Complex interconvertibility of nitrogen oxides (NOX): impact on occupational and environmental health

Oxides of nitrogen have been implicated in a vast number of environmental and occupational health effects, some of which lack concrete mechanisms. Whereas certain compelling pieces of evidence link a particular nitrogen oxide to a particular adverse health effect, other reports seem to implicate virtually all the oxides in one form of toxic process or the other. Such diversity has probably emerged because each oxide of nitrogen possesses a different oxidation state, in which each form exerts different important levels of biological activity. Most important, each nitrogen oxide readily interconverts into another oxide at a very fast rate. This property of rapid interconvertibility poses problems to researchers in identifying with great confidence the actual oxide of nitrogen that is responsible for a specific occupational and environmental health effect. This paper reviews the complex nature of the nitrogen oxides (represented collectively as NOX) to explore the extent to which their acute or chronic exposure could be associated with toxicity. The nomenclature of the nitrogen oxides is outlined as a necessity for clarity and simplicity in understanding their reactions and interconvertibility and how they affect health. The natural occurrence and sources of occupational and environmental exposures and effects are critically evaluated and analyzed.