Which agency sets legal limits on the concentration levels of harmful contaminants in potable water distributed to customers?

The agency responsible for setting legal limits on the concentration levels of harmful contaminants in potable water is the Environmental Protection Agency (EPA). The EPA establishes enforceable standards for drinking water quality under the Safe Drinking Water Act. This act mandates that public water systems must monitor water quality and comply with specific regulations to ensure that the water provided to consumers is safe and free from harmful substances.

In this context, the roles of the other agencies are different. The Food and Drug Administration (FDA) primarily oversees the safety of food products and bottled water, while the Centers for Disease Control and Prevention (CDC) focuses on public health and disease prevention rather than regulating water quality standards. The Occupational Safety and Health Administration (OSHA) is involved in workplace safety regulations and does not regulate drinking water quality standards for consumers. Thus, the EPA's specific responsibility for managing and regulating drinking water quality makes it the correct choice for this question.