question: Hello, What oxygen levels are considered dangerous? My father has had a C Pap machine for 8 years now. I noticed the last time that I was visiting, that he was falling asleep in the middle of the day, after sleeping for 8 hours. And he seemed extremely tired. I asked if he had his machine checked , which he said that he had it checked. I told my mom to get an oxygen meter and to check his levels, especially when she see s him getting sleepy or falling asleep. When should they be concerned or I should ask, what levels are dangerous that they should reach out to his doctor?