This measurement refers to the NO and NO2 levels in the air, both of which are toxic.  Indoors, these compounds primarily result from combustion reactions (gas stoves, gas heaters, tobacco smoke).  Outdoors, NO and NO2 come primarily from emissions (car exhaust, or industry).  They pose a threat to human health both in their NO and NO2 forms, and because NO2 is a contributor to ozone production. Nitrogen dioxide is formed at high temperatures as N2 is broken up in the air. ​

Learn more about nitrogen oxides