The National Ambient Air Quality Standards established under the United States Clean Air Act originally identified six major criteria air pollutants known as the standard pollutants, but regulatory tracking often encompasses expanded lists of hazardous air pollutants, precursors, and specific chemical categories. The primary regulated criteria pollutants include carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide, which are monitored continuously to protect public health and environmental quality.