As marijuana legalization becomes a hot topic in more and more states, new issues are rising to the surface that need solving. One of the biggest issues is DUIs. While blood tests can detect how much of marijuana’s various components are in a person’s system, there isn’t a standardized acceptable amount in a person’s system to determine whether they’re too impaired to drive.

While states use .08 as the limit of blood alcohol content allowed to drive, there isn’t a similar level for marijuana. In Colorado, a person with five nanograms of THC per milliliter of blood is considered “impaired” according to the law. But scientists say that limit is arbitrary and there’s no evidence supporting that people with that level of THC are too impaired to drive.

“We just don’t know whether or not that means they’re still intoxicated or impaired or not,” says Tara Lovestead, a chemical engineer at the National Institute of Standards and Technology in Boulder, Colorado. “There’s no quantitative measure that could stand up in a court of law.”

One of the biggest issues is that marijuana’s effects on the body aren’t as consistent as alcohol. When drinking, ethanol is released into the body which leads to dull thinking and reflexes. It also dissolves in water, and since humans are mostly water, the ethanol can be cleared from the body in a few hours.

THC doesn’t work the same way. Instead of dissolving in water, it dissolves in fat, meaning it can linger in the body longer than ethanol. Other factors such as gender, body fat, frequency of marijuana use and product type can all affect how long THC stays in the body and affects a user’s impairment. In fact, a person can have a THC-level above 5 nanograms, the amount used to determine a DUI in Colorado, several days after smoking. This makes blood testing for marijuana DUIs incredibly unreliable.

Breathalyzers are also not a viable alternative. Unlike alcohol, THC and other cannabinoids degrade quickly and only a small amount will stay in a person’s breath.

Scientists around the country are working to develop more accurate tests as well as determine a more strict level of THC that causes impaired driving. But until then, police officers will use their subjective judgment to determine DUI offenses. And while some of them are training themselves to make educated determinations, it’s still a non-objective test that can affect a person being charged with a very serious crime.