CHEMICAL RISKS

Overview
        Chemical risks present in drinking water may be naturally occurring or are inadvertently added through other means. Sources of chemicals may be classified in the following categories:
- Source water: Some chemicals present in source water are naturally occurring such as potassium-40, a radio nuclide found in natural bodies of water.
- Source water contamination: Surface waters may be contaminated through chemical spills, a situation more easily monitored than infiltration of chemicals such as benzene or MTBE to groundwater from leaking underground gasoline storage tanks.
- Chemical addition to drinking water: Fluoride addition to drinking water has proven a very effective oral health measure in the prevention and control of tooth decay.
- Disinfection by-products: Disinfection of drinking water, in particular, chlorination has been associated with the presence of halogenated organics in water.
- Distribution system contamination: Lead and copper pipes in distribution or hot water systems may elevate levels of the metals in distribution water.

        This section discusses the issues associated with these chemical risks in drinking water and includes fact sheets providing general information and health effects of thirteen chemical compounds.

Disease Types
        In general, all substances in large enough doses will have harmful effects on humans. Many, in fact, are beneficial in smaller quantities and only become toxic over a certain threshold level. It is therefore necessary to determine the levels at which a chemical may be detrimental to one's health. Animal studies and studies of human populations provide guidelines on safe exposure limits and are the basis on which regulatory limits are set. Some specific disease endpoints that are associated with exposure to chemicals potentially found in drinking water are discussed below. Individual chemicals and their health effects are discussed in detail in the later sections.

        Cancer. Many studies have focused on determining the carcinogenic potential of chemicals. At least thirty organic and inorganic chemicals have been positively determined to cause cancer. Many others are suspected carcinogens. These known and suspected carcinogens are found everywhere in the environment. Some are naturally occurring while others are synthetic and inadvertently introduced into the environment from chemical spills, leaking storage tanks, etc. Generally their presence is well below cancer causing levels. Carcinogens which may be present in drinking water include trihalomethanes (a disinfection by-product of chlorination) which have been associated with bladder cancer, and benzene (contaminant of groundwater from leaking underground gasoline storage tanks) which has the potential to cause leukemia.

        Reproductive and Developmental Effects. Reproductive and developmental effects refer to any adverse pregnancy outcomes such as cardiac/neural tube defects, oral clefts and low birth weights in infants. Reproductive effects also include those which affect fertility, for e.g., low sperm counts. Recently these effects have received greater attention in light of concern over endocrine disruptors (synthetically produced chemicals which can interfere with functions controlled by hormones such as reproduction). Other chemicals known or suspected to cause developmental effects are lead and fluoride.

        Neurotoxicity. Chemicals that exhibit neurotoxicity are those that cause detrimental effects on the brain and central nervous system. Diseases include Alzheimer's which has been associated with exposure to aluminum, and lead is known to affect the central nervous system and has been linked with reduced intelligence in children.

        Hematological Effects. Chemicals which have been identified to cause harmful changes to the bloodstream are said to have hematological effects. Lead can cause anemia (low red blood cell count) and exposure to benzene affects the blood, possibly resulting in leukemia.

        Other Effects. Normal functions of all the organs in the body are subject to disruption by various chemicals. Copper is known to cause liver and kidney damage, and lead disrupts the immune system.

Cancer: Causes and Prevention
Endocrine Disruptors
Disinfection By-Products
Volatile Organic Chemicals
Methyl-Tert-Butyl Ether (MTBE)
Fluoride
Aluminum
Lead
Copper
Arsenic
Radionuclides



Source: http://www.ci.sf.ca.us/puc/wqfs/tocsec4.htm