The Clean Air Act requires EPA to set National Ambient Air Quality Standards for pollutants that are considered harmful to people and the environment. The Clean Air Act established two types of national air quality standards. Primary standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, which means protection against decreased visibility and harm to animals, crops, vegetation, and buildings. The EPA Office of Air Quality Planning and Standards (OAQPS) has set National Ambient Air Quality Standards for six common pollutants, which are also called the "criteria" pollutants: ground-level ozone, particle pollution, lead, sulfur dioxide, carbon monoxide, and nitrogen dioxide.