Part 75 of Volume 40 of the Code of Federal Regulations (CFR) requires continuous monitoring and reporting of sulfur dioxide (SO₂), carbon dioxide (CO₂), and nitrogen oxides (NO_X) emissions. Most of these emissions are measured with continuous emission monitoring systems (CEMS). The CEMS monitor the amount of pollution emitted from a smokestack (pollutant concentration) and the volume of exhaust gases (stack gas volumetric flow rate). The regulation also specifies quality assurance and quality control tests to ensure the CEMS are performing properly. Emission sources with lower emissions may use less rigorous methods, with safeguards in place to ensure that emissions are not under-reported.