Rev Invest Clin
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A large world population resides at moderate altitudes. In the Valley of Mexico (2240 m above sea level) and for patients with respiratory diseases implies more hypoxemia and clinical deterioration, unless supplementary oxygen is prescribed or patients move to sea level. ⋯ The frequency of other altituderelated diseases at moderate altitude, described in skiing resorts, remains to be known in visitors to Mexico City and other cities at similar or higher altitudes. Residents of moderate altitudes inhale deeply the city's air with all pollutants and require more often supplementary oxygen.