Emergency crews, including a hazardous materials team, were called to the Travelodge on Carling Avenue (near Kirkwood Avenue) around 7:30 p.m.
Ottawa Fire spokesman Marc Messier says 40 hotel guests — most of them children — showed symptoms consistent with chlorine exposure.
But he says so far, no one has needed hospitalization.
Messier says staff were treating the hotel's pool and set off a chemical reaction.
He says the area is ventilated and chlorine levels appear normal.
"We're basically sending all the guests back up to their rooms to shower, then they'll come back down and they'll get reassessed by paramedics, just to make sure that everything's normal," he said.