Every summer, numerous otherwise healthy friends here at Flagstaff complain of feeling weak or winded while exercising in midday summer heat.
Some have even sought medical help at the E.R. Along with cyclists, hikers, and 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 above sea level, 2) increasing temperature, and to a lesser degree, 3) increasing humidity.
Have you noticed your (non-turbocharged) car’s poor passing performance here on hot summer days? We pilots study density altitude because it can be a significant factor in whether our airplanes 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 reaches 90°F here, 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 far up Humphreys Peak. 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 on a warm day at high elevations!
Based on 7,000 feet elevation above sea level and standard atmospheric pressure:
- At 90°F air temperature, your body operates at a density altitude of 10,500 feet.
- At 80°, your body operates at a 9,900-foot density altitude.
- At 70°, 9,200 feet.
- At 60°, 8,600 feet.
- Only near 35°F, does our density altitude here at Flagstaff approximate our actual 7000-foot elevation! (Based on 59°F at sea level.)
The bottom line? At high elevations, it is healthier as well as 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.)
Now our bodies are a bit more adaptable than engines, due to flexible lung capacity and oxygen exchange. But my wife the Clinical Pharmacist advises that it takes our bodies 72 hours or more to adapt to altitude changes through blood-vessel dilation, and 21 days to produce more oxygen-carrying red blood cells. So a hot afternoon hike immediately upon arriving from the much-lower Valley of the Sun is not a great idea.
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.)
Sincerely, Greg
PS: Calculate your current density altitude with the lower section of this online calculator, where “pressure altitude” is your elevation above sea level.
Or with this chart: 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…
©2010, 2024 Gregory N. Brown