WHO SEMINAR PACK FOR DRINKING-WATER QUALITY Inorganic Constituents and Aesthetic Parameters Session Objectives
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چکیده
• To describe the process of setting Guideline Values for inorganic parameters and describe the narrow divide between toxic and essential elements. • To describe some basic physico-chemical characteristics of water. • To provide some examples of inorganic chemicals to illustrate the uses of GVs for inorganics and highlight priority substances. • To describe the basis of monitoring of physical and chemical parameters. All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy. Introduction Many of the inorganic and aesthetic constituents evaluated in the Guidelines are known to be essential for life. Chromium, copper, fluoride, iodine, manganese, molybdenum, and selenium are essential elements in human nutrition; arsenic and nickel are considered by some researchers as essential elements. Of the aesthetic constituents, iron, chloride, calcium and magnesium (hardness), sodium and zinc are essential elements. A classification into "essential" and "toxic" elements is fraught with difficulties since as science advances, there is a constant shift of the elements from one group to the other. Toxicity is inherent in all elements, and is a function of the concentration to which humans are exposed. Paracelsus' statement remains valid, "The right dose differentiates a poison and a remedy". Ordinary salt, calcium, magnesium and iron, are all toxic above certain doses. This is illustrated in Figure 1 below. The plateau of "safe and adequate intake of essential elements" are mainly a matter for nutritionists to decide. Notes:NOAEL: no observed adverse effect limit; LOAEL: lowest observed adverse effect limit Threshold chemicals: only toxic above certain dose, therefore have a NOAEL Non-threshold chemical: toxic at any dose, therefore do not have a NOAEL WHO SEMINAR PACK FOR DRINKING-WATER QUALITY 2 No attempt has been made in the Guidelines to define a minimum desirable concentration of essential elements in drinking-water. The Guidelines are concerned with the quantification of toxic effects. A few of the inorganic chemicals and aesthetic constituents of common interest to many countries will be discussed here to illustrate the approach taken in deriving, or not deriving, guideline values (GV). A complete list of inorganic substances for which GVs have been derived is given in Table A2.2 (A) in Annex 2 of Volume 1 of the Guidelines. Asbestos in drinking-water: no health hazard Because of numerous inquiries from governments, industry and academia on the potential adverse health effects from asbestos in drinking-water, WHO issued the attached …
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