Five-hundred people were evacuated from National Jewish Health following a hydrochloric acid spill that released toxic fumes into the air, according to CNN.

The spill happened at an indoor therapy pool at the Kunsberg School, where two 15-gallon drums of acid spilled into the pool, mingling with bleach, spreading a cloud through campus and along internal tunnels that run under Colorado Boulevard near Colfax Avenue.

The hospital specializes in treating people with severe respiratory problems, and at least 44 people were transported to another hospital for treatment, according to 7News. Students at the school will return to classes today. 9News has a slideshow documenting the confusion.