Twenty-one people were taken to hospital after a carbon monoxide leak at the Keybrand Foods plant in Kitchener, Ont., according to the local fire department.

Fire Chief Tim Beckett said when firefighters arrived at the plant on Victoria Street North, carbon monoxide readings were extremely high.

"Within about 10 minutes of our arrival, we had the readings back to zero," said Beckett. "We turned off a suspected source, which was an oven, and we locked out the appliance for further inspection."

Mark Karjaluoto, the director of communications at Grand River Hospital, said workers from the plant began arriving around noon.

Four of the 21 workers who attended the hospital have been admitted as inpatients for further treatment.

"We believe that they will stay with us for a little while longer," said Karjaluoto. "We'll be monitoring what their symptoms may be and conducting blood testing to determine level of exposure, those sorts of things."

Karjaluoto said the hospital did initiate a code orange, which allows for the deployment of additional staff and resources to respond to a mass injury event. But no additional care providers were required.

Health Canada said those exposed to low levels of carbon monoxide might have a headache, nausea, feel tired or short of breath, or find their motor functions impaired.

At higher levels of exposure, or at lower levels for a long time, symptoms might include chest pain, feeling tired or dizzy, and having trouble thinking.

Convulsions, coma and death are possible with high levels of exposure.