Cannabis is becoming more popular across the US. Four states have legalized recreational marijuana and five more, including California, will vote on the issue next Tuesday. This is good news for many, but there is one big downside: legal pot can be a public safety nightmare.

Several studies have shown that stoned driving can be dangerous. But how can law enforcement test if a driver is stoned and impaired? Traditional blood tests that might work for alcohol don’t work for marijuana. This creates legal quandaries when it comes to determining how much weed is okay for safe driving.

A solution to the problem might come from “marijuana breathalyzers,” handheld devices that can capture THC (marijuana's active component) in the breath. But peer-reviewed, academic research on marijuana remains scarce, and it’s not clear how soon accurate and reliable devices might become available.

Here’s what you need to know about this issue that’s going to populate your news feeds more and more.

What’s wrong with just using a quick blood test to check for marijuana if you’re pulled over? Why do we need new technology?

The big problem is that, as anyone who had to pass a drug test knows, cannabis can stay in the body for weeks after you last smoked. That doesn’t mean that you’re still high weeks after having a joint. There is almost no correlation, in fact, between marijuana in blood and actual impairment, says Mary Celeste, a Colorado judge who specializes in cases of impaired driving. This can lead to a lot of false positives at the roadside.

Everyone’s looking for a number when it comes to stoned driving

In Colorado, where recreational marijuana is legal, the legal limit if you’re pulled over for driving is 5 nanograms of THC in the blood. But that figure is tricky. “There’s very little science backing these levels in terms of how that correlates with impairment,” says Eric Sevigny, a professor of criminal justice at Georgia State University. “Sometimes policymakers will put out something they think is in the interest of public health and safety when all the evidence isn’t there, and then adjust at a later point.”

Someone might have 5 nanograms in their blood because they were smoking at home a week ago. Also, heavy users who have built up a tolerance can have that much in the blood and not that impaired.

So what’s the solution to keeping roads safe, while not sending innocent people to jail? Many think it’s “marijuana breathalyzers.” But the issue is complicated.

How does a marijuana breathalyzer work, and what does it take to develop one?

Marijuana breathalyzers are, in theory, handheld devices that could be used by a cop to measure the THC content in a driver’s breath. The idea is that THC can only stay in the breath for a few hours, so if it shows up, the person was almost certainly smoking recently and might still be impaired.

Early studies suggest that marijuana breathalyzers are possible, but the data is limited. In a study published this June, Olof Beck, a pharmacology professor at the Karolinska Institute in Sweden, found that it is possible to accurately capture and measure how much THC is in breath.

“It might take some more time and effort to develop.”

Beck’s team asked subjects to smoke a cannabis cigarette, and then measured the amount in their breath up to three hours after they last smoked. They used a complicated and sensitive lab instrument called a mass spectrometer. While the researchers didn’t directly correlate THC particles in breath with impairment, they correlated it with physiological change like a faster pulse and dilated pupils, which act as a proxy.

This is only one study, so more research needs to be done. But it is a promising step. The challenge, Beck says, will be to produce cheap breathalyzers that are accurate enough. “I think maybe there is some more time needed to get the sensitivity that we have in the laboratory,” says Beck. “It might take some more time and effort to develop.”

It’s 2016, why don’t we have marijuana breathalyzers already?

One reason is that marijuana wasn’t legal until recently, so there was little incentive to develop one. But this ties into a more general issue: there’s been very little research into marijuana because of strong federal restrictions. Cannabis is a Schedule I controlled substance, the most restrictive classification. The Drug Enforcement Administration says Schedule I drugs have no acceptable medical uses. This means it’s in the same category of heroin and monitored very carefully.

Until recently, only the University of Mississippi was allowed to grow marijuana for research purposes and all researchers had to get their marijuana from that farm. In August, the DEA rejected an appeal to stop classifying cannabis as Schedule I drug, but it did say it would give out licenses for other universities to grow the drug for research.

There are a couple ways that these strict requirements have hurt research. First, having only one institution provide all the marijuana creates a bottleneck. Many scientists have waited a very long time to get the amount that they need. Second, only specific strains of marijuana are approved for research use. But in real life, people smoke marijuana of all different kinds, with all different amounts of THC concentration. That means that some of the work that has been done is still pretty limited in scope.

At Washington State University, emeritus professor Nicholas Lovrich and chemistry professor Brian Clowers have run into obstacles while working on a marijuana breathalyzer. It took them three years to get approval from the university to do their experiments, says Lovrich, and that has slowed down their work. Their handheld breathalyzer is now six years in the making. “The research is not there largely because we haven’t been able to do this kind of research on college campuses,” says Lovrich.

Who else is developing these handheld breathalyzers?

We are working toward determining impairment carefully, accurately and in a timely fashion. https://t.co/7PbhidwIdN #marijuana pic.twitter.com/gyFvKZiF7K — Hound Labs Inc. (@HoundLabsInc) November 1, 2016

Apart from Lovrich and Clowers at Washington State, there are many startups in the business, including Lifeloc, the maker of the alcohol breathalyzer; Vancouver-based Cannabix Technologies; and California-based Hound Labs.

All of these companies are trying to make the same basic thing: a portable, handheld device for law enforcement to use. You would blow a few breaths into it — the exact amount of breath varies — and then it would quickly come up with a number telling you how much THC there was in the breath.

Hound CEO Mike Lynn claims that the prototype can detect edibles as well as joints, and the company has already begun field testing its handheld device with police departments in California. But we don’t know yet exactly how accurate they are.

In the cities where Hound’s breathalyzer is being tested (Lynn declined to say which ones), the police ask a driver they suspect is stoned if they’re willing to blow into the device. For now, Lynn says, there have been no negative results — meaning that all drivers who used the breathalyzers resulted having THC in their breath. (Further pilot studies will begin in 2017.)

Breathalyzers won’t be the be-all and end-all

Cannabix also has a prototype ready, says Kal Malhi, the company’s CEO. He says the team has been doing scientific trials in-house and is gathering the evidence necessary for the tool to be certified by a court and used by law enforcement. They need to do a certain number of tests to prove that it is accurate. Once that’s done, they will submit the information to the Minister of Justice in Canada and the National Highway Traffic Safety Administration in the US. The goal is to have this done by next spring.

The downside, though, is that the information from these businesses is proprietary, so we don’t know how exactly the devices work. And that potentially could create some problems. For scientific evidence to be allowed in a court of law, there has to be a general consensus that the methods used — in other words, how exactly the breathalyzer operates — are accurate. If startups are secretive about their devices, it’s possible their results would be tossed out.

So are breathalyzers the future?

Maybe. Experts agree that even if we did have a working weed breathalyzer, it wouldn’t be the be-all and end-all of fixing the problem of impaired driving. Just like police use an alcohol breathalyzer and make people try to walk in a straight line, the breathalyzer is just one tool that should be used with other techniques.

“It would certainly help in some capacity with assisting with roadside safety and determinations with police,” says Sevigny, the GSU professor, “but it can’t be a single arbiter.”

Celeste, the Colorado judge, agrees. “Everyone’s looking for a number, but it’s not like alcohol. Some people may be impaired and some people may not be impaired with higher amounts,” she says. “There’s a whole bunch of variables related to that, like how somebody tolerates the drug, how somebody metabolizes the drug, even gender may play a role.”

Right now, law enforcement use roadside drug recognition experts who have been specifically trained to tell when someone is under the influence of drugs like marijuana and heroin. These experts aren’t perfect, either, but should continue to be used, with the breathalyzer as an additional screening tool.

We also need better data on fatal crashes that involve drugs, says Sevigny. Without that data, it’ll be hard to pass better laws. And ultimately, fewer federal restrictions on marijuana research could answer so many lingering questions about the drug and its effects. We don’t know what the future holds, but we’re going to need to find out quickly.