Question by Aditya Prakash Singh

On Earth, the atmosphere exists all around us, and with it comes atmospheric pressure. The air column above us is being pulled down by gravity (which is why we can say that there is such a thing as the “weight of the air”). One hundred kilometers of air directly above a square inch on our shoulder has a total mass of around 7 kilograms. So why don't we perceive it? It certainly doesn't feel like we are constantly holding multiple bowling balls on our shoulders. To answer this question, we must look at some properties of fluids.

First of all, the weight of the air is not the only force acting upon us as we stand in this atmosphere. Atmospheric pressure is exerted on the human body in all directions, thereby exerting a force equal to the pressure times the surface area upon which it is acting. So essentially, the air around us is not just trying to crush us down, but actually trying to crush us in.

If the atmosphere were made of solid material, the pressure would be felt only downwards and not sideways. However, a situation like this cannot exist in a fluid, which cannot sustain shear stresses and must flow towards lower pressure whenever it feels such. (It does not flow up, towards the lower pressure above, because gravity prevents it (cf. pressure gradient)). The key here is that fluids at a finite temperature have an internal pressure, even in the absence of a gravitational force. Naturally, this pressure must act uniformly in all directions, else there would be a net force. Thus, in the case of the atmosphere, the pressure balances itself around an object (except for a small vertical component). 

Let’s imagine we’re holding out our hand with the palm facing down. While it's true that the air is pushing down on our palm with a lot of force, the air is also pushing up on the back of our hand with nearly equal force. Even when the force of the air on the back of our hand cancels the force of the air on the palm of our hand, our hand is still getting crushed in the middle between these two opposing forces.

Fortunately, there is also pressure inside our hand directed outwards which cancels the inward force due to air pressure. The air inside of our lungs is at the same pressure as the air outside, so it presses your chest cavity outward with the almost same force as it is being pressed inward (slightly less, because there is more surface area on which pressure is applied outside than in, and that is why we exhale when we relax).

As for the rest of our body, it's mainly made up of water, which is a liquid and not a gas. Liquids are relatively incompressible: they push back with the same pressure that is pressed upon them by themselves. Note that even if the internal body pressure and the external air pressure are not exactly equal and therefore don't cancel each other out, most tissues in our body are strong enough to withstand the resulting minute net force.

As a result, there is no appreciable force on the surface of our hand (apart from gravity, of course!).

That is why it does not feel like we are constantly lugging around ten bowling balls on the palm of our hand, and that is why the long air column above us does not pin our hand to the ground. Similarly, while air presses down on the top of our head with a lot of force, it also presses up on our chin and neck with nearly the same amount of force. In this way, the fluid nature of air makes it so that the weight of the atmosphere pushes everywhere in on us, but does not push us down.

Now, if we were on Venus it would be a different question!

The atmospheric pressure on Venus is 92 times that of the Earth. However, from what we know, we can claim that the (~60%) water in our body, being an incompressible fluid, should be able to push back against this increased pressure, thereby balancing out the atmosphere’s crushing effect. However, while water is incompressible, air is not (at least, it can’t be considered to be, under such high pressures). Most of the body has liquids and bone which don’t compress well, but the voids with air cavities will definitely collapse under this pressure. This means that even if (theoretically) the high pressure on Venus should feel just like extreme deep sea diving, it would definitely crush our lungs. And this, along with the other nasties in Venus’s atmosphere, would be quick to eliminate us.

And how would a space journey without a spacesuit feel like?

Even though outer space represents a lack of air pressure, which usually counters the internal pressure in our bodies, our tissue is strong enough to handle the imbalance. Our skin is almost completely gas-tight and strong enough to withstand a pressure differential of well over one atmosphere. Long-term exposure to the vacuum, of course, is deadly, but we could recover from an exposure of, say, 15-30 seconds. We don't explode, and our blood doesn't immediately boil, because the pressure is held in by our skin. There are other nightmares such as lack of oxygen, exposure to radiation, and low temperature. Humans can remain fully conscious and useful for 9-12 seconds after being exposed to a vacuum.

References-

1. https://wtamu.edu/~cbaird/sq/2015/09/14/why-dont-i-feel-the-miles-of-air-above-me-that-are-crushing-me-down/
2. https://www.quora.com/Is-the-air-above-us-pushing-down-on-us-or-is-it-just-gravity?share=1
3. http://scienceline.ucsb.edu/getkey.php?key=1711
4. https://www.physicsforums.com/threads/how-much-atmospheric-pressure-is-exerted-on-the-human-body.867521/
5. https://wtamu.edu/~cbaird/sq/2012/12/17/how-long-can-a-human-in-outer-space-last-without-a-spacesuit-before-exploding/
6. https://physics.stackexchange.com/questions/549532/why-would-the-pressure-on-venus-crush-you
7. https://www.sciencefocus.com/space/what-would-happen-if-you-were-in-space-without-a-spacesuit/