While the risks and dangers of secondhand smoke from cigarettes and tobacco are well-known, medical research has yet to explore the effects of secondhand smoke from cannabis flower and other products. Accidental and/or undesired inhalation of secondhand smoke from cannabis has been theorized to induce a contact high or a ‘second-hand high,’ but is such a phenomenon scientifically possible?

For non-users of cannabis, the risks of experiencing a contact high, however slim, can be especially problematic. If you’ve ever wondered whether or not a contact high is plausible, here’s what current scientific literature has to say about the phenomenon.

What is a Contact High?

A contact high, also known as a secondhand high, is the idea that non-users of cannabis can experience the effects of THC or embed THC within their bodies from inhaling the secondhand smoke produced by cannabis flower and other cannabis products.

While the possibility is daunting, the reality is less discouraging.

Can Secondhand Cannabis Smoke Affect You?

Before diving into whether or not a contact high is possible, it’s important to understand how cannabis affects the body.

THC within cannabis induces its psychoactive effects by acting on the human body’s endocannabinoid system. The ECS is an endogenous bodily system that mediates everything from our hunger and energy levels to our libido and mood by creating, managing and removing endocannabinoids within the body.

In the case of smoking or vaping cannabis, the THCA within cannabis undergoes decarboxylation to transform into its more bioavailable variant, THC.

The THC, terpenes, and other cannabinoids and compounds found within cannabis are inhaled into the lungs, absorbed into the bloodstream and assimilated into the ECS through its endocannabinoid receptors.

When THC is ingested, it’s processed by the liver through a process known as first-pass metabolism, which transforms THC into 11-Hydroxy-THC, a metabolite of THC that produces more potent effects by penetrating the blood-brain barrier more quickly and effectively.

Airborne THC

Cannabis produces its effects by entering into the lungs and being absorbed into the bloodstream. Still, a prior study published in the British Journal of Anesthesia in 1999 found that only an estimated 50% of THC and other cannabinoids are inhaled and absorbed. A more recent study published in 2005 in the Journal of Pain Research and Management pegs the bioavailability and absorption rate of THC at a much lower percentage of just 30%.

So, given the low absorption rate of THC from inhaled cannabis smoke, exhaled cannabis smoke can still retain some THC, but is this enough to enter into the bloodstream to induce a contact high?

THC in the Bloodstream

In August 2018, mere months before cannabis legalization in Canada, a systematic review of second-hand cannabis smoke was published by the Canadian Medical Association Journal produced findings that suggested more research into second-hand cannabis smoke was needed before the medical community can say anything conclusive about its effects.

According to the report, an individual exposed to second-hand cannabis smoke in a closed and poorly ventilated room could test positive for THC, with amounts being detectable in the body after just 15 minutes of second-hand exposure.

While the review did not reproduce a real-world exposure environment, Dr. Clement, the study’s lead researcher, described a comparable real-world exposure environment in an interview with Toronto City News as an “…enclosed space maybe the size of a regular kitchen, all of the windows closed, relatively poor ventilation, and a joint being passed around while you’re having a conversation. “

Dr. Clement went on to say that “people who are passively exposed, so not smoking themselves, can test positive in their blood or urine for THC levels above 5 nanograms per millilitre, which is being put out as the legal limit, with relatively modest exposure conditions.”

A similar study conducted in 2015 by the Drug and Alcohol Dependence Journal had researchers recruit six non-smokers and six cannabis smokers to measure their body’s THC levels after being exposed to cannabis smoke for an hour in an unventilated chamber.

The researchers ran two experiments; one where non-smoker participants sat with smoker participants as they consumed cannabis cigarettes for an hour in an unventilated room and another where they repeated the experiment in a ventilated room.

In the first unventilated experiment, researchers discovered detectable THC levels in both the blood and urine of the nonsmoker group. Additionally, researchers also found that the nonsmoking participants experienced a minor increase in heart rate, mild to moderate subjective drug effects, and minor but detectable performance impairment levels on some behavioural and cognitive assessments.

In the second ventilated experiment, researchers failed to find detectable levels of THC in both their bloodstream and urine. Participants in the second experiment also did not report any increases in subjective drug effects.

Is a Contact High Real? Being Cannabis Considerate

Based on the two experiments referenced above, a contact high is plausible. However, it should be noted that in both studies, participants were only able to get ‘high’ and have detectable levels of THC within their bodies only when they were exposed to cannabis smoke in extreme conditions.

So, according to these two very rudimentary studies, a contact high is indeed real. However, a statement issued by The Canadian Lung Association affirms that “there are still many unknowns about cannabis and its long-term effects on lung health, but we do know that the inhalation of smoke is harmful to lung health as the combustion of materials releases toxins and carcinogens” and acknowledged that more research is needed before any conclusions can be reached.

Whether a contact high is possible or not, cannabis users should be considerate about where, when and with who when they are consuming cannabis. Exposing others to secondhand smoke against their will is never acceptable and, depending on where you are, might be against Canada’s federal cannabis regulations.