The threshold for the driving-under-the-influence standard that is part of the new Washington state marijuana law may be too high, a government expert told the Seattle Post-Intelligencer.

Some marijuana users say the standard under the new law—a 5-nanograms-per-milliliter limit for the active ingredient in the drug, THC—is too low, and will unfairly criminalize people who use medical marijuana. However, recent research conducted by scientists from the National Institute on Drug Abuse (NIDA) suggests the standard may be too high to capture drivers impaired by marijuana, the newspaper states.

Marilyn Huestis of NIDA, who conducted a recently published study on marijuana use and psychomotor function, says active THC quickly falls below the 5-nanogram limit within 24 hours. “The level of 5 nanograms per mil is pretty high,” she said. “We know that people are impaired at lower levels than 5, but the balancing act is trying to find a number that can reliably separate (the impaired from the not-impaired), which is almost impossible to do.”

An analysis of nine studies, published last year, found driving under the influence of marijuana is associated with an increased risk of a motor vehicle crash, especially for fatal collisions. The analysis found driving under the influence of marijuana was associated with almost twice the risk of a motor vehicle crash compared with unimpaired driving. The studies in the analysis included nearly 50,000 people.