Criteria air contaminants (CACs), which are the focus of this page, refer to a group of pollutants that include sulphur oxides (SOx), nitrogen oxides (NOx), particulate matter (PM), volatile organic compounds (VOC), carbon monoxide (CO) and ammonia (NH3). In addition, ground-level ozone (O3) and secondary particulate matter are often referred to among the CAC because they both are by-products of chemical reactions between the CACs that take place in the atmosphere.