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Chlorination remains the most common form of wastewater disinfection in North America due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. The presence of chlorine gas in the atmosphere can pose immediate and serious hazards to the health of any person breathing the air. Large schale declorination operations are generally conducted indoors. All rooms in which chlorine is to be stored or handled should be adequately ventilated to the outside. Since chlorine is heavier than atmospheric air, fans are generally used to exchange the air. <ref>http://www.usace.army.mil/publications/armytm/tm5-814-3/chap17.pdf (accessed 5/9/2008)</ref>Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment.<ref>