Long path infrared spectroscopy of air pollutants is operationally classified into several system types: Long folded path samplers, two station remote instruments using either a remote source or a remote retroreflector, and one station remote instruments using either natural sources, laser backscatter, remote fluorescence, or passive measurement of sample thermal emission. The operational requirements of each system are described, as are their advantages, disadvantages, and one example each of their results. A comparison of their overall performance reveals that the best system now available would be a two station remote device with an array of distal retroreflectors, employing a correlation Fourier Transform Spectrometer as a receiver. The main application of such a system would be the detection of widely dispersed pollutants at extremely low concentrations.