Researchers have recently figured out a way to measure the vapor pressure of THC, the main chemical found in marijuana. This discovery means that someday law enforcement might be able to test to see if someone is under the influence of marijuana based on a person’s breath.
Right now, police officers utilize urine and blood tests to detect THC, which are reliable methods that can detect if you have used a drug in the past. They aren’t very good at figuring out if you are currently impaired by THC. Some states do currently allow blood testing suspected DUI drivers. But traces of THC can stay in your blood stream for up to a week, meaning if you smoked on Sunday and get pulled over on Tuesday for a DUI, you could potentially test positive for THC.
Researchers have been working on developing a reliable THC Breathalyzer for some time and they are definitely not there yet. The science behind vapor testing is ongoing. To understand the science, imagine a half empty bottle of liquor. Some of the alcohol is in the liquid, while there is also alcohol molecules in the air space above. If you know how much pressure is produced by the vapor, you can figure out how much alcohol is left in the bottle based on the properties of both gases. This same logic applies to THC as it leaves the body when one exhales.
The biggest question is how long can THC vapor be detected after you have smoked. The initial research has demonstrated THC vapor can be detected from thirty minutes and up to two hours. Interestingly, since marijuana is considered a Schedule I by the DEA, it is quite difficult to conduct research on the drug.