Nitric oxide is a colourless odourless gas that is quite chemically active. In normal concentrations it is not counted as toxic, in fact, nitric oxide is present in the human body and is also used in the treatment of asthma. Nitric oxide is chemically neutral and not readily soluble in water. It is formed in two basic ways in a combustion system: from combination of atmospheric nitrogen with oxygen. Through nitrogen compounds to be found in the fuel. Gas and light oil burners will mostly produce atmospheric nitric oxide, solid fuels and heavy oils will tend to produce more fuel-based nitric oxide. Production of atmospheric nitric oxide is very dependent on combustion temperature. Higher temperatures will produce higher concentrations. The big problem with nitric oxide is that it will later oxidise further to nitrogen dioxide, an unpleasant pollutant partly responsible for smog formation.