I used to supervise a crew of health inspectors specializing in housing issues. Seems that every fall CO poisonings would be reported. I can't over emphasize the importance of maintenance and inspection of all gas fired appliances, water heaters and heating. I recall a call from a concerned apartment tenant, she complained of headaches and her pet parakeet had died unexpectedly.
Despite the fact that all of the appliances, in her unit, were electic, we conducted an investigation. The gas fired water heater, for the building, was located in a storage room below her unit, mattresses and furniture had been stuff into this room, dislodging the water heater flue. CO made its way around pipes, conduit and cover plates into the wall and out into this ladies unit. Co levels were measured at dangerous levels. You may never know when something goes wrong. CO is colorless and oderless, however it is usually present in smoke and when soot is noted. GET A GOOD DETECTOR. Most Gas Companies have outstanding, procedures to test for, advise on and abate CO hazards. Give them a call.