ST. LOUIS PARK, Minn. (AP) — A small chlorine spill at a swimming school inside a mall in suburban Minneapolis sent 11 people to the hospital Sunday, but officials called it a precaution and said nobody became seriously ill.

Emergency responders were called to the Foss Swim School at Knollwood Mall around 3 p.m. Sunday. City spokesman Jamie Zwilling said about 40 to 50 children and adults experienced respiratory symptoms such as coughing.

School founder and CEO Jon Foss said “just about a cup of bleach” spilled on the ground during some regular maintenance in the pump area during the last class of the day. When a parent opened an emergency door intending to let in some fresh air and clear away the smell, the zero-degree air from outside mixed with the 85-degree, humid air inside, forming a water vapor cloud that Foss said caused most of the coughing.

That door normally would never be opened in such cold weather, Foss said. He said this was the first time it had happened at any of his eight schools, though he knew of similar incidents elsewhere that had caused coughing.

Since school staffers weren’t initially sure what was happening, Foss said, they followed their emergency plan, evacuated the pool and called the fire department.