Disclosed herein are systems and methods for producing an illumination pattern in a gas tube of a facially-fitting device, which is used in conjunction with a capnograph. The illumination pattern is determined by at least one illumination parameter, derived at least from measured CO2 data, such that the illumination pattern is indicative of at least one breath-related/physiological parameter and/or one or more of the respiratory/physiological conditions determined/assessed based at least on the CO2 data.
