Air pollution is of great concern, and the Air Quality Index (AQI) is a helpful tool that tells you how clean or polluted your air is, and what associated health effects might be a concern for you. The Environmental Protection Agency (EPA) created the AQI, which provides information on five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. A health-based National Ambient Air Quality Standard – the level EPA has set to protect public health - exists for each of these pollutants. Ground-level ozone and airborne particles are the two pollutants that pose the greatest threat to human health in the United States.