The term "emerging contaminant" generally refers either to a contaminant recently introduced into the environment that therefore poses a new or emergent threat to the environment, or to a contaminant previously present in the environment at such low levels that available analytical techniques have not been able to detect its presence. Rather than describing a specific contaminant, the term generally refers to a class of compounds. For example, pharmaceuticals and endocrine disrupting compounds have been labeled emerging contaminants in drinking water. Some of these chemicals were detected in U.S. waterways as early as the 1970's, but more recently they have been identified in some drinking water supplies.