NOVELTY - The system has a sensor cavity (190) sealing a gas entry path from outside ambient gaseous atmosphere through filter material (150) i.e. activated carbon cloth. An integrated circuit package is provided with a sensor housing. A package lid is coupled in sealing relationship to a sensor housing (102) to form the sensor cavity and separate a filter cavity (192) within the package. A gas entry opening (106) is defined in the lid and in fluid communication with the filter cavity. The filter cavity is in fluid communication with the ambient gaseous atmosphere through the gas entry opening.    USE - Packaged integrated circuit gas sensor system for sensing presence and/or concentration of target gas such as carbon monoxide and methane (all claimed) or gas characteristics e.g. humidity present in ambient gaseous environment i.e. closed building.    ADVANTAGE - The system utilizes a microcontroller to perform calculations, determine gas concentration from sensed measurements and provide digital output at an output pin of the package so as to provide detected concentration to a user in an efficient manner. The filter material can be arranged within the filter cavity so as to filter out, absorb, reduce or remove contaminant constituents/gases that interfere performance of the system. The system ensures that an insert component can be optionally placed inside the filter cavity with the filter material so as to adjust interior size of the filter cavity, thus providing convenience to accommodate the filter materials and filter material components with differing sizes and thicknesses.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a method for packaging an integrated gas sensor system    (2) a method for sensing target gas.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a packaged integrated circuit gas sensor system.    Sensor housing (102)    Gas entry opening (106)    Filter material (150)    Sensor cavity (190)    Filter cavity (192)