Recently there has been increasing commercial activity at altitudes of 3500-6000 m. Examples include new mines in northern Chile at altitudes of about 4500 m. Because the workers come from sea level, intolerance of the high altitude is a major problem. This degree of hypoxia reduces work capacity, mental efficiency and sleep quality. One novel solution is to raise the PO2 of the room air by adding oxygen to the room ventilation. This is remarkably effective. For example, at altitudes of 4000-5000 m, increasing the O2 concentration by 1% (e.g. from 21 to 22%) reduces the equivalent altitude by about 300 m. Thus raising the O2 concentration by 5% at the new mines reduces the equivalent altitude to 3000 m which is easily tolerated. The introduction of oxygen concentrators (molecular sieve) which require only electrical power makes O2 enrichment feasible. The fire hazard is less than in air at sea level. Everybody now expects that the ventilation of a room will provide a comfortable temperature and humidity. Control of the oxygen concentration can be regarded as a further logical step in man's control of his environment.