The safe limit for 8 hour exposure to chlorine dioxide gas in air is 0.1 PPM.
The safe limit for 15 minute exposure to chlorine dioxide gas in air is 0.3 PPM
The threshold (for most people) of detection is 0.1 PPM.
This means that when the concentration is high enough that you just start to detect it, you are at the safe level. Anything over that starts to attack your respiratory system. Note that respiratory issues can show up any time up to 48 hours after exposure.
I believe only 1 person is recorded to have died from chlorine dioxide exposure, and that concentration was 19 PPM. However, there is no indication of what the exposure was when the person died. The 19 PPM was measured when they found the person.
Fortunately, chlorine dioxide is very reactive and if you open up doors and windows you can quickly reduce the concentrations to safe levels.