Doctors from the Ohio State University said they see a new rash of people who are allergic to hot tubs.

The diagnosis is rare, but experts say it's growing, according to a news release from the school. They believe a popular chemical called potassium peroxy-mono-sulfate deserves the blame.

It causes skin to get "just intensely itchy, red, flaky -- to the point where (patients) scratch so much that they start to bleed, because it's so intensely itchy," said dermatologist Dr. Matthew Zirwas.

Zirwas diagnosed the condition in Bill Mumford. Before that, there were only two cases on record of people being allergic to hot tubs and pools.

"I've now seen a half-dozen patients who've all had allergy to this chemical, and it's made them get severe rashes whenever they've gotten into their hot tubs," said Zirwas.

The problem is most patients don't know they're allergic. So even if you only use hot tubs once in awhile, Zirwas said to start slow, putting your feet in first to see if there is a reaction. If your skin turns red and itchy within five minutes, stay out.