A cabin 
pressure altitude monitor and 
warning system provides a warning when a detected cabin 
pressure altitude has reached a predetermined level. The 
system is preferably embodied in a portable, 
pager-sized device that can be carried or worn by an individual. A 
microprocessor calculates the 
pressure altitude from signals generated by a calibrated pressure 
transducer and a temperature sensor that compensates for temperature variations in the signals generated by the pressure 
transducer. The 
microprocessor is programmed to generate a warning or alarm if a cabin pressure altitude exceeding a predetermined threshold is detected. Preferably, the 
microprocessor generates two different types of warning or alarm outputs, a first early warning or alert when a first pressure altitude is exceeded, and a second more serious alarm condition when either a second, higher pressure altitude is exceeded, or when the first pressure altitude has been exceeded for a predetermined period of time. Multiple types of alarm condition indicators are preferably provided, including visual, audible and tactile. The 
system is also preferably designed to detect gas concentrations and other ambient conditions, and thus incorporates other sensors, such as 
oxygen, 
relative humidity, 
carbon dioxide, 
carbon monoxide and 
ammonia sensors, to provide a more complete characterization and monitoring of the 
local environment.