NOx are oxides of nitrogen formed during the combustion of fossil fuels. They also arise during combustion by the direct combination of atmospheric nitrogen and oxygen at high temperature. The main component of NOx is nitric oxide (NO), with a smaller proportion of nitrogen dioxide (NO₂). The most commonly used unit of measurement of NOx is the mg/kWh. 

Permissible NOx levels are defined differently according to location. Urban areas already suffering from a high level of pollutants have the strictest levels. For example, the Greater London Authority (GLA) considers Low NOx technologies to be those whose NOx emissions are < 40 mg/kWh. Ultra Low NOx applies where NOx levels are < 15 mg/kwh. 

Until 2018, CHP manufacturers specified their equipment according to German standard TA Luft for areas with light pollution (< 250 mg/kWh), or Half TA Luft for highly polluted areas (< 125 mg/kWh). 

From September 2018, the European maximum NOx limits are now 56 mg/kWh for gas and LPG boilers, and 240 mg/kWh for CHP using gas.