The toxic gas monitoring system is used to quantitatively detect the presence of both AsH3 and PH3. It provides an instantaneous readout of concentration and initiates both warn and alarm signals whenever the preprogrammed concentration level for a particular gas and location is exceeded. A continuous gas detection system must be provided to detect the presence of hazardous (toxic or highly toxic) gases at or below the permissible exposure limit or the ceiling limit. The detection system must initiate a local audible and visual alarm as well as transmit a signal to a constantly attended plant emergency control stations. Activation of the alarm/monitoring system must automatically close the shutoff valve on highly toxic or toxic gas supply lines.