On Wednesday, Massachusetts’ highest court ruled that a failed hair drug test is not enough to disqualify an applicant for the Boston Police Department. The hair drug test has long been controversial and we've covered numerous court cases challenging it. WGBH Radio’s Gabrielle Emanuel spoke with WGBH’s Arun Rath about the ruling. This transcript has been edited for clarity.

Arun Rath: So bring us up to speed first on why hair drug testing is so controversial.

Gabrielle Emanuel: Hair drug testing has been used, and it's been pretty common, since the 1980s. It's done in big city police departments. Major employers use it and they use it for two big reasons. First, hair has a record of what drugs you've used going back many months. Urine only goes back a few days. And the second thing is it's much harder to doctor a hair drug test. But, it's controversial.

The issue is that hair testing measures exposure. So that means if you use the drug, it shows up in your hair. But also, if you've just been in the room and somebody else has been using a drug, it might show up in your hair. There are two other problems that exist. One is that these drugs, like cocaine or heroin, seem to bind to the melanin in your hair. And people who have darker hair tend to have more melanin. And they've done really cool tests on this with people who have gray hair. If you look at their dark hair, they look like a heavy drug user. If you look at their white hair, it's as if they don't use drugs. So that's the first problem. The second is that certain hair products that are mainly used by African-Americans tend to make it more likely that drugs will bind to your hair. So there have been numerous lawsuits challenging the hair drug test and saying it's ineffective and racially biased.

Read more: State's Highest Court To Weigh In On Controversial Hair Drug Test Used By The Boston Police

Rath: So between the hair color and the hair product issue, it seems like there could be a bias against African-Americans or darker skinned people.

Emanuel: That's exactly the concern.

Rath: Now, what about this particular case that we're talking about here. What happened with this individual?

Emanuel: So this case is about a man named Michael Gannon, who about 10 years ago was denied a job at the Boston Police Department because his hair drug test came back positive for cocaine. But Gannon insists he never used cocaine. It was heard in April, but today the justices came down in a 6 to 1 ruling. The state's highest court sided with the Civil Service Commission, which was backing up Michael Gannon's case, and they sided against the police department. They basically said it is unreasonable to reject an applicant to the Boston Police Department solely based on a failed hair drug test. They said it's just too unreliable. The Boston Police Department should have to provide additional evidence of misconduct and in this case, they didn't do that. I should add this is not the first time a court has ruled against the hair drug test.

I spoke to Oren Sellstrom about this ruling. He is at the legal advocacy group Lawyers for Civil Rights. and he has a similar case that’s pending in federal court right now.

“It is a very powerful ruling," he said. "It is yet another court that is highlighting how scientifically unreliable this test is. In our view, this is the death knell for this discredited, discriminatory test. And the Boston Police Department should discontinue its use immediately.”

Rath: Well, that sounds pretty clear cut, pretty unequivocal. What did the police department have to say?

Emanuel: I spoke with Sergeant Detective John Boyle and he said they've passed this ruling onto their lawyers and they're reviewing it. But I will say in the past, they've refused to stop using the hair drug test. Records show that Boston has spent millions in legal fees and back-pay because of this test. And lawyers like Oren Sellstrom say continued use of the test calls into question just how sincere the city is about wanting diversity in its police department, because, as you said, many of the false positives happen to African-Americans.