Recently, numerous healthy friends here at Flagstaff have complained of feeling weak or winded while exercising in midday summer heat. Along with bicyclists, hikers, and cardio-tennis types, were people doing regular household activities. For those who aren’t familiar, I thought it worth sharing a pilot’s perspective on high-elevation exercise.
The term “density altitude,” refers to the fact that atmospheric air density (and hence oxygen availability per breath of air) decreases based on factors including 1) increasing elevation and 2) increasing temperature. Pilots study density altitude because it can be a significant factor in whether planes get off the ground or not. So imagine the effects on our bodies.
Flagstaff’s physical elevation is 7,000 feet — already trying for recent arrivals from lower elevations. But when the air temperature here reaches 90°F, the physiological density altitude climbs to 10,500 feet. So although 90°F may feel comfortable enough, exercising at that temperature is equivalent to doing so halfway up Humphreys Peak. Have you noticed your (non-turbocharged) car’s poor passing performance here on hot summer days? Put another way, airplane pilots are counseled to wear supplemental oxygen when flying (comfortably seated) at altitudes above 10,000 feet. No wonder climbing even minor hills feels daunting here on a warm day!
Based on 7,000 feet above sea level and standard atmospheric pressure:
- At 90°F air temperature, your body operates at a density altitude of 10,500 feet.
- At 80º, your body operates at a 9,900-foot density altitude.
- At 70º, 9,200 feet.
- At 60°, 8,600 feet.
- Only near 35ºF, does our density altitude here at Flagstaff approximate our actual 7000-foot elevation!
The bottom line? At high elevations, it may be healthier and more comfortable to exercise early or late in the day when temperatures are cooler, and thereby reduce density-altitude effects. The difference between early morning and mid-afternoon temperatures could equate to exercising as much as 2,000 feet lower! (Note that due to lower partial pressures, we also dehydrate more quickly at high density altitudes and therefore require more water.)
I’m far from qualified to offer medical or exercise advice. But as a pilot I recognize that these atmospheric effects impact all of us regardless of health, and are worth considering when planning our workout routines. (If you do have health concerns, consider consulting your doctor before exercising at high elevations.) ©2010 Gregory N. Brown
PS: Calculate your own density altitude with this chart used by pilots to predict aircraft performance; it’s equally valid from a physiological standpoint. Simply reference the diagonal “pressure-altitude” line equalling your location’s elevation (accurate under standard pressure conditions) against air temperature. Note from the corrections table that low atmospheric pressure can further raise density altitude by over 1000 feet.
PPS: Read/hear NPR’s “Weather Data Sheds New Light on Mt. Everest Mystery,” about how the 1924 disappearance of George Mallory and Andrew Irvine on Mt. Everest may have resulted from oxygen starvation due to an extreme low-pressure area that moved in while they were on the mountain − density altitude at its ultimate…