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Chlorides: (US: MCL = 250 mg/l). Chlorides are normally associated with salty water. Sodium chloride is common table salt and also is the salt found in seawater. High chloride levels can cause human illness and also can affect plant growth at levels in excess of 1000 mg/l. Taste threshold is about 250 mg/l for most people.

Fluorides: (US: MCL = 1.4 to 2.4 mg/l, function of climate). The optimum level of fluorides in water for reducing dental cavities is about 1 mg/l. Higher levels could cause mottling of the teeth. Reduced MCL values in a hot climate are justified by increased daily intake of drinking water in warm weather. Controversy over negative and positive effects of adding even small amounts of fluorine to drinking water make it very difficult to accurately summarize its effect on the human body.

Nitrates: (US: MCL = 10 mg/l as N). Nitrates are present in water particularly in regions where agricultural fertilization or organic waste disposal may be polluting water sources. The nitrate level in drinking water is extremely important with infants, because of their high intake of water with respect to body weight. Nitrates in the infant are converted by the body to nitrites that oxidize blood hemoglobin to methemoglobin. The altered blood cells can no longer carry oxygen, which can result in brain damage or suffocation. The upper limit for nitrates in drinking water is 10 mg/l as nitrogen. This is about 45 mg/l of the nitrate ion. Epidemiological studies show a correlation between high nitrate levels and gastric and stomach cancers in humans.

Organic compounds: (variable MCL). Organic compounds include a wide range of substances, all of which contain carbon. The common types of industrial organic substances found in water are petroleum products, solvents, pesticides, and halomethanes. These are generally referred to as either hydrocarbons or organic halides (usually chlorinated hydrocarbons). Most organic halides, especially the man-made compounds, have been found to be toxic--acutely at high concentration and chronically at very low concentrations. These types of organic compounds run from methylene chloride (CH 2C 12) to DDT (1,1,1-trichloro-2, 2-bis (p-chlorophenyl)ethane). Most volatile (or purgeable) chlorinated organic chemicals can cause cancer. High concentration symptoms include nausea, dizziness, tremors, and blindness.The testing methods for these chemicals are very complex, expensive, and time consuming. Usually gas chromatography with mass spectroscopy and a computer search involving expensive equipment and highly trained operators are required. As a result, the average homeowner cannot afford this complete test.

Radionuclides: radium-226 and radium-228 (US: MCL = 5 pCi/l); tritium (hydrogen-3, MCL = 20,000 pCi/l); strontium-90 (MCL = 8 pCi/l). These doses are based on not exceeding 4 millirem/year (rem stands for roentgen-equivalent-man, a radiation dosage unit) of net ,, and photon radioactivity. Excessive levels could cause radiation sickness or bone disease. The presence of radium in drinking water is not of great concern because it is not retained in the body.

Total dissolved solids: (US: TDS, MCL = 500 mg/l). TDS represent mostly the total mineral content of the water (deposits left after evaporation of a water sample), primarily salts, carbonates, and metals. Organic compounds may also be dissolved solids. A high concentration of TDS is an indicator of possibly high volume contamination and further investigation may be recommended.

Sulfates: (US: MCL = 250 mg/l). Sulfates are associated with gypsum formations (f. e. North Thueringia, Germany). High sulfate water can cause diarrhea, and in fact was commercially sold as a laxative in the past.