At least four people have been treated for ammonia inhalation at the Exceldor poultry processing plant in south shore Saint Bruno de Montarville.
Factory workers say alarms sounded at around 3 AM and the plant quickly filled with the fumes.
Longueuil firefighters who responded to the early alarm, called in the Montreal fire department's specialized hazardous materials team.
It is believed the leak occurred in a cooling system.
The plant is located in an industrial park and no other evacuations were necessary.