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Safety is relative not absolute. For example, an aspirin or two may help a headache, but if you took a whole bottle at once, you'd probably die. So, is aspirin safe? When setting drinking water standards, federal regulatory agencies use the concept of reasonable risk, not risk-free. Risk-free water would cost too much. So the answer to the question is, no, drinking water isn't absolutely safe. But the likelihood of getting sick from drinking water that meets the federal standards is very small, typically one chance in a million.

One difficulty the US Environmental Protection Agency has when trying to determine reasonable risk relates to the problem called susceptible population. Not all people who drink water are the same from a health point of view, that is, some people are more susceptible to getting sick than others. For example, only babies three months old or younger are affected by nitrates in drinking water, so for that contaminant they are the susceptible population: They are susceptible to getting sick from too much nitrate in their drinking water. The standard for nitrate, therefore, was chosen to protect these infants. With other contaminants, identifying the susceptible population is not as easy. Are they the elderly, those undergoing cancer treatment, those in nursing homes, all babies, those who are HIV positive, or others? For each standard, the federal regulatory agencies must balance the risk to all these groups against the cost of treatment and arrive at a standard that will protect as many people as possible and that can be afforded. This is called "the greatest good for the greatest number."

(See Questions 33, 37, and 53 for related information.)