Medical crews are evaluating visitors after complaints of chemical exposure at a water park in Gulfport, Mississippi.

American Medical Response in Gulfport told CBS News that four ambulances were sent to Gulf Islands Waterpark, and 15 patients were treated for breathing issues.

Two people were taken to the hospital earlier in the day after a piece of equipment malfunctioned and dumped a large amount of a chlorine-type chemical into the water at one of the attractions.

“They [park officials] called it a chlorine bubble,” Gulfport Fire Chief Michael Beyerstedt said. “[The park’s general manager] told me it is the exact same thing that happened this morning.

They’re working on the system, and there was some chlorine trapped in the system, and when they opened the valve again, all the chlorine was released at once.”

The substance released was a kind of liquid bleach, sodium hypochlorite, which Beyerstedt described as much stronger than regular household bleach.

Several visitors reported the strong bleach smell and were checked by an medical response crew. Gulf Islands spokeswoman Betty Wallace said the incident happened in the back of the park near the lazy river attraction.

“Lifeguards smelled it and turned off the chlorine immediately. It’s part of our routine to constantly adjust chemicals, checking the water and the pH balance to keep the water pure,” Wallace explained.