Secondhand cannabis smoke (SHS) is the mixture of smoke from a burning cannabis product and the smoke exhaled by a user. As cannabis use becomes more common, a frequent concern is whether passive inhalation of this smoke can lead to a person feeling intoxicated or cause a positive result on a drug test. This concern stems from the fact that SHS contains tetrahydrocannabinol (THC), the main psychoactive compound in cannabis. Scientific evidence has explored the degree to which this passive exposure affects non-users, looking closely at the conditions necessary to produce an effect.
The Potential for Psychoactive Effects
The possibility of experiencing a subjective “high” from secondhand cannabis smoke depends almost entirely on the environment and the duration of exposure. Under typical, well-ventilated social conditions, the THC in the smoke is rapidly diluted in the ambient air, making the concentration too low to cross the threshold for impairment. Researchers have found that non-smokers who spent hours in a well-ventilated room with people casually smoking cannabis showed detectable but very low levels of THC in their blood, insufficient to cause impairment.
However, studies simulating extreme conditions show that a psychoactive effect is possible. When non-smokers were exposed to high-potency cannabis smoke in a small, completely unventilated space for an extended period, they reported mild subjective effects, sometimes described as a “contact high.” This scenario, sometimes called “hot-boxing,” resulted in the non-smokers displaying mild impairments on motor tasks.
The strength of the psychoactive effects reported by passive inhalers increases with the THC content of the cannabis being smoked. The effects experienced by non-users are consistently weaker than those reported by active smokers, but the pattern of intoxication is similar.
In everyday, open-air, or even moderately ventilated settings, the concentration of THC inhaled passively is generally too low to cause intoxication. This indicates that only under very specific and intense exposure conditions can enough THC be absorbed to produce noticeable mind-altering effects in a bystander.
The Risk of Failing a Drug Test
The primary concern for many non-users is the risk of failing an employment or legal drug screening. Most standard urine drug tests look for THC metabolites, specifically THC-COOH. These tests are designed with a specific cut-off threshold to minimize false positives from environmental exposure, with the most common cut-off for initial screening being 50 nanograms per milliliter (ng/mL).
Research indicates that casual exposure to secondhand smoke is highly unlikely to result in a positive test at this 50 ng/mL level. One study placed non-smokers in an unventilated, sealed room with heavy cannabis smoke for an hour, and none of the participants tested positive using the standard workplace threshold. However, under the most extreme, unventilated conditions using high-potency cannabis, some non-smokers’ urine samples have shown levels that approach or briefly exceed the threshold immediately following exposure.
The risk of a positive result hinges on the test’s sensitivity and the environment of exposure. While detectable amounts of THC metabolites can be found in the blood and urine of passively exposed individuals, these trace amounts are usually below the cut-off levels designed for workplace screening. Therefore, a positive result from secondhand smoke is rare for the majority of people under normal circumstances, although the risk increases significantly with prolonged exposure in highly confined spaces.
Environmental Factors Determining Exposure
The amount of THC a non-user absorbs from secondhand smoke is heavily influenced by several environmental and procedural variables. Ventilation is the most significant factor, as open-air exposure or a room with circulating air dramatically reduces the concentration of THC in the atmosphere. In contrast, enclosed spaces, such as a small car or a sealed room, allow the smoke and its compounds to accumulate, greatly increasing the potential for absorption.
The concentration of THC in the smoke is also determined by the duration of the exposure and the proximity of the non-user to the source. Spending a long time close to the smoker leads to higher levels of absorption than a brief, distant encounter. Furthermore, the potency of the cannabis product being consumed directly correlates with the amount of THC released into the air and, subsequently, the body of the passive inhaler.
Other variables, such as the volume of the space and the number of cannabis products being smoked simultaneously, also mediate the effects of passive exposure. Understanding these variables is key to assessing the actual risk of both psychoactive effects and a positive drug test result.