A CMOS gas sensor that uses MTJ elements to capture/store gas concentration level data at high temperatures for later readout at low temperatures. Each MTJ element includes a storage layer whose magnetic orientation is switchable between parallel and anti-parallel directions relative to a fixed reference when heated above the storage layer's blocking temperature, whereby the MTJ element is switchable between low and high resistance states. During operation, reaction heat generated by a gas sensing element raises the MTJ element's temperature above the blocking temperature when ambient target gas exceeds a minimum concentration level, whereby an applied magnetic biasing force causes the storage layer's magnetic orientation to switch relative to the fixed reference, whereby the MTJ element captures measured concentration level data for later readout. In one embodiment, multiple MTJ elements connected in a NAND-type string switch at different concentration levels to provide highly accurate quantitative measurement data.