An air pollutant for which acceptable levels of exposure can be determined and for which an ambient air quality standard has been set. Examples include: ozone, carbon monoxide, nitrogen dioxide, sulphur dioxide, and PM10 and PM2. 5. The term "criteria air pollutants" derives from the requirement that the U. S. EPA must describe the characteristics and potential health and welfare effects of these pollutants. The U. S. EPA and CARB periodically review new scientific data and may propose revisions to the standards as a result. View our health and air pollution website.