Not to be outdone by legislators in Colorado, Washington state officials are considering setting a legal limit for the amount of THC in a driver's blood. Nevada and Ohio have two-nanogram limits for THC. Pennsylvania has a five-nanogram "guideline," but not a limit. Twelve states have a zero-tolerance policy for driving under the influence of any illicit drug, including marijuana. A group of 11 international scientists published a comprehensive study on the effects of THC on driving ability. They concluded that drivers do not pose a crash risk until they reach between 10-20 nanograms of THC per milliliter. They also determined that 18-20 nanograms is the equivalent of a blood-alcohol content (BAC) of .08, the current legal limit in most U.S. states.