Infrared detection of air pollutants has been extended to the parts-per-billion sensitivity range. The increased detection sensitivity results from the use of the scanning Michelson interferometer, cooled solid state detectors, the fast mini-computer, and the multiple pass long path cell. Forming ratios of spectra with the aid of the computer extracts obscure information and minimizes the interferences of atmospheric water and carbon dioxide. Almost all of the significant gaseous pollutants can be measured by the method described, even at concentrations smaller than one part pollutant to one billion parts air.