A carbon monoxide gas detection and warning system for a portable phone device is disclosed which includes a phone case dimensioned and configured to receive a portable phone device, the phone case having a raised outer shell portion that includes at least one sensing port communicating with a corresponding sensing chamber formed within a wall of the shell, and a sensing circuit embedded in the shell of the phone case, the sensing circuit including a carbon monoxide sensor operatively associated with the sensing chamber of the at least one sensing port for generating a gas detection signal upon detecting the presence of carbon monoxide gas in the sensing chamber, and a controller for receiving the gas detection signal from the carbon monoxide sensor and for periodically sending an output signal to the portable phone device by way of a wireless transmitter.