Chlorine dioxide is a chemical compound with the formula ClO2.  It is a synthetic, green-yellowish gas with a chlorine-like, irritating odour. Chlorine dioxide is a neutral chlorine compound, which is very different from elementary chlorine, both in its chemical structure as in its behaviour. Chlorine dioxide is a small, volatile and very strong molecule. In diluted, watery solutions it is a free radical. At high concentrations it reacts strongly with reducing agents. Chlorine dioxide is an unstable gas that dissociates into chlorine gas (Cl2), oxygen gas (O2) and heat. When it is photo-oxidised by sunlight, it falls apart. The end products of chlorine dioxide reactions are chloride (Cl-), chlorite (ClO-) and chlorate (ClO3-). At –59°C, solid chlorine dioxide becomes a reddish liquid. At 11°C chlorine dioxide turns into gas. It is 2 – 4 times denser than air. As a liquid, chlorine dioxide has a bigger density than water. One of the most important qualities of chlorine dioxide is its high water solubility, especially in cold water. It does not hydrolyse when it enters water; it remains a dissolved gas in solution. Chlorine dioxide is approximately 10 times more soluble in water than chlorine. Chlorine dioxide can be removed by aeration or carbon dioxide. 
The major use of chlorine dioxide is as a bleach in a number of industries: in cleaning and de-tanning of leather, and as a bleaching agent for wood pulp, fats and oils, cellulose, flour, textiles, and beeswax. Chlorine dioxide is registered as a bactericide, fungicide and algaecide. It is used to disinfect human drinking water systems, commercial water-cooling tower systems, and metal cutting fluids. It may also be used to disinfect dairy farm animals and milking equipment, in eating establishments and food processing/handling areas and around the house. It is used extensively in Europe for disinfecting drinking water, and its use there is increasing as well as in North America and Australia, as an alternative to chlorine due to lesser problems with disinfection by-products. Furthermore, it is an approved food additive in Australia (No. 926).
In the Environment 
Chlorine dioxide is a very reactive compound and breaks down quickly in the environment. In air, sunlight rapidly breaks down chlorine dioxide into chlorine gas and oxygen. In water, chlorine dioxide quickly forms chlorite. Chlorite in water may move into groundwater, although reactions with soil and sediments may reduce the amount of chlorite reaching groundwater. Neither chlorine dioxide nor chlorite build up in the food chain.
Sources of Emission & Routes of Exposure
Sources of Emission 
- Industry sources: Paper and allied products industries; textile mills; chemical industries: food processing industries; drinking water treatment plants; and commercial water-cooling tower systems.
- Diffuse sources: Residues from food and drinking water that have been treated with chlorine dioxide.
- Natural sources: Because of the nature of its high chemical reactivity, it is unlikely to occur naturally.
- Consumer products: Possibly in some disinfectants and bleaches. Foods and drinking water that have been treated with chlorine dioxide
Routes of Exposure [3,4]
Chlorine Dioxide can be absorbed into the body by inhalation, ingestion of food that has been treated with chlorine dioxide, or skin contact. Chlorine dioxide is added to drinking water to protect people from harmful bacteria and other microorganisms. Most people are exposed to small amounts of chlorine dioxide and chlorite by drinking treated water. In addition, Individuals who are employed at pulp and paper mills, municipal water treatment facilities, and other facilities that use chlorine dioxide and chlorite as a disinfectant may have high exposures to chlorine dioxide and chlorite (ions or salts).
Health Effects [2,4]
Acute exposure of the skin to chlorine that originates from the decomposition of chlorine dioxide, causes irritations and burns. Eye exposure eyes to chlorine dioxide causes irritations, watering eyes and a blurry sight. Chlorine dioxide gas can be absorbed by the skin, where it damages tissue and blood cells. Inhalation of chlorine dioxide gas causes coughing, a sore throat, severe headaches, lung oedema and bronchio spasma. The symptoms can begin to show long after the exposure has taken place and can remain for a long time.
Chronic exposure to chlorine dioxide causes bronchitis.
Development and reproduction
Chlorine dioxide is thought to have effects on reproduction and development. However, there is too little evidence to ground this thesis. Further research is required.
The Ames test is used to determine the mutagenity of a substance. The Ames test uses Salmonella bacteria that are genetically modified. No bacterial colonies are formed, unless they come in contact with a mutagenic substance that alters genetic material. Tests show that the presence of 5-15 mg/L ClO2 increases the mutagenity of water. It is difficult to prove the mutagenity of chlorine dioxide and chlorine dioxide by-products, because the substances are biocides. Biocides usually kill the indicator organisms that are used to determine mutagenity.
There are no studies on cancer in humans exposed to chlorine dioxide. Based on inadequate information in humans and in animals, the International Agency for Research on Cancer (IARC) and the EPA have determined that chlorine dioxide are not classifiable as to human carcinogenicity.
First Aid Measure
- Inhalation: Remove victim from area of exposure – avoid becoming a casualty. Remove contaminated clothing and loosen remaining clothing. Allow patient to assume most comfortable position and keep warm. Keep at rest until fully recovered. If patient finds breathing difficult and develops a bluish discolouration of the skin (which suggests a lack of oxygen in the blood – cyanosis), ensure airways are clear of any obstruction and have a qualified person give oxygen through a facemask. Apply artificial respiration if patient is not breathing. Seek immediate medical advice.
- Skin Contact: If spilt on large areas of skin or hair, immediately drench with running water and remove clothing. Continue to wash skin and hair with plenty of water (and soap if material is insoluble) until advised to stop by the Poisons Information Centre or a doctor.
- Eye Contact: If in eyes, hold eyelids apart and flush the eye continuously with running water. Continue flushing until advised to stop by a Poisons Information Centre or a doctor, or for at least 15 minutes. Continue to wash with large amounts of water until medical help is available.
- Ingestion: Immediately rinse mouth with water. If swallowed, do NOT induce vomiting. Give a glass of water. Seek immediate medical assistance.
Exposure Controls & Personal Protection
Ensure ventilation is adequate and that air concentrations of components are controlled below quoted Workplace Exposure Standards. If inhalation risk exists: Use with local exhaust ventilation or while wearing suitable mist respirator. Keep containers closed when not in use.
Personal Protective Equipment
The selection of PPE that should be used is dependant on a detailed risk assessment. The risk assessment should consider the work situation, the physical form of the chemical, the handling methods, and environmental factors. The recommended personal protective equipment includes:
- chemical goggles,
- full face shield,
- elbow-length impervious gloves,
- splash apron or equivalent chemical impervious outer garment, and
- rubber boots.
- If inhalation risk exists, wear air-supplied mask meeting the requirements of AS/NZS 1715 and AS/NZS 1716.
- Always wash hands before smoking, eating, drinking or using the toilet.
- Wash contaminated clothing and other protective equipment before storage or re-use.
OSHA: OSHA Permissible Exposure Limit (PEL):
- General Industry: 0.1 ppm, 0.3 mg/m3
- Construction Industry: 0.1 ppm, 0.3 mg/m3 TWA
ACGIH: American Conference of Governmental Industrial Hygienists Threshold Limit Value (TLV): 0.1 ppm, 0.28 mg/m3 TWA; 0.3 ppm, 0.83 mg/m3 STEL
NIOSH: National Institute for Occupational Safety and Health: Recommended Exposure Limit (REL): 0.1 ppm TWA; 0.3 ppm STEL
Australian Drinking Water Guidelines (NHMRC and ARMCANZ, 1996): Maximum of 1 mg/L (i.e. 0.001 g/L)
Safe Work Australia: Maximum time weighted exposure (TWA) level: 0.1 ppm 0.28 mg/m3 and Maximum short term exposure level (STEL): 0.3 ppm 0.83 mg/m3