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Minimal concentrations of CO2 are mixed with pressurized air to provide a gas mix effective for stabilizing breathing of target patients or users. CO2 concentrations below about 2% and preferably between about 0.5% and 1.25% are employed. A gas modulator includes a gas mixing module, a sensor and a control processor. The gas mixing module mixes plural gases, including CO2, into a gas mix, for delivery to a substantially leak-proof patient face mask. The sensor, located substantially at the face mask, measures CO2 concentration in the face mask. The control processor, based on a signal from the sensor, controls the CO2 concentration in the gas mix.