Nearly 500 people including students with serious breathing problems were evacuated from National Jewish Health Hospital in Denver and an affiliated school after maintenance workers inadvertently mixed two water-cleaning chemicals causing toxic fumes.

National Jewish Health Hospital is a medical facility that specializes in treating patients with breathing disorders.

Forty-five people were sent to area hospitals with nausea and respiratory distress, according to Lt. Phil Champagne of the Denver Fire Department. They were expected to recover soon.

The source of the toxic fumes is a mixture of hazardous chemicals chlorine and hydrochloric acid during a maintenance routine of a swimming pool on the campus of National Jewish Health Hospital. The fumes spread throughout the campus through a series of interconnected tunnels causing respiratory distress in people coming in close contact with the vapors.

Kunsberg School spokesman William Allstetter said an estimated 400 staff members and 92 children were evacuated as a precaution. Kunsberg School is affiliated with the adjacent National Jewish Health and teaches kindergarten through eighth grade.

Students were taken to the Denver Fire Department headquarters where their parents could pick them up, Champagne said. Classes were also expected to resume on Tuesday.

“National Jewish Health has resumed full operations as of Monday afternoon, except for Kunsberg School, which will remain closed for the remainder of the day,” the campus said.