Abstract: A portable stress monitor is provided for monitoring conditions under which physiological activity is occurring. The conditions monitored may be environmental, such as ambient temperature and humidity, or physiological, such as heart rate or body temperature. The monitor includes a main body portion and a hinged cover. The hinged cover includes a removable sensor module. One or more sensors are attached to the sensor module, each sensor may be pivotally mounted on a mast, such that the sensors extend outwardly from the hinged cover when the sensor module is in the deployed position and may be pivoted to sit substantially flush with the hinged cover when the sensor module is in the storage position. A central processing unit is contained within the main body, and is operable to process data acquired by the sensors according to a heat strain algorithm. A display is attached to the outer surface of the main body for providing read out information, and an input device is provided for user input.