A breathalyzer is a portable device commonly used by law enforcement to quickly estimate a person’s blood alcohol concentration (BAC) from a breath sample. This technology relies on a direct and predictable relationship between the alcohol content in the lungs and the blood. While alcohol breathalyzers are a standard, legally accepted tool, reliable devices for detecting tetrahydrocannabinol (THC), the main psychoactive compound in cannabis, are still an emerging technology. The development of a cannabis breathalyzer faces significant scientific and pharmacological hurdles that make it a complex challenge. The core differences between how the body processes alcohol and THC explain why a simple roadside test for cannabis impairment remains difficult to implement.
The Principles of Alcohol Detection
The reliability of the alcohol breathalyzer stems from the unique properties of ethanol, which is highly volatile and water-soluble. When alcohol is consumed, it is absorbed into the bloodstream and circulates throughout the body, including the lungs. As blood flows through the alveoli, the tiny air sacs in the lungs, the alcohol evaporates into the exhaled breath. This process is governed by a consistent physical principle that dictates the concentration of the substance in the air above a liquid.
This consistency allows for a standardized conversion factor. The concentration of alcohol in the deep-lung air has a uniform ratio to the concentration in the blood, typically averaged at 2,100:1. A breathalyzer measures the alcohol in the breath and then multiplies that reading by this ratio to accurately estimate the BAC. The stability and predictability of this blood-to-breath ratio makes the technology rapid and legally reliable for determining intoxication.
Why THC Breath Detection is Difficult
THC presents a fundamentally different challenge because its pharmacological profile is the opposite of alcohol’s. Unlike water-soluble alcohol, THC is highly lipophilic, meaning it is fat-soluble and rapidly leaves the bloodstream to be stored in fatty tissues throughout the body. This characteristic results in very low concentrations of active THC circulating in the blood and consequently in the exhaled breath.
The amount of THC that transfers from the blood into the deep-lung air is minuscule, requiring a detection device with extreme sensitivity. THC lacks a stable and predictable blood-to-breath ratio that is comparable to alcohol’s consistent 2,100:1 factor. The ratio for THC is highly variable and unstable, making it nearly impossible to accurately infer a blood concentration from a breath sample.
A major scientific hurdle is correlating the detected level with actual impairment. Even if a minute amount of THC is detected in the breath, that level does not consistently correlate to a specific degree of cognitive or motor impairment. Furthermore, the small amounts of THC must be distinguished from inactive metabolites or residual contamination, which can linger long after the intoxicating effects have faded. The primary obstacle is reliably linking the detected concentration to the specific window of impairment necessary for legal purposes.
Current Methods for Detecting Impairment
Since a standardized THC breathalyzer is not yet available, law enforcement relies on alternative methods to determine cannabis impairment. The initial assessment often involves Standardized Field Sobriety Tests (FSTs), such as the Walk-and-Turn or One-Leg Stand. While these tests assess balance and attention, their reliability for cannabis impairment is often considered subjective since they were developed for alcohol detection.
Officers specially trained as Drug Recognition Experts (DREs) perform a more thorough 12-step evaluation. This includes checking vital signs, observing physical signs like pupil dilation, and behavioral cues. For objective chemical confirmation, two primary methods are used: oral fluid tests and blood tests.
Oral fluid or saliva tests are non-invasive and quickly detect THC in the mouth, indicating recent use. The most definitive method remains a blood test, which requires a sample to be drawn and analyzed in a laboratory. Blood testing provides the most accurate measure of active THC concentration. However, this process is slow, often taking hours, and the THC level detected may not reflect the driver’s impairment level at the time of the stop.