You should leave the hotel room or Airbnb immediately. Even if you're not experiencing any symptoms of CO poisoning, it's better to err on the side of safety. When staying at a hotel, go to the lobby and insist that they call the fire department or appropriate local agency to have the room checked. If you feel fuzzy headed or otherwise ill, request that the hotel summon medical assistance, and go outside for fresh air. If staying at a vacation rental, leave the apartment or house immediately. Don't just leave the room in which the alarm went off, go outside the building into fresh air. Call the fire department yourself to have the home checked, and request medical assistance if you feel ill. Airbnb and other rental platforms do want to know as soon as possible if you experience any problems, so give a call them when you're able to, and give the host a heads-up about the issue as well. If it seems like a hotel or vacation rental is not taking the situation seriously, don't be bullied into returning to a room you feel unsafe in. Be sure to take notes and names, and if at all possible, simply switch lodgings. With enough documentation, you may be able to file a chargeback on your credit card for the original accommodations.