Chlorine dioxide (ClO₂) is a reddish-yellow gas, which is an oxidized form of elemental chlorine. Chlorine dioxide is unstable in the gas and liquid states, and in the gas state it can be explosive.

Chlorine dioxide is used in the disinfection of water and bleaching of wood pulp. It can also be used for air disinfection, and was the principal agent used in the decontamination of buildings in the US after the 2001 anthrax attacks. It is effective against viruses, bacteria and protozoa – including the cysts of Giardia and the oocysts of Cryptosporidium.

Chlorine dioxide, ClO₂ is produced by reducing sodium chlorate in a strong acid solution with a suitable reducing agent (for example, sodium chloride, sulfur dioxide, or hydrochloric acid).

2ClO₃^- + 2Cl^- + 4H⁺   →   2ClO₂ + Cl₂ + 2H₂O

References

National Library of Medicine (US) (http://toxnet.nlm.nih.gov/)