Incomplete oxidation occurs due to inefficiencies in the combustion. The carbon flow for a typical combustion process can be described as follows:

• Most carbon is emitted as CO₂ immediately.
  
• A small fraction of the carbon in the fuel escapes without being immediately oxidized to CO₂ during combustion. Most of this fraction is emitted as non-CO₂ gases such as methane (CH₄), carbon monoxide (CO), and non-methane volatile organic compounds (NMVOCs). The carbon in these gases, though, is assumed to ultimately oxidize to CO₂ in the atmosphere and is therefore integrated into the overall calculation of CO₂ emissions (i.e., the carbon content value). Therefore the carbon in these non-CO₂ molecules is intentionally “double-counted” because it is eventually transformed into a CO₂ molecule.[2]
  
• The remaining part of the fuel carbon is unburned (i.e., unoxidized) and remains as soot and ash. In general, this fraction of the fuel carbon is assumed to remain stored indefinitely (i.e., not emitted in gaseous form).