Book excerpt: Chlorine dioxide (ClO2) exists as a greenish yellow to orange gas at room temperature. It is used in the paper and pulp bleaching industries as a sterilizing agent, in hospitals as a biocide in water treatment, and as an improving agent in flour. This document focuses on exposures via routes relevant to occupational settings principally related to the production of chlorine dioxide, but also contains environmental information. The health effects and environmental fate and effects of chlorine dioxide used in the treatment of drinking-water, together with those of halogenated organics produced by the interaction between the disinfectant and other materials present in the water are covered in a recent Environmental Health Criteria publication (EHC No. 216 2000) and are not dealt with in detail here. Chlorine dioxide is an irritant and it seems likely that health effects would be restricted to local responses. The few ecotoxicity data available show that chlorine dioxide can be highly toxic to aquatic organisms.