Consequently John Joyce learned that a 'harmless' part of NOx emissions, nitric oxide (NO), in the presence of hydrocarbons (such as household aerosol propellants, possible gas leaks and ingress of vehicle exhaust fume), converts to NO2. This was found to be the case in the New South Wales school investigation. In a scientific sense it had become practice to calculate both NO + NO2, when measuring oxides of nitrogen levels in emissions. Hence the now commonly used reference to "total NOx".