And, if you have any appliances that produce carbon monoxide — gas stoves, gas fires, log burners — then get a carbon monoxide alarm. If it's any good, it'll give you temporary shift of hearing when you test it. It *has* to be that loud, because carbon monoxide will make you drowsy, thus keeping you in place while it kills you.
Carbon monoxide alarms are cheap. I paid a little over £20 for ours in (I think) 2016, and it should last ten years.