To put it simply, it is not possible to test how many nanograms of THC is too many to drive. This is an area of some confusion among law enforcement officials as there are no real tests that can be taken and no definitive guidelines to go by. The test for alcohol bears no applicability to THC, the active compound found in Cannabis. This is because alcohol dissolves in water, while THC dissolves in fat. So while a driver may be clearly stoned, there is no way to test for it, and no real available guidleines as everyone reacts so differently to the substance.
Alcohol v THC
When people drink, alcohol spreads through the saliva and the breath. It saturates the lungs and the blood evenly. When you measure the volume of alcohol in one part of the body you measure the levels in all parts of the body, most notably the brain. This fact helped formulate the maximum levels of blood alcohol levels that are permissible – .08 nanograms. This standard is the result of over 3 decades of epidemiological research. This is not the case with marijuana.
To add more complexity to marijuana, the height of intoxication is not the moment that your THC levels peak. The high is not uniform based on how much THC enters and leaves bodily fluids, meaning that even if officials could test how much THC was in the body, it might not be an indication of how high the individual was at the given time. Though it is reasonable to suggest that after a certain level of THC the person should not be driving anyway. But someone with lower levels of THC who is under the limit may be quite high/intoxicated whereas someone with higher levels of THC might be quite sober
In terms of road crashes and fatalities marijuana is a clear winner against alcohol. Statistics show that in every state in the US where marijuana has been legalized, there have been fewer crashes and fatalities. The reasons for this are unclear. Some suggest is that it leads to lower levels of alcohol consumption. Others say it is due to the fact that marijuana legalization leads to less use of more serious drugs such as meth and heroine, which are drugs associated with many crashes. The most probable reason is that stoned drivers are more aware of the fact that they are stoned, and drive more cautiously. Alcoholics have less awareness and tend to be more reckless. Despite this, much research does suggest that when THC levels are very high, in the range of 13 nanograms per milliliter, then stoned driving is just as dangerous as driving drunk.
THC in the Body
Fatty tissues in the body act like sponges to THC, and the brain is made predominantly of fat. You can still measure THC in the brain even if it is no longer in the blood. This makes the matter even more complicated, as testing for brain levels of THC is almost impossible for officers to do without specialized tests, while blood levels are much easier to measure. THC levels also differ from user to user. If you are an occasional user, you can smoke a lot of marijuana,and a couple of hours late it will have left the blood but still be in the brain, meaning that you would still be quite intoxicated. And all of these problems are compounded by the fact that everything changes in terms of those who occasionally smoke and those who are daily users. Heavy cannabis users have so much THC built up in their fatty tissues that it will take weeks to get detoxified. And these users will also have a steady amount of THC in their bodies even when they have not been smoking.
Eating THC as opposed to inhaling it will have different effects on the body. When cannabis is eaten, the blood does not contain as much THC. It also takes far longer for the THC to peak. The chronic effects of smoking weed also changes the brain, reducing the density of cannabis receptors. Users are cognitively impaired for up to 28 days after their last use and drivers might be impaired for that length of time, and perhaps even longer.
People are looking for a definitive number so that tests can begin and drivers can be held accountable. Washington have gone with 5 nanograms of THC in the bloodstream. Any drivers with levels above this limit are given a DUI. But simply picking an arbitrary number like this without research to back it up may not be the most logical approach to the issue. Even still, there are many potential obstacles. Bloods tests are usually taken 1.5-4 hours after an accident, by which time THC levels would have fallen considerably, and these people might have been impaired yet still pass the test. And in Washington, a heavy user might get a DUI even though he or she has not smoked in weeks.
No Solution in Sight
While legislators are eager to get testing underway little can be done as scientists have not come up with a valid solution to the problem. There does not seem to be any way around it in the meantime, and it is frustrating for many officials. People are working on breath tests, saliva and other blood markers and behavioral tests, but nothing has stuck so far. With deadlines for marijuana legalization and implementation looming in States across the US, a viable solution is quickly needed, though there is none in sight.