Can a Negative Breath THC Result Exclude Cannabis Inhalation an Hour Earlier?
A negative result from one breath-collection device cannot categorically exclude cannabis inhalation an hour earlier. In a small impaction-filter study, THC was not quantifiable in every post-use sample. Some samples collected before the experimental use also contained quantifiable THC.
What the collection method captured
Jeerage and colleagues studied 18 participants and analyzed 42 breath samples collected across six batches. Sampling included an intake visit and a later experimental visit with collections before and approximately one hour after participants inhaled a legal-market cannabis flower product.
The device used an impaction filter to collect material from exhaled breath. Participants followed a specified breathing maneuver, and the laboratory measured THC recovered from the collection device. These quantities were reported per device, not as a universally standardized blood-equivalent concentration.
Positive and negative results both need context
THC was quantifiable in 31% of intake extracts, 36% of pre-use extracts and 80% of post-use extracts. The denominators reflect the samples available in each sampling condition. The higher post-use proportion supports further study of the collection approach, but the incomplete recovery prevents treating a negative result as definitive exclusion.
The baseline findings also prevent the opposite shortcut: assuming every positive result proves the experimental episode of use. The requested abstinence was not independently established through continuous observation. The study could not resolve every possible reason for baseline detection.
Collection is part of the measurement
A breathing maneuver, capture efficiency and the amount of breath sampled can influence what reaches the analytical instrument. A result below quantification does not prove that the person exhaled no THC at all. It means the recovered amount did not meet the method’s quantitative criteria.
This was a pilot collection study, not a validation against on-road impairment. Its findings should not be used to assign a safe-driving interval. The separate limits of interpreting blood THC reinforce the broader point: a chemical measurement requires evidence specific to its matrix and intended interpretation.
Primary research
THC in breath aerosols collected with an impaction filter device before and after legal-market product inhalation—a pilot study. DOI: 10.1088/1752-7163/acd410.
