This CO2 level of 1604ppm was at a dentist’s office. There was an air purifier in the room, but it had been unplugged. I decided not to take off my mask, nor wait around to see how much higher it would get.
If I hadn't been carrying a CO2 monitor (in a mesh bag, clipped to my purse), I wouldn't have known the air was bad.
A friend of a friend who masks everywhere except the dentist's, came down with COVID after an appointment there.
6/n