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A team at Washington State University is working to develop a breath test that could quickly determine whether a driver is under the influence of marijuana.

Law-enforcement officers already use preliminary breath tests in the field to estimate drivers’ blood alcohol content. But no similar portable tool exists to test for marijuana impairment using a breath sample.

Stoned drivers have become an increasing concern since Washington voters legalized recreational use of marijuana in 2012. A quarter of blood samples taken from drivers in 2013, the first full year the initiative was in effect, came back positive for pot.

WSU chemistry Professor Herbert Hill said that existing technologies — including those already used by airport security and customs agents to detect drugs and explosives — can be re-purposed to test breath for THC, the psychoactive component of marijuana.

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Hill said he and WSU doctoral student Jessica Tufariello are working on a handheld device that uses a technique called ion mobility spectrometry to detect THC in someone’s breath.

Right now, officers and prosecutors rely on blood tests to determine how much active THC is present in a driver’s blood. Those test results aren’t immediately available to patrol officers who suspect someone is driving high.

Initiative 502 set 5 nanograms of active THC per milliliter of blood as the legal limit at which a driver is automatically determined to be impaired.

Initially, the marijuana breath test under development at WSU probably won’t be able to pinpoint the level of THC in the body; it will only tell officers that some active THC is present, Hill said.

Still, Hill said such a tool could prove helpful to officers as they decide whether to arrest a suspected impaired driver.

“We believe at least initially that it would lower the false positives that an officer would have,” Hill said. “They would have a higher level of confidence in making an arrest.”

Law-enforcement agencies still would have to obtain follow-up-test results to use as evidence in court, just as they do after a positive preliminary breath test for alcohol impairment.

Hill said he and his research team plan to finish laboratory tests with a prototype marijuana breath test this year, then start testing human breath between January and June 2015.

After that, the researchers plan to test a version of the device out in the field, he said.

Some lawmakers at a Nov. 21 meeting of the Senate Law & Justice Committee appeared impressed by the research.

“WSU is going to be at the forefront, it seems to me, of supplying this kind of science and the technology that’s based on it to police all over the country,” said Sen. Adam Kline, D-Seattle.

Bob Calkins, a spokesman for the Washington State Patrol, said the agency would “welcome anything that will help us get impaired drivers off the road.”

He said the State Patrol wouldn’t want to use any new technology until it is fully developed, though.

“It needs to be rock solid before we’ll adopt it,” Calkins said.

Some state officials have expressed concern about increasing numbers of drivers testing positive for marijuana impairment since the drug was legalized in Washington.

In 2012, 18.6 percent of blood samples taken from suspected impaired drivers in Washington tested positive for active THC, according to the Washington State Toxicology Laboratory.

That number rose to 25 percent of tested blood samples statewide in 2013, the first year I-502 was in effect.