons may also have questions about second-hand smoke. Whether being around smoke could cause them to fail a workplace drug test and how to limit exposure.

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The Second Hand High

Imagine you’re on a long car trip and it’s cold outside, so the windows are closed, and another passenger lights up a marijuana cigarette. You’re likely to get a second hand mild contact high from the smoke under these “extreme” conditions, but it’s going to take more than walking past a smoker on the street to feel any significant side effects. Unless you plan on sitting in a small room with the door closed or the aforementioned car with the windows rolled up, you’re unlikely to get high. Some of the other long-term effects of second-hand smoke exposure are still up for debate, including potential harm to the lungs. Those who have to get screened for drug use for their work or for other reasons are advised to stay away from heavily smokey or unventilated spaces to be safe because THC can show up in certain tests following such exposure. If you’re looking to get more than a contact high, second-hand exposure isn’t going to do the trick.

Smoking young man is losing his head by smoke iStock / alice in otherland

How Does Cannabis Smoke Cause a High?

When someone smokes marijuana, THC, the intoxicating ingredient, passes from the lungs into the bloodstream and the blood then carries the chemical to the brain and other organs, according to the National Institute on Drug Abuse. The THC over-activates parts of the brain with a high number of receptors that ordinarily react to natural THC-like chemicals and this is responsible for the high. The effects of a high depend on how much marijuana is smoked, whether a person has eaten recently and other factors. It can range from an altered sense of time, euphoria, impaired body movement and changes in mood. According to a 2017 literature review, there’s evidence that the THC in marijuana causes effects on people passively exposed, but the degree to which non-smokers are affected depends on several factors, including the amount of smoke in the environment, the number of cigarettes lit, air volume, ventilation and the number of smokers present.

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What the Research Says

A small 2010 study from the Netherlands looked at secondhand smoke exposure in what the authors called “real-life conditions.” Eight volunteers were exposed to cannabis smoke for three hours in a well-attended coffee shop and had blood and urine samples tested both before and after exposure. The results showed that all the volunteers absorbed THC, but the concentrations were small. After six hours, THC wasn’t detectable anymore in blood. A 2015 study from Johns Hopkins looked at several “worst-case scenario” situations, including an enclosed vehicle. They found that secondhand exposure under these conditions can cause nonsmokers to feel the effects of the drug, have minor problems with memory and coordination, and even test positive for the drug in a urinalysis in some cases. In the experiment, six smokers and six nonsmokers spent an hour sitting in a 10 by 13-foot room in different sessions, including one with ventilation fans turned on and one without fans. Following exposure, blood, urine, saliva, and hair were tested for THC at regular intervals. Researchers found that nonsmokers in the unventilated room had detectable levels of THC in urine and blood.

Young People Smoking Weed At Home On Sofa iStock / monkeybusinessimages
The good news for people who aren’t looking to passively get high is that the kind of exposure needed for THC to show up in tests is so obvious you’d be aware and could leave the area. Lead author Even S. Herrmann, a postdoctoral fellow in psychiatry and behavioral sciences at Johns Hopkins, said in a report on the Johns Hopkins website: “the scenario we looked at was almost a worst-case scenario. It could happen in the real world, but it couldn’t happen to someone without him or her being aware of it.” The authors said their report is the most comprehensive study of secondhand smoke effects since the 1980s, when researchers found that THC could be detected in nonsmokers’ bodies after an hour in “extreme” conditions with heavy smokers in an unventilated space. The study had a few limitations, including its small size and lack of a placebo trial, where participants would be exposed to cannabis that had no THC. The 2017 review published in CMAJ Open noted that the simulated environments in some of the studies weren’t representative of what people would likely face in the real world, outside of smokey cars and closed rooms. “In the context of legalization, people may be exposed to second-hand marijuana smoke outside, in parks or in passing on the sidewalk. This type of exposure may not result in cannabinoid metabolites in bodily fluids, as the exposure may be shorter and less intense than in unventilated areas,” the report notes. It’s a good bet that breathing in momentary smoke in the park won’t cause you to fail your drug screen or get you high, but the authors do recommend that people with respiratory ailments and children limit their overall exposure, especially in more enclosed spaces. That means if you want to smoke at home, open the windows and kick the kids and pets outside.