The concept of a “secondhand high,” often called a contact high, refers to experiencing the psychoactive effects of cannabis by passively inhaling the smoke produced by others. This occurs when the active compound, delta-9-tetrahydrocannabinol (THC), is absorbed into the bloodstream of a non-user from the surrounding air. Scientific studies confirm that while trace amounts of THC can enter the body through passive smoke exposure, the likelihood of actually feeling intoxicated is extremely low under typical, real-world conditions. For a non-smoker to absorb enough THC to feel genuinely “high,” a specific and intense set of environmental circumstances must be present, which dictates both the potential for impairment and the risk of detection in drug screening.
Environmental Conditions Required for Exposure
The external environment plays a role in determining whether passive smoke inhalation can lead to measurable THC absorption. A normal setting with open airflow, such as an outdoor space or a large, well-ventilated room, quickly disperses the cannabis smoke, making any significant exposure almost impossible. The THC concentration in the air must reach exceptionally high levels for a non-smoker to inhale a pharmacologically active dose.
The scenario that can facilitate a secondhand high is characterized by extreme conditions, often referred to as a “hotbox.” This involves an extended period of exposure in a small, completely sealed, and unventilated space, such as a closet or a small vehicle. Studies simulating these environments required non-smokers to be exposed for an hour to the smoke generated by multiple high-potency cannabis cigarettes, as modern strains often contain significantly higher THC percentages than those used in older studies.
In these extreme, controlled settings, the air within the space becomes so saturated with smoke that it causes eye irritation and general discomfort. Experiments where ventilation fans were operating showed non-smokers reported no psychoactive effects, confirming that the physical containment of the smoke is the single most important factor. The lack of air exchange allows the concentration of airborne THC to build up enough for the lungs to absorb a sufficient quantity.
Physiological Effects and Impairment
If the environmental conditions are severe enough to force the inhalation of concentrated smoke, the non-smoker can absorb a detectable amount of THC into their plasma. Studies conducted in sealed rooms using high-potency cannabis have shown that non-smokers can achieve plasma THC concentrations up to 7.3 nanograms per milliliter (ng/mL) shortly after exposure. This level confirms that the drug has entered the systemic circulation and is biologically active.
However, this passively absorbed concentration is substantially lower than the levels associated with active intoxication. For an individual to experience a “peak high” from direct use, plasma THC concentrations typically range from 75 ng/mL to over 165 ng/mL. The mild intoxication reported by non-smokers in controlled studies, described as feeling tired or less alert, corresponds to the lower-end plasma concentrations achieved through passive inhalation.
Researchers have used cognitive and motor performance tests to measure objective impairment following extreme passive exposure. While some participants showed mild deficits on tasks like measures of sustained attention, the level of impairment was consistently minimal. This suggests that while a non-smoker may subjectively feel a slight psychoactive effect under the most intense conditions, they are unlikely to reach the level of cognitive or motor impairment seen in active users.
Detection on Standard Drug Tests
A major concern for individuals exposed to secondhand smoke is the possibility of failing a drug test. Standard workplace drug tests typically screen for the non-psychoactive metabolite, 11-nor-9-carboxy-THC (THC-COOH), rather than the active THC compound itself. The presence of THC-COOH confirms that THC has been processed by the body.
For urine drug screens, a common cut-off concentration for a positive result is 50 nanograms of THC-COOH per milliliter of urine (50 ng/mL). Under realistic, well-ventilated conditions, passive exposure is highly unlikely to cause a positive result at this standard cut-off level. However, laboratory studies simulating the most extreme passive exposure—an hour in an unventilated space with high-potency cannabis—did produce maximum THC-COOH concentrations in non-smokers ranging up to 57.5 ng/mL.
This means that a non-smoker subjected to a worst-case “hotbox” scenario could potentially test positive, especially if the testing laboratory uses a lower initial screening cut-off of 20 ng/mL. A positive result at the 50 ng/mL standard is rare and requires an extraordinary, concentrated exposure. For oral fluid (saliva) tests, the risk of a positive result from passive inhalation is limited to a very short window, often less than 30 minutes, due to the rapid clearance of airborne THC particles from the mouth.