Both NO₂ and NO play an important role in photochemical-smog-forming reactions. In sufficient concentrations NO₂ is deleterious to health, agriculture, materials, and visibility.

In combustion processes, significant amounts of NO may be produced by combination of atmospheric nitrogen and oxygen; at ambient temperatures, NO can be converted to NO₂ by oxygen and other atmospheric oxidants. Nitrogen dioxide also may be generated from processes involving nitric acid, nitrates, the use of explosives, and welding.

1.1 This test method covers the manual determination of the combined nitrogen dioxide (NO₂) and nitric oxide (NO) content, total NO_x; in the atmosphere in the range from 4 to 10 000 μg/m³ (0.002 to 5 ppm (v)).

1.2 The maximum sampling period is 60 min at a flow rate of 0.4 L/min.

1.3 The values stated in SI units are to be regarded as standard. The values given in parentheses are for information only.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.