Boiler Pollution, Nitrogen Oxide Emissions (NOx) the new emissions level of 56mg/kWh will be enforced for all natural gas boilers from the 26th of September 2018. So what is NOx? it is the Nitrogen Oxide that is within the boiler's emissions, these are the products of combustion that come out of the boiler flue. so the fumes that come out the chimney. The lower amount of NOx in mg per kWh, the cleaner the emissions, meaning the cleaner the air will hopefully be around your home.