For residents of our nation’s capital, news of a fire on the city’s rapid transit system—the Washington Metro—is not surprising. It catches fire and smokes quite regularly. At some points last year, there were reports of more than four fires per week (although there’s some dispute about that rate). There’s even the handy site—IsMetroOnFire.com—to check the current blaze status.

Yet, despite the common occurrence, residents may be surprised to learn a potential contributor to the system-wide sizzling: their own hair.

According to a safety specialist with the Amalgamated Transit Union (ATU), a thick, felt-like layer of human hair, skin, and other debris has collected on the aging tracks of the city’s rails. In particular, hair has built up on insulators supporting the transit system’s electrified third rails, which run cables carrying 750 Volts of electricity to power the trains. The hair coating delivers a real threat of electrical sparks and fire.

“I was flabbergasted” at the amount of hair in the Metro, ATU specialist Brian Sherlock told a local NBC news station. “The amount of debris is just beyond vulgar to think of.”

Arcing and smoking insulators is a problem that has dogged Metro for years. In 2015, an arcing insulator was linked to a smoke incident that left one passenger dead and more than 80 others sickened by thick smog.

“A lot of the issues with the insulators is actually fiber and hair that literally comes off of people and clothing, and gets sucked up” and into the tunnels, Paul Wiedefeld, general manager of Washington Metropolitan Area Transit Authority, told NBC News.

However gross, the collection of hair from riders may not be that surprising. The system provided 97 million rides in 2016. A single healthy person sheds anywhere from 10 to 200 hairs per day. But those with health issues or hair loss can shed far more. Stress—a common problem around DC—can also up the head shedding.

In addition to hair, riders also let loose dead skin cells as they commute on the rails. A healthy person sheds about a thousand skin cells per centimeter squared of skin every hour. That works out to about 500 million cells a day. And each one of us sheds our entire outer layer of skin every two to four weeks.

Metro spokesperson Dan Stessel told NBC that Metro is working to boost track cleaning considering the hairy situation.