How do the Flashes breathe utilizing their super speed? The air pressure would be extreme. Do they run in a pressurized bubble?
Most of the Flashes get their powers by channeling an extra-dimensional energy known as "The Speed Force." In fact, most DC characters with speedster-type powers are said to ultimately draw their quickness from this ultimate energy source.
As a result, the Flash in particular can use the the Speed Force to do more than just move quickly. Wally West once pointed out that he creates an aura of protective energy around himself when he hits high-speeds. This aura basically cocoons the Flash from friction-related effects.
In other words, the Flash has to create a personal environmental bubble just to shield himself from burning up due to high velocity friction. Not suffocating is simply a lower level use of his Speed Force shielding ability.
It's simple. He doesn't breathe. Appearances to the contrary, when true speedsters in the DC Universe utilize their powers at their maximum effect, they stop existing solely as beings of flesh and blood and become forms of living energy. Supporting evidence follows.
Barry ponders his powers and how they work.
Panel 1: "I wait until I am clear of the city limits before I hit the sound barrier. That's one I figured out the hard way a few times. Flying glass and pedestrians don't mix. I really crank it up when I hit the open road."
Panel 2: "Incredible. At this speed, I vibrate my atoms at a frequency that allows pesky things like bugs and telephone poles to pass through my mass. It's like coming purposefully unglued as you're shot from a high caliber rifle. I'm a swarm of atoms rocketing towards the one thing I can't live without. The reason I keep my identity secret."
What most people don't know about Barry Allen was while he adjusted to having powers quickly, he used his very scientific mind to understand his powers, methodically exploring them in the early days. This meant his understanding of his powers was not nearly adept as he is now.
The panel hints at his early use of his powers may have included his "frictionless speed aura" but not the included protection from sonic booms.
He also mentions he is out of phase with the world when he is running. At the speeds he is moving, he is vibrating his molecules to be able to pass through everything that might be in his way, likely including air. That means he is also not likely to be breathing either.
Barry Allen, on the first day he gets his powers, he was a natural. No suit, no boots, and still managing to run on water in his street clothes.
The Speed Force or "Why the Flash gets to ignore the laws of physics"
The DC Universe's speedsters are notorious for defying the laws of physics and when the Flashes are moving at superspeed, they are not completely in phase with the universe. Yes, I know it is difficult to consider, but unless they are somehow transformed into energy, what they do is simply beyond the physics of our universe. DC Comics breaks their users of superspeed into two groups, the Speed Force users (true speedsters) and everyone else.
True speedsters, beings who have a connection to the "speed force" dimension, are able to tap that dimension for access to the ability to defy the laws of physics. Forces such as gravity, inertia, energy, mass all become variable and able to be manipulated by speedsters.
DC Comics stories indicate only true speedsters can utilize superspeed which is drawn from the Speed Force. But I find that to be an incomplete conclusion. There is no other reasonable explanation for speedsters to have access to the Speed Dimension exclusively when there are plenty of other metahumans capable of performing feats of incredible superspeed such as Superman, Wonder Woman, Captain Marvel, etc.
Speedsters consider their manipulation of speed as a crossing of barriers. Each barrier releases a speedster into another realm of capability: The sound barrier, the light barrier, the time barrier, the dimension barrier and ultimately the "speed force" barrier.
People who are able to utilize superspeed but are not speedsters are thought to be able to access the speed dimension but do so unconsciously and uncontrollably, so their results vary widely. This may simply be part of a metagene connection allowing the non-speedster the ability to move very fast but limited to some or all of the laws of physics regarding superspeed interactions.
Wally West was such an individual early in his career. He seemed to possess all of the powers of the Flash during his time as Kid Flash but he lacked the knowledge, discipline and intuitive understanding Barry Allen seemed to have as the Flash.
It was theorized after the death of Barry Allen, the Silver Age and most well known Flash was believed killed during the first Crisis on Infinite Earths, Wally's powers were linked to Barry's. When Barry died, Wally's connection to the Speed Force was reduced.
During that time, Wally could not top the speed of sound and was forced to consume a higher calorie diet to utilize his powers. This limitation, believed to be a psychological one, was eventually overcome allowing Wally to access the "speed force" for far greater control of his powers than even Barry knew was possible.
Wally questions the loss of his powers.
The doctor in the clip indicates from her understanding of the metahuman abilities Barry Allen possessed, he was completely outside of the laws of physics.
When Barry was using his powers he was selectively affected by states of matter, could vibrate himself to be able to pass through walls, able to move at near light speed, be unaffected by friction, inertia or even gravity if he so desired.
This ability to deflect or selectively utilize the effects of speed was first called his "friction-less aura."
It was later expanded to include any speedster whose powers utilized super-speed, violated laws of conservation of energy, momentum or motion and became the "speed-force aura" or later a connection to the "Speed Force" dimension, the source of all speedster metahuman abilities.
It must also be assumed the "Speed Force" makes it possible for Flashes to communicate, see, hear and interact with other speedsters as well as being aware of the universe at large while utilizing superspeed.
The portrayals of the "Speed Force" are inconsistent and these days spoken about less often than they used to be during the days of Wally West as the Flash.
Flash Fact: Barry Allen ran so fast at one point during the Crisis on Infinite Earths, he was believed to have died. Instead, he crossed the dimensional barriers merged with the "Speed Force" and moved backward through time to eventually become the lightning bolt that struck him in his lab and gave him his powers. While many writers like the idea that Barry generates the "Speed Force" I prefer to think of it as another natural phenomenon like the Source and he is merely able to tap into its limitless potential.
What I think the speed force does is it syphons off oxygen from the surrounding atmosphere and channels it into the 'aura' for the conduit to breathe. The drawback to this, however is that if there is no atmosphere, then the conduit will quite literally run out of oxygen-and possibly suffocate.
Case in point is that nearly every example I have seen of the Flash running in space, there is always some outside force providing both oxygen and a surface for him to run on-even in 'The Human Race,' where he is running through a time-space wormhole tunnel. The one time he ran out into space without such was when an evil clone of Supergirl tricked him into following him out of the Watchtower onto the surface of the moon-and the same thing happened that would happen to you or me, and he nearly died.