A method and system for gas sensor drift correction is disclosed, appropriate for use in a system placed in an environment that with some regularity reaches a background gas concentration. A background gas concentration range is defined. When the gas sensor readings drop below the background gas concentration range for at least a defined short dwell time, an upwards calibration adjustment is effected. When the gas sensor readings stay above the background gas concentration range for a defined long dwell time (longer than the maximum expected interval between excursions to the background gas concentration), a downwards calibration adjustment is effected.