Nitrogen dioxide is a brownish gas with an acidic, unpleasant odour. It is highly toxic, producing respiratory distress in very small quantities. It is chemically active as an oxidising agent and is highly water soluble. Nitrogen dioxide is produced in small quantities during combustion in a similar way to nitric oxide, partly from oxidation of the nitrogen in the air and partly from nitrogen compounds in the fuel. Nitrogen dioxide is the major component of smog and is hence extremely undesirable. The small amount of nitrogen dioxide produced directly during combustion is increased by oxidation of the nitric oxide in the presence of ozone to form further nitrogen dioxide. Generally, an electrochemical sensor is used to measure nitrogen dioxide.