What is the difference between ambient environmental-quality standards and emission standards?

In environmental law, the distinction between ambient environmental-quality standards and emission standards lies in what they regulate and where they apply.1

• Ambient environmental-quality standards (or ambient standards) fix the maximum allowable concentration of a pollutant within the receiving environment itself, such as a body of air or water. These standards are often used to define a goal for overall environmental health, which can be expressed numerically (e.g., a specific concentration of a pollutant over a set time) or narratively (e.g., ensuring water is suitable for swimming). In the United States, for example, the National Ambient Air Quality Standards (NAAQS) set such limits to protect public health and welfare.12

• Emission standards (or discharge standards) regulate the amount of a pollutant that a specific "source"—such as a power plant or industrial facility—is permitted to release into the environment.1

Comprehensive environmental laws typically employ both types of standards, coordinating them to ensure that the cumulative impact of individual emissions does not exceed the broader environmental-quality goals set for a region.1