NFPA 99B Standard for Hypobaric Facilities defines an oxygen-enriched atmosphere to be any atmosphere in which the concentration of oxygen by volume is greater than 23.5 percent. It further defines an atmosphere of increased burning as any atmosphere containing a percentage of oxygen, or oxygen and nitrous oxide, greater than the quotient of 23.45 divided by the square root of the total pressure in atmospheres. Atmospheres of increased burning are required by the NFPA to be protected with a water fire-extinguishing system capable of delivering 305.6 L/min/m2 (7.5 gal/min/ft2). This requirement was developed by the NFPA using results from testing in 100 percent oxygen. This paper describes testing performed to determine the total water demand required to suppress fires in atmospheres of less than 100 percent oxygen. Testing was accomplished in a 7.1 m3 (250 ft3 ) hypobaric chamber using polyurethane foam and cotton fabric in atmospheres from 6.9 to 101.3 kPa (1 to 14.7 psia) and oxygen concentrations from 21 to 100 percent. The results indicate that the maximum water demand required to extinguish the fires was well below the NFPA requirement.