And it could get you fired.

Open up your wallet. Pull out a bill. There’s cocaine on it.

It’s well-known in government circles that traces of cocaine mar most banknotes in circulation. In the US, it’s 90%. In the UK, forensic analysts say the drug gets on every single note within two weeks of it entering circulation. It’s so bad, British police don’t even bother to test bills for cocaine anymore.

The reason for the contamination? Sometimes it’s because people have actually used the bills to snort cocaine, but more often it’s traces of the drug on the hands of someone who’s recently used it, or contact with other contaminated notes.

“When I was a young kid, my mom told me the dirtiest thing in the world is money,” Yuegang Zuo, author of the US study, told CNN. “Mom is always right.” Zuo says cocaine binds to the green ink in American dollars. He says fives, tens, twenties and fifties are more likely to have cocaine on them than humble one-dollar bills.

What’s in YOUR wallet? | Giphy

It’s gross to think about, but it can also be a real problem. Just ask Alan Bailes of Bristol, England. In 2015, the 55-year-old bus driver underwent a routine saliva test for drug use. He tested positive for cocaine, but insisted he’d never used any illegal drug. After 22 years of service, the father of two was fired. Even after he paid for a hair follicle test that proved he hadn’t used drugs in at least 90 days, he didn’t get his job back. After Googling possible sources of contamination, he found an article like this one.

Bailes realized he’d handled lots of cash the day of the test and hadn’t washed his hands. During the test, he had handled the swabs he used to take a saliva sample, possibly tainting them with cocaine from his unwashed hands. Armed with the facts, he sued his former employer and won “substantial” compensation.

The case showed how saliva drug testing is a flawed method, given the amount of cocaine on banknotes, and US researcher Zuo agrees. It also gives you another reason to wash your hands after handling money, which health agencies have long said you should do, anyway, for sanitary reasons. (Maybe credit cards, too, come to think of it.)

While UK cocaine use is on the rise, experts say it’s fruitless to estimate actual usage based on traces of coke on banknotes. Bills come into contact with other bills in wallets and bill-counting machines, and only a small amount is needed to register positive in a test, said Martin Barnes, who runs drug information charity DrugScope, in an interview with the Daily Mail.

There’s a better indicator of how much cocaine people are doing, and it’s a lot more gross. Within 2 to 6 hours of taking it, cocaine leaves your body — in your pee. A recent EU study of wastewater across Europe not only found coke in sewage, it allowed researchers to determine which cities took more of the drug. Londoners were the region’s leading coke fiends on weekdays — but on weekends, residents of the Belgian city of Antwerp edged them out.