In breath, researchers found that they could measure THC in picograms (parts per trillion) – that's 1,000 times smaller than the nanograms measured in fluids. "A picogram is a trillionth of a gram," says Joe Heanue, CEO and co-founder of Triple Ring Technologies, a strategic partner of Hound Labs, a THC Breathalyzer developer out of California. "The way to think about that is a raisin weighs about a gram, so picture taking a raisin and cutting it in a trillion parts and trying to detect one of them... That gives you an idea of the kind of challenge we're facing in these devices."
Tara Lovestead, a research chemical engineer for the National Institute of Standards and Technology, has been working on vapor characterization and trace vapor detection for a decade. She says that while law enforcement is eager to have an analogous device to the alcohol breathalyzer for determining cannabis intoxication, THC has a much lower vapor pressure than ethanol. That means it's much more challenging to detect in breath than alcohol.
"Picture cutting a raisin into a trillion parts and trying to detect one of them" says one scientist.
"Not only is the vapor pressure of THC really low, it is predicted to be approximately 100 million times less than ethanol's vapor pressure," says Lovestead. "Thus, a breathalyzer designed to detect THC will have to be extremely sensitive, able to capture a needle in a haystack."
Current THC detection methods include fluid samples, like blood and oral fluid. In addition to being more invasive than breath testing, they pose other challenges. "Alcohol distributes evenly throughout the body and in the water that's present in our blood." says Huestis. THC, on the other hand, has to be carried around the blood by transport proteins. That means that THC concentrations in blood, urine and oral fluids can remain for long periods, and aren't necessarily a benchmark for saying whether someone is high. This is particularly true for daily users. In studies, even when chronic, frequent users had abstained for study purposes, THC remained in their blood for weeks..