Passive infrared is an emerging method for the remote detection of hazardous vapors particularly where warning is the primary consideration. Detection of gases, vapors and aerosols is based on the difference, (Delta) T), between the temperature of the target cloud and the effective radiometric temperature of the background. Computer simulation of spectra has been used to predict the detection limits for several target gases with a low angle sky background. The simulation is based on a 3 layer model that uses MODTRAN, which includes 6 standard atmospheric models, to compute background radiance and atmospheric transmittance. The detection limits, at 2 cm-1 resolution, for sulfur hexafluoride (simulant), Sarin, trichlorethylene, methyl isocyanate (the Bhopal gas), mustard gas, methyl chloride, and sulfur dioxide are discussed for selected cases with the U.S. Standard, and the sub-arctic winter and the tropical models. In this paper the method is illustrated with methyl isocyanate.