Last year, my friends and I went hiking in the awe-inspiring mountains in Denver, Colorado.  The five of us are all active people, but certainly not elite athletes. As we left the car and glanced out at the beginning of the trail, I thought, “That’s barely a mild incline; this will be easy.”  We grabbed our water bottles and set out for the first peak. But scarcely halfway to the first resting point, I began to gasp for air. I willed my quadriceps to keep pushing up the trail, but each step left me weary and feeling weak.  “How could I possible be this out of shape?” I wondered.  After training as a ballet dancer for 20+ years, I had considered myself reasonably physically fit, but this mountain was showing me otherwise.  

We stopped to catch our breath, and a “lightbulb” went on as a tidbit of science popped into my head.  I remembered the one critical reason why hiking in Denver is so challenging: Denver, the “mile-high” city, is elevated far above sea level and consequently has lower air pressure than many other cities.

Photo by Dana Simmons

What does low air pressure mean for us?  

To begin, let’s first explore what’s in the air we breathe and why it’s so important for us.  The air we breathe contains 20.95% oxygen, 78.08% nitrogen, 0.93% argon, 0.04% carbon dioxide, and tiny amounts of methane, neon, helium, and water vapor.  As humans, the important part for us is the oxygen. As we inhale and air fills our lungs, oxygen enters our bloodstream where it is snatched up by hemoglobin molecules.  Hemoglobin travels through our body and delivers oxygen to our organs and muscles, allowing them to work. During demanding physical activities such as hiking, our muscles require more oxygen than they do during sedentary activities, such as binge-watching TV shows.  So, when we are active, our body helps us increase the flow of oxygen to our muscles by making us breathe faster and more deeply.

FACT: If you feel a sharp pain in your side, or a cramp, during physical activity, it means that you aren’t inhaling a sufficient amount of oxygen for your muscles to comfortably perform that activity.  Try to breathe deeply to reduce the pain.
FUN FACT: Atmospheric methane primarily comes from cow farts.

So what makes hiking in Denver so different from anywhere else?  It’s the low air pressure. Denver, the mile-high city, is named as such because it sits approximately one mile above sea level.  And as we move away from the center of the Earth, or increase our altitude, the air pressure decreases. Air pressure is a measure of how condensed all the molecules in the air are, so when the pressure decreases, all the molecules in the air, including oxygen, spread apart, making it harder for us to breathe.  Air pressure is measured in several different units including millibars, atmospheres, millimeters Mercury, and pounds per square inch. At sea level, the standard air pressure is 1013 millibars. The air pressure in Chicago on December 31st at 2:00 pm was 1010 millibars.  In contrast, the air pressure in Denver was only a measly 831 millibars of pressure.  

Although the percentages of the components of air are the same in both Chicago and Denver, the overall density of the air in Denver is lower.  This means that to get the same amount of oxygen to our muscles in Denver as we do in Chicago, we must breathe extra deeply or extra quickly.  

Remembering all of this on the mountainside was at first a relief: I was not, in fact, as out of shape as I had thought.  However, as the hike became more vertical and the path became more rugged, I knew that I would have to do some serious breathing to keep up.  

FUN FACT: Many elite athletes train or sleep in low pressure environments to enhance their abilities in races taking place at regular atmospheric pressure or to prepare for competitions at high altitudes.

Since this is the ISC’s 100th blog post (woohoo!), let’s do a quick thought experiment!  Keeping in mind how difficult physical activity is in Denver at 833 millibars of air pressure, what do you think would happen if the air pressure around you dropped from 1013 millibars to only 100 millibars?  

The average breathing rate for an adult is 12-16 breaths per minute.  In this environment, where the air pressure is one tenth that of normal, you would have to increase your breathing rate by ten times to consume the same amount of oxygen.  In other words, you would have to breathe 120-160 times per minute in order to take in a normal amount of oxygen. However, breathing that quickly is impossible. In fact, when a person hyperventilates – typically during a panic attack or asthma attack – he or she breathes more than 20 times per minute and gets dizzy because they’re producing carbon dioxide faster than they can get rid of it.  To treat this, people can breathe into a paper bag so they inhale recycled carbon dioxide instead of oxygen in order to restore their proper oxygen to carbon dioxide balance.

CBS’s Sheldon from The Big Bang Theory hyperventilates.

Try it: Count how many fast, deep breaths you can take in one minute.  I guarantee you that it won’t be enough. You would have to take a minimum of two deep breaths every second to inhale a normal amount of oxygen at a 100 millibars.  

Simply put, at 100 millibars of atmospheric pressure, we could not survive.  Humans have adapted over the course of evolution to exist in a world of around 1013 millibars of pressure because that’s what’s here. Life is fragile, and we are very sensitive to changes in our environment.  Take a breath of fresh air and marvel at the intricacies of our biology.

FUN FACT: Sherpas in the Andes and Himalayas have adapted to their environments over hundreds of generations.  In a published study from the Johns Hopkins University School of Medicine, researchers discovered that Sherpas’ mitochondria convert oxygen into energy more efficiently compared to people living at lower altitudes.


  • Dana Simmons

    Dana Simmons is a Co-Editor-in-Chief of Science Unsealed and holds a Ph.D. in neurobiology from the University of Chicago. She is an active participant in the global SciArt community, and her innovative neuron art has been exhibited around the world. Dana is a medical writer for a Chicago agency that serves pharmaceutical and biotechnology companies. SciArt website: Twitter: @dhsimmons1

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