14 CFR § 460.11 - Environmental control and life support systems.
(a) An operator must provide atmospheric conditions adequate to sustain life and consciousness for all inhabited areas within a vehicle. The operator or flight crew must monitor and control the following atmospheric conditions in the inhabited areas or demonstrate through the license or permit process that an alternate means provides an equivalent level of safety -
(1) Composition of the atmosphere, which includes oxygen and carbon dioxide, and any revitalization;
(2) Pressure, temperature and humidity;
(3) Contaminants that include particulates and any harmful or hazardous concentrations of gases, or vapors; and
(4) Ventilation and circulation.
(b) An operator must provide an adequate redundant or secondary oxygen supply for the flight crew.
(c) An operator must
(1) Provide a redundant means of preventing cabin depressurization; or
(2) Prevent incapacitation of any of the flight crew in the event of loss of cabin pressure.