health agencies. EPA has set standards for over 90 contaminants organized into six groups: microorganisms, disinfectants, disinfection byproducts, inorganic chemicals, organic chemicals and radionuclides. States and territories must implement rules that are at least as stringent as EPA's to retain primary enforcement authority (primacy) over drinking water. Many states also apply their own state-specific standards, which may be more rigorous or include additional parameters. Many countries look to the standards set by the EPA in the United States for appropriate scientific and public health guidance and may reference or adopt US standards. World Health Organization Guidelines The World Health Organization (WHO) Guideline for Drinking-water Quality (GDWQ) include the following recommended limits on naturally occurring constituents that may have direct adverse health impact: Arsenic 10 μg/L Barium 10 μg/L Boron 2400 μg/L Chromium 50 μg/L Fluoride 1500 μg/L Selenium 40 μg/L Uranium 30 μg/L Organic species: Benzene 10 μg/L Carbon tetrachloride 4 μg/L 1,2-Dichlorobenzene 1000 μg/L 1,4-Dichlorobenzene 300 μg/L 1,1-Dichloroethane 30 μg/L 1,2-Dichloroethene 50 μg/L Dichloromethane 20 μg/L Di(2-ethylhexyl)phthalate 8 μg/L 1,4-Dioxane 50 μg/L Edetic acid 600 μg/L Ethylbenzene 300 μg/L Hexachlorobutadiene 0.6 μg/L Nitrilotriacetic acid 200 μg/L Pentachlorophenol 9 μg/L Styrene 20 μg/L Tetrachloroethene 40 μg/L Toluene 700 μg/L Trichloroethene 20 μg/L Xylene 500 μg/L Comparison of parametric values The following table provides a comparison of a selection of parameters for concentrations listed by WHO, the European Union, EPA, and Ministry of Environmental Protection of China. Notes " indicates that no standard has been identified by editors of this article and ns indicates that no standard exists. μg/L = micrograms per litre, or 0.001 ppm; mg/L = 1 ppm, or 1000 μg/L. * means action level; not a concentration standard. A public water system exceeding the action level must implement "treatment techniques" which are enforceable procedures. ** TT (treatment technique). The public water system must certify that the combination of dose and monomer level does not exceed: acrylamide = 0.05% dosed at 1 mg/L (or equivalent); epichlorohydrin = 0.01% dosed at 20 mg/L (or equivalent). See also Water pollution References Further reading CDC Water Quality and Testing European Drinking Water Directive Drinking Water Regulations: Overview - US EPA Drinking water regulation Drinking water quality Water treatment