But the risk isn't just that someone who is not impaired could be convicted based on fluid detection levels; there's also a risk that impaired drivers might be missed. That's because even though THC remains in the blood for long periods, THC levels drop very rapidly after use. Huestis says that toxicologists have long known this, but her research team set about to determine exactly how quickly those levels dropped. Using human subjects and a driving simulator, researchers looked at not only how drivers performed while stoned, but also how the THC levels in their blood changed over time.
According to Huestis, 30 minutes following the driving test, the THC concentration in subjects was 74 percent lower than it was at the time of driving, and at 1.4 hours it was 90 percent lower. In the U.S., she says, the typical time to get a blood sample for suspected impairment after either a traffic stop or a crash is between 1.4 and four hours.
That's why Huestis thinks per-se laws are inadequate. "It's a terrible number because, if you're an occasional smoker who doesn't have any tolerance, 5 nanograms [a common legal threshold for THC] is going to be below that level in only two and a half hours. And the occasional user is going to still be very impaired at two and a half hours."
Policy makers constantly press Huestis for a number – a content level, like a blood alcohol level – that can definitively say that someone is impaired. "Now that we have done so much research on chronic, frequent cannabis users, I tell them, there is no one number that will clearly distinguish between impaired and non-impaired people," she says.