Nitrogen Oxides (NOx) is a generic description for a family of gases.
The two most relevant for air pollution are, Nitric oxide (NO) and Nitrogen Dioxide (NO2). The former is a highly reactive gas and is a toxic environmental pollutant. It undergoes spontaneous oxidization to Nitrogen Dioxide NO2 and is then toxic to the respiratory tract. NO2 forms as a result of motor engine combustion and from burning fossil fuels such a Natural gas, also from natural processes such as lightning. It contributes to the formation and modification of other air pollutants such as Ozone and particulate matter.
Long term exposure limit (NO) (8 hours TWA) 0.5ppm. Short term exposure limit (15 minutes) 0.5ppm.