When fuels are burned in air, the nitrogen and oxygen are combined together. The products are commonly referred to as oxides of nitrogen - NO, NO2 and are generally known as NOx. These are pollutants that contribute to acid rain and smog, but are not considered greenhouse gases. Nitrous oxide, N2O, also known as laughing gas, is a combustion by-product and is considered a greenhouse gas. Although produced in small quantities, it has a global warming potential about 310 times greater than CO2.