As many of you may have heard, a hotel in Maine (not a Marriott) and a Legal Seafoods Restaurant in NY both suffered carbon monoxide poisoning. My apartment complex replaced all our smoke detectors with combo smoke/carbon dioxide detectors this year). I asked the maintenance man if carbon dioxide poisoning could occur if windows are kept open and he said no. So I am safe (I have to keep my apt. heating level to a certain norm so the pipes don't freeze but hate a hot apartment -- I want cold for sleeping.
But the subject occurred to me regarding hotels (and in today's case restaurants). If windows cannot be opened (usually not in US hotels; often so in European ones), this is a potentially serious issue. Even in Venice in December and January I had all my 4 windows open at least slightly and my heat turned off (actually it's automatic if you open the windows).
But for all of you out there, check that you have a CO2 detector and/or keep some windows open.