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The World Health Organization (WHO) and various national agencies have drinking water quality standards that specify the acceptable microbial, chemical, and radiological characteristics of safe drinking water.

Excessive amounts of microbes or chemicals derived from human and animal wastes, agricultural runoff, industrial chemicals, and even natural pollutants, make some water unsafe to drink and cause water-related diseases. If water sources are not protected, or are unexpectedly contaminated for any reason, the quality of drinking water suffers.

Contamination can occur at the source of the water both at the surface and in the ground. Once the water is in the distribution system, there are additional opportunities for drinking water to be contaminated. If pipes are not successfully protected from contaminants, the quality of drinking water suffers. Improper storage can also result in unsafe drinking water.