A sensor system that provides an adjustable threshold level for the sensed quantity is described. The adjustable threshold allows the sensor to adjust to ambient conditions, aging of components, and other operational variations while still providing a relatively sensitive detection capability for hazardous conditions. The adjustable threshold sensor can operate for extended periods without maintenance or recalibration. A portable monitoring unit working in communication with the sensor system provides immediate communication of conditions detected by the sensors. The portable monitoring unit allows building or complex management to be in communication with a sensor system at all times without requiring someone to be physically present at a monitoring site. The portable monitoring unit can be equipped with an auditory device for alerting management or a screen for displaying pertinent information regarding an occurring situation so that management can quickly identify and resolve the problem. In addition, the portable monitoring unit can also be equipped with function keys that allow the portable monitoring unit to send instructions back to the sensor system. In one embodiment, the portable monitoring unit also includes a second transceiver for communications over a short wave radio frequency, or with a cellular phone system.