In the early days of mining, caged canaries were hung in the tunnels. The birds, being so small, were especially susceptible to poisonous gases because of their high metabolism and ventilation rate. If the bird died, it was an alarm telling the miners get out of the mine. Now electronic CO detectors are available for the home.
--- Make sure fuel-burning equipment is working correctly and is properly exhausted. This includes furnaces, hot water heaters, and propane and gasoline powered vehicles used indoors.
--- Stop smoking.
--- Don't leave your car running in the garage, especially if it is attached to your house. It could poison you in the car, and CO could get into the house and afflict others.
--- If you are in heavy traffic, keep your windows rolled up. This is especially true if you are caught in traffic in an enclosed space like a tunnel.