If you can detect the odor of chlorine dioxide, the concentration is high enough to possibly do damage to your respiratory tract. The higher the concentration, the quicker the damage occurs.
Since it is very difficult for the average person to measure the concentration of chlorine dioxide in the air, it is very difficult to determine how much is safe.
Everyone's sense of smell is a little different so the threshold of odor detection is not exact, but it does serve as a general guideline.