A “contact high” refers to the involuntary inhalation of secondhand cannabis smoke, leading to the concern that this passive exposure might cause a positive result on a drug test. Under typical social conditions, the amount of tetrahydrocannabinol (THC) absorbed by a non-user is minimal, making a positive result extremely unlikely. This low likelihood is primarily due to drug tests utilizing high cutoff thresholds designed to filter out trace environmental exposure.
Scientific Basis of THC Detection
Standard drug tests measure a specific non-psychoactive byproduct known as THC-COOH, or 11-nor-9-carboxy-THC, rather than the psychoactive compound THC itself. After THC is absorbed, the liver metabolizes it into this compound, which is stored in fat cells and excreted in the urine over time. THC-COOH is the primary molecule laboratories screen for because its presence confirms the body has processed THC.
Drug testing protocols incorporate “cutoff thresholds,” which are specific concentration limits that must be exceeded for a result to be considered positive. For the common urine test, the standard initial cutoff is 50 nanograms per milliliter (ng/mL) of THC-COOH. This threshold is set high enough to ensure that trace amounts absorbed from passive inhalation are not flagged as active use. A result below this threshold is reported as negative.
Passive Exposure vs. Active Use
The difference in absorbed THC concentration between actively smoking and passive exposure is substantial. Active users inhale high concentrations of THC directly, leading to peak blood levels ranging from 75 to 165 ng/mL, which metabolizes into significant amounts of THC-COOH. Passive exposure involves inhaling smoke diluted by the air, containing only a fraction of the original THC.
Studies show that when non-users are exposed to cannabis smoke, the levels of THC and its metabolites in their blood and urine remain extremely low. The amount of THC absorbed through passive exposure is often less than 100 times the amount absorbed through active use. While a sensitive test might detect trace amounts of THC-COOH, these levels generally fall far below the standard 50 ng/mL cutoff threshold for a positive result.
Conditions Required for a Positive Result
A positive result from casual passive exposure is highly improbable, but it is not scientifically impossible, requiring extremely specific conditions. For a contact high to register above a standard cutoff, the exposure must take place in an environment with minimal or no ventilation. The room must be small and sealed, allowing the smoke concentration to build up significantly over a prolonged period.
Controlled experiments resulting in positive tests involved non-users exposed to the smoke from multiple cannabis cigarettes (up to 16) for an hour or more in a small, unventilated chamber. In these highly concentrated conditions, a few participants produced urine samples that slightly exceeded the 50 ng/mL cutoff, but only for a few hours immediately following exposure. These conditions are rarely encountered in typical social settings where ventilation is usually present.
Test Type Sensitivity to Contact High
The likelihood of a positive result from passive exposure is influenced by the type of drug test administered. The urine test, the most common screening method, detects the long-lasting metabolite THC-COOH. It relies heavily on the established cutoff threshold to prevent false positives from environmental exposure. While a positive result is unlikely at the standard 50 ng/mL cutoff, the risk increases if a lower cutoff, such as 20 ng/mL, is used.
Saliva tests measure the parent THC compound rather than the metabolite, reflecting very recent exposure (usually within 24 to 72 hours). Since the parent THC compound is quickly cleared from the oral cavity, passive smoke inhalation is unlikely to leave enough residual THC to trigger a positive result unless exposure was immediate and highly concentrated. Hair follicle tests measure drug use over months as metabolites are incorporated into the growing hair shaft. These tests are virtually impervious to passive exposure, as the mechanism requires a systemic concentration level from sustained use that secondhand smoke cannot provide.