A fire detector with a maximum average response time of less than 1.5 minutes is obtained by combining a smoke detector with a CO.sub.2 detector that uses NDIR sensor technology. The smoke detector is used to detect smoldering fires and to help prevent false alarms attributable to the CO.sub.2 detector. The CO.sub.2 detector is used to rapidly detect fires by measuring the rate of change of CO.sub.2 concentration. A signal processor generates an alarm signal when a smoldering fire is detected or alarm logic indicates that a fire has been detected.