It's getting to be heating season, and we've already had our first local deaths from carbon monoxide (CO) poisoning.
CO results from incomplete combustion combined with poor ventilation. Symptoms are headaches, nausea, sleepiness, dizziness and disorientation. In high concentrations, carbon monoxide can cause loss of consciousness and death. Many victims die in their sleep. It kills over 200 people every year in the US, and is a factor in fire deaths because it incapacitates people when they need to escape.
So if you have a gas or other kind of heater that has a flame, make sure there are no obstructions in the airflow going in or out. In particular, the flue might be blocked by bird nests or otherwise. If you don't feel qualified, call your handyman, heating repairman, apartment manager or whoever.
In any case, spring for a CO monitor and install it as directed. You don't need as many of these as is recommended for smoke alarms, but there should be one near the likely sources.
And if you won't listen, you can just drop dead.