Fire and ambulance crews were called at 10:43 a.m. when kitchen staff began feeling sick and reported smelling a strange odor, the Globe and Mail reported.
Carbon monoxide is odorless, but can be part of other leaks in natural gas systems that are scented to alert people of a leak.
Paramedics said one of the five kitchen workers was in serious condition, while the other four had minor exposure to the gas.
None of the residents were injured. The Toronto Transit Commission delivered six buses to shelter evacuees outside the seniors' home as firefighters worked at ventilating the building, CP24 TV reported.
District Fire Chief Ron Smith said 10 parts per million of carbon monoxide in the air is considered acceptable, but crews found the concentration in the kitchen at 400 parts per million.
Hours later, the source of the leak was still under investigation.
The residential parts of the building have carbon monoxide detectors that are checked monthly, a spokesman for the Leisureworld facility said.