The German Federal Water Management Act requires that substances be evaluated for negative influence on the physical, chemical or biological characteristics of water. These are classified into numeric water hazard classes (WGK or WHC depending whether you use the English or German acronym). Additional Info: The water hazard potential is based on the properties of the substances (in particular acute oral toxicity for mammals), toxicity for aquatic organisms (mostly fish and bacteria) as well as biodegradability and bioaccumulation. The ranking is assigned by the Commission of the Evaluation of Water Hazardous Substances (KBwS) in which the German federal government, the German States and industry are represented. Class C: nwg "nicht wassergefährdend", Hazard Potential HP: Not hazardous (formerly WGK 0); C: 1, HP: Slightly hazardous to water; C: 2, HP: Hazardous to water; C: 3, HP: Extremely hazardous to water. Classification of substances into Water Hazard Classes is discussed in the third link under Further Reading below. You'll need to know German to read the flowchart. MSDS Relevance: Although not required by the U.S. OSHA, the WGK abbreviation commonly appears on MSDS's for products that are sold in the European Community (EC). Most ANSI-compliant MSDS's contain more detailed information on the environmental/ecological fate and aquatic toxicity in addition to the WGK class. See also: acute toxicity, ANSI, Biolgical Oxygen Demand, CHIP, Heavy Metal