A number of states have legalized marijuana for recreational purposes in recent years and more states may follow in the coming year. Law enforcement is now faced with a new dilemma, how to tell when drivers are "over the limit." If a suspect has had too much to drink, an officer can use a breath test to determine the suspect's blood alcohol content. If a police officer has pulled over a person that is suspected of driving under the influence of marijuana, there is no quick test the officer can give to determine how high that person is. However, this may soon change as police departments across the country are testing out new methods to detect THC levels.

According to the National Highway Traffic Safety Administration, THC is the ingredient in marijuana that is "believed to be responsible for most of the characteristic psychoactive effects of cannabis." Currently, in order to determine a driver's THC level, law enforcement can obtain a warrant for a blood sample. But, as NPR reported, "it can take weeks for results to come back from the toxicology lab." Upon hearing about the problem law enforcement is facing Washington State University professor Herb Hill he asked, "Why don't we have a Breathalyzer for that?" He then set to work on developing just such a device.

The technique that Hill and his graduate student Jessica Tufariello came up with in order to measure the THC levels in a person's blood uses differential mobility spectrometry (DMS). According to Bloomberg, this "generates two electric fields designed to keep THC ions flowing to a sensor at the end of a tube." The researches first tested a pure THC sample and detected the THC using the device. In order to test for false positives, Tufariello would eat and drink different types of food to see if any of those food produced similar signals to THC. So far, none have.

Testing out a marijuana breathalyzer was a bit tricky for Hill and Tufariello because of federal restrictions, as marijuana is still illegal under federal law. The test subjects could only be paid for their time and the researchers couldn't provide any weed to the subjects. In addition, the testing could not take place on campus. However, they worked out a system. Tufariello would test volunteers before and after they smoked. The volunteer would call her up and she would head over to grab a sample. In addition, the test subject who went to grab the weed from the dispensary got additional compensation for being "a more dedicated volunteer." Subjects all smoked the same type of weed, a popular strain called Blue Dream. According to Bloomberg, "[t]he DMS analysis identified THC in 81 percent of the samples."

Then the company that is funding the research, Chemring Group, sent over new equipment to help them continue testing including a "breath-capturing device that looks like a glue gun." The researchers repeated the tests and found even more promising results. In the second round, "THC was detected 89 percent of the time." There are still more tests that need to be conducted. The next tests that the researchers are conducting is "to correlate the THC levels found in the lab with blood samples drawn at a hospital." These tests will tell Chemring what is the "smallest amount of THC we can detect." The company hopes to have a marijuana breathalyzer prototype that police officers can start field testing in the fall.