Too much pee and too little fresh air probably made employees of an indoor water park sick, the Centers for Disease Control and Prevention report. Public health departments should keep a close watch on the growing indoor water park industry, the agency says — because you never know what could be in the water.

This time, it was most likely pee, along with sweat, dead skin cells, and lotions, the CDC reported today in its Morbidity and Mortality Weekly Report. The agency was called in to investigate an indoor water park in an unnamed Ohio city after the local health department started getting complaints. People reported that their eyes were burning, their noses were irritated, they were having trouble breathing, and they were throwing up — all after splashing around at that water park.

Public aquatic facilities are gross. Routine inspections of public pools turned up health or safety violations at 80 percent of them, the CDC reported last year. The water can harbor bacteria, viruses, and parasites — and cleaning can make matters even worse. Chlorine disinfectants react with molecules in swimmers’ pee, sweat, dead skin, and lotions to produce irritating chemicals called chloramines. Splashing wafts these irritants into the air, where they form a nasty layer that hovers over the water’s surface.

Complaints in Ohio started in July 2015; the CDC investigators arrived in January 2016. The investigators surveyed employees, tested the water, and inspected the water park’s ventilation system. Their tests came back negative for the infectious diseases they suspected, and only turned up low levels of bacterial toxins. But the agency measured high levels of chlorine compounds, which include those nasty chloramines. This testing was in the winter, too, when fewer visitors were peeing, sweating, and shedding skin in the water.

Generally, levels of chlorine compounds in the water park’s air were low, but the investigators couldn’t actually test the air the employees were breathing. It would have been too hard for employees to work while public health researchers sampled their airspace, the study says.

Since employees working inside the water park were sicker than the ones working outside, and chloramines are known to cause eye and lung irritation, the CDC settled on these chemicals as the likely culprit. The investigators also discovered that the water park’s ventilation system didn’t actually work. So that cold, moist, probably chloramine-laden, and definitely irritating air was just accumulating inside the building — burning peoples’ eyes, hurting their lungs, and sending a few to the doctor’s office.

The solution was pretty simple, mostly. The water park needed to fix the ventilation system, encourage park visitors to shower before getting into the water, and respond quickly when employees start complaining about being sick. There was also one piece of advice that even pro-swimmers might have trouble following: don’t pee in the pool.