Adaptation to Moderate Altitude Hypoxemia: The Example of the Valley of Mexico.

A large world population resides at moderate altitude. In the Valley of Mexico (2,240 m above sea level), its inhabitants, breathe approximately 29% more on average and have 10% increased hemoglobin concentrations compared to sea level residents, among other differences. These compensations reduce but not eliminate the impact of altitude hypoxemia. The objective of the manuscript is to review and describe the information available on health and disease at moderate altitudes, mainly with data in Spanish language from Latin-American countries. Young adults in Mexico City have an SaO2 between 92% and 94% versus 97% at sea level, frequently decreasing below 90% during sleep and intense exercise. It is likely that among the population living at this altitude, lung growth, and development during pregnancy and infancy are enhanced, and that after residing for several tens of thousands of years, more important adaptations in oxygen transport and utilization have developed, but we are not certain about it. For patients with respiratory diseases, residing at moderate altitudes implies increased hypoxemia and clinical deterioration, unless supplementary oxygen is prescribed or patients move to sea level. Hyperventilation increases exposure of residents to air pollutants compared to those living in cities with similar concentrations of pollutants, although at sea level. Humans evolved at sea level and lack the best-known adaptations to reside at moderate or high altitudes. Residents of moderate altitudes breathe deeply the city´s air with all its pollutants, and more often require supplementary oxygen.