NOx compounds are oxides of nitrogen, generally considered to be Nitric Oxide (NO) and Nitrogen Dioxide (NO2). Primary emissions of NOx are predominantly from combustion, such as vehicle exhaust emissions. But, NOx is also formed in the atmosphere, by NO reacting with Ozone (O3) to form NO2 and Oxygen (O2) – these are Secondary emissions. With some engines known to be exceeding official pollution limits in ‘real world’ running conditions, NOx and its human health impacts, specifically respiratory issues, have moved up the policy agenda once again.