The Toronto Fire said crews responded to LA Fitness near Yonge Street and Sheppard Avenue around 5:20 p.m.  for a hazmat call.
Several people needed medical attention as they may have inhaled chlorine fumes while in the pool due to excessive chlorination, the Toronto fire said.
Ten people were evaluated at the scene and five were transported by Toronto paramedics to the hospital.
Of the five, one suffered serious but non-life-threatening injuries, rescuers said, adding that the children’s injuries were described as mild.
The Toronto Fire later cleared the spot and handed it over to police.  The Ministry of Labor and the Technical Standards and Safety Authority are also committed.