A “contact high” refers to the involuntary passive inhalation of cannabis smoke that results in psychoactive effects for the non-user. Scientific evidence indicates that a true, noticeable high from secondhand smoke is extremely rare under typical social circumstances. The possibility of absorbing enough psychoactive material to feel impaired depends entirely on a combination of extreme environmental factors and the potency of the cannabis being used. Understanding the pharmacological hurdles of passive inhalation helps clarify why this phenomenon is not a common occurrence.
The Pharmacology of Secondhand Cannabis Smoke
The main compound responsible for the psychoactive effects of cannabis is Delta-9 tetrahydrocannabinol (THC). When cannabis is smoked, THC is present in both the mainstream smoke inhaled by the user and the sidestream smoke released into the surrounding air. For a contact high to occur, a sufficient amount of this airborne THC must successfully transfer from the smoke, through the passive inhaler’s lungs, and into their bloodstream.
The bioavailability of THC, or the amount that reaches the systemic circulation, is significantly lower from passive inhalation than from active smoking. In ambient air, the THC quickly disperses and degrades, presenting a major physiological hurdle to absorption. When absorbed, the THC is rapidly metabolized, first into the psychoactive compound 11-hydroxy-THC and then into the non-psychoactive carboxy-THC (THC-COOH).
Conditions Required for Significant Exposure
Achieving a high enough concentration of THC in the air to cause impairment requires specific, extreme environmental conditions. Scientific studies designed to test the limits of passive exposure often simulate a “hotboxing” scenario. This involves placing non-smokers in a small, sealed chamber with multiple people actively smoking high-potency cannabis for an extended period, typically one hour.
In these unventilated, high-concentration settings, researchers have demonstrated that non-smokers absorb detectable levels of cannabinoids in their blood. Conversely, when the same experiment is performed with adequate ventilation, the cannabinoid levels in the non-smokers’ blood drop substantially, often becoming negligible. The volume of air, the number of users, the duration of exposure, and the lack of air exchange are the primary factors determining significant passive exposure.
Assessing Impairment and Subjective Effects
Under the most extreme, unventilated conditions, non-smokers in research studies have reported minor to moderate subjective drug effects, such as mild sedation. This exposure is also associated with minor, measurable cognitive impairment, such as a reduced performance on psychomotor tasks requiring working memory and attention. These effects are consistently described as significantly weaker than those experienced by the active smokers in the same room.
In common, real-world social settings, the air volume and natural ventilation prevent THC from concentrating to psychoactive levels. Any perception of a “high” in a well-ventilated or open environment is often attributed to the individual’s expectation or anxiety, which is known as a placebo effect. Measurable cognitive impairment is only observed when the exposure is so dense and prolonged that it would be immediately obvious to the exposed individual.
Contact High and Drug Testing Results
A major concern is whether passive exposure can lead to a positive result on a drug test. Drug screening typically targets the non-psychoactive metabolite THC-COOH in urine. Standard workplace or legal testing uses a cutoff threshold, often 50 nanograms per milliliter (ng/mL), to distinguish active use from environmental exposure.
Studies show that even under the extreme conditions required for a contact high, non-smokers’ THC-COOH levels rarely exceed this common 50 ng/mL threshold. While trace amounts of the metabolite are detectable, a positive result at the standard cutoff is exceptionally rare. A positive test from passive exposure is almost always limited to the hours immediately following a prolonged, unventilated exposure scenario. Forensic analysis can distinguish between the low, transient levels from passive inhalation and the higher, sustained levels found after active consumption.