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The effect of toxic contaminants on human health can be classified as either acute or chronic. The reaction to a substance causing serious illness or death in an individual within 48 hours after exposure is considered acute toxicity. Chronic toxicity is a longer term effect on health due to frequent exposure to small amounts of a toxic substance. Chronic reactions to chemicals are difficult to study and our knowledge of the chronic toxic effects of nearly all chemicals is very poor. Examples of chronic health effects would be kidney and liver disease, cancer, mental illness, etc.

Based on epidemiological evidence and experimentation on laboratory animals, the Public Health Services have established maximum contaminant levels (MCLs) allowable in drinking water. Most of these levels allow a sufficient margin of safety; however, one must remember that acceptable contaminant levels vary widely among individuals and population groups. For example, high sodium, which may be harmless for many people, can be dangerous for the elderly, hypertensives, pregnant women, and people having difficulty in excreting sodium. Specific symptoms of different contaminants are presented in more detail in later sections.