NOx is the catchall for Nitrogen Monoxide(NO) and Nitrogen Dioxide(NO2). NO is colorless and tasteless but when exposed to air it becomes NO2, poisonous, red-brown, and stinky.

Like Clauser says, running hot is the main culprit in the generation of NOx emissions. This can be due to fuel mixture (running too lean), poor EGR, unusually high compression, overadvanced ignition.

Kuan