It is interesting to read about the Powell River chlorine dioxide spill.
They released 600000 liters of chlorine dioxide solution in the middle of a working pulp mill and there were no fatalities or serious injuries.
I believe there was some "issues" with aquatic life where the chlorine dioxide was eventually pumped to, but that is all that I could find.
I was unable to determine the concentration of the chlorine dioxide gas in the tanks, but it appears that it was below the 5 PPM that causes major issues in the air.