“Typically, if you have five nanograms in a regular smoker, you probably won't see any behavioral effects,” said Carl Hart, a professor of psychology and psychiatry at Columbia University. “Whereas, with five nanograms in someone who’s never smoked, you might see a lot of effects.”

With breath, Lynn explains, the THC signal only sticks around for a few hours. Some studies have found that THC stays on a person’s breath for as long as four hours. The company doesn’t yet know if the breathalyzer can detect if someone ate a marijuana product recently.

The path to creating a breath test has not been simple or easy. Lynn said THC is found in quantities “somewhere between a million and a billion times less concentrated” than alcohol in breath. To create a test that could isolate THC in breath and measure it accurately, Lynn enlisted the help of a team of scientists led by Dan Fletcher, a professor at the University of California, Berkeley who specializes in bioengineering. They looked at everything from large devices used to detect small quantities of substances to scanners that look for residues found in explosives to try and find a solution. Eventually, they figured out how to filter breath through the right chemicals to create a reaction so a device can tag and count THC molecules.

Lynn wouldn’t discuss the actual chemistry, saying that it was “proprietary.” He said the process has been successfully tested in the lab, and that his company will start trials at the University of California, San Francisco, and San Francisco General Hospital early next year. They will also be testing versions of the breathalyzer with law enforcement around that time, and they hope to have the product ready for deployment by the end of 2016. Initial law-enforcement tests will be carried out in Oakland and Berkeley.

Hart expressed skepticism about the idea behind the technique, calling it a “dumb idea.” “The chemistry of alcohol is such that there is essentially no blood-brain barrier for alcohol … so what is in your lungs or blood is the same amount, typically, as what’s in your brain,” he said. “With marijuana or any other drug, we can’t do that. We have no idea what's in the brain based on some measure in your mouth or in your lungs.” He said the THC doesn't transport itself along the blood-brain barrier like alcohol, so what’s in the breath probably doesn't represent what’s in the brain.

Many outside of Hound Labs are concerned people who may have smoked recently, for medical purposes or otherwise, might get labeled as intoxicated when they aren’t. “If you were to take somebody that is a heavy cannabis user, they’re certainly going to have a much higher test result than someone who occasionally uses it … How do you quantify that?” said Jason Thomas, a former Denver detention officer and Marshal’s Deputy. Tolerance will also be difficult to assess.