In a recent episode of MythBusters, Adam and Jamie revisited the 'escape from an underwater car myth'. I have to say, this one was quite exciting. They put Adam in a car (with a safety diver) and dumped it in a lake. There were cables to prevent the car from going deeper than 15 feet, but this was still a rather scary myth. I have to say that I consider myself to be fairly comfortable in underwater situations, but I was nervous for Adam in this case. It just looked scary.
The first shot they showed of Adam escaping the car looked fine. However, it was then revealed that he cheated. He had to use some of the air from the emergency diver. This is part that also worries me. One of the most important rules that no one ever uses when scuba diving is: don't hold your breath while ascending. I am not saying Adam held his breath, it just wasn't clear that he didn't. In fact, this was also a concern of mine when they were doing the spinning around tests in a 10 foot deep pool.
So, why is there is no breath holding rule in scuba diving? First let me start with pressure where pressure is the force per unit area:
As you go deeper in water (or even in the atmosphere), the pressure of that fluid (or gas) increases. Why? Well, there are several ways to think of this. Suppose we think about this in terms of floating. A block of water in water should float, right? Here is a picture of water floating in water.
If this block of water is at rest and stays at rest, then the net force on it must be the zero vector. The net horizontal force must be zero, this means that the two forces from the pressure on the sides must have the same magnitude. That is fine, they are at the same depth (even though this changes along the side of the cube, they are the same on each side).
The force from the water pushing down on the top must be less than the force from the water pushing up. Why? Gravity, that is why. The net force in the vertical direction must be zero, this means I can write the vertical components as:
If this is an actual rectangular cubic chunk of water, then the top has the same area as the bottom. Rewriting that equilibrium equation, I get:
Right here, you can see that the pressure of the water at the bottom must be greater than at the top. But wait! We can do more. What if the cube has an area on the top and bottom of A and a height of d? In this case, I can use the density of the water to find the mass. I will use ρ for the density.
As you get deeper, the pressure increases. And? Where am I trying to go with this? Ok, now assume that I have a balloon with air in it. Suppose I put this balloon in some water and pull it underwater. If the temperature stays constant, then I can write the following expression comparing the pressure and volume of the balloon at the surface and at some depth (using the ideal gas law).
As you take the balloon down deeper, the pressure increases and the volume decreases. Now imagine this balloon is your lungs. Really, they are quite similar. If I take a deep breath at the surface and go down to a depth of 5 meters, my lungs will decrease in volume (because there is a finite amount of air in them). This really happens. I couldn't find a good picture of this on the interwebs, so I made one myself.
Ok, we are good. Now what if you do something different? What if you go down 5 meters and breath from a scuba tank? One of the very important things about a scuba regulator (the thing that attaches to the tank) is that it regulates. Really, it does. It regulates the pressure of the air getting to the mouth. It provides air to the diver at about the same pressure as the water. Does this matter? Shoosh yeah. The next time you go to a pool, try this. Take a 2 foot long pipe (pvc or something is fine). Go all the way underwater with one end of the pipe in your mouth and the other out of the water. Try to breath. It is no simple task. Why? Here is a picture.
When you inhale, you want your lungs to expand. The problem is that since the pressure outside is greater than the pressure inside your lungs, you muscles have to really push. If you lungs don't expand, you can't bring in more air. It is like some big fat guy is sitting your chest. Bring back the scuba regulator and it is quite easy to breath since the pressure outside and inside your lungs are about the same - no matter what the depth. This is why I tell new scuba divers that breathing from a regulator is way easier than breathing through a snorkel.
I still haven't answered the question, have I? Why can't you hold your breath while scuba diving? Ok, let's go back to Adam. Suppose he is 5 meters underwater in the inverted car. He is stuck, so he takes a couple of breaths from a scuba regulator. The pressure of the air in his lungs is the same as the pressure of the water at the 5 meter depth. Now what happens if he ascends while holding his breath? The opposite of the free diver going down. Instead of his lungs getting smaller, they would get larger - if only they could. However, they probably can't get any larger - especially if he took a full breath. This means that the lungs themselves have to exert extra pressure on the air and this only goes so far.
An ascending, breath holding diver can have one of two very bad things happen. The first is an air embolism. Basically (and I am not a medical doctor here, so there is that) air from the lungs gets pushed into the bloodstream. Air bubbles in the blood is bad. These bubbles can cause all sorts of bad problems - let's just leave it at that. The second problem has a name that I can't remember. Basically, your lungs break or tear. Again, not a good thing.
Can divers hold their breath while using scuba gear? Sure, as long as they don't ascend while doing this. The badness level of holding a breath and ascending is high enough that divers are usually told - "just never hold your breath". If you need to ascend while not breathing, breath. Breath out at least. This will allow the expanding air in your lungs to escape. Actually, the common recommendation is to do something similar to a slight humming sound. This will allow the air to escape. It can be an unnatural thing to do. You don't want to die, so you feel like you should keep the air in your lungs.
I am sure Adam and the MythBusters are quite skilled at these kinds of things, but it still scares me.
One more thing. How deep is too deep to hold your breath? Clearly, the deeper you go, the greater the change in pressure. How about I look at the change in pressure over 1 meter change in depth for different depths. Here is a plot of the percent change in volume as you go up that one meter.
So, if you are 1 meter underwater and you surface your lungs (if they were perfect balloons) would have almost a 10% increase in volume. If you are at 30 meters, it is less than a 3% increase in volume. Why? Atmosphere, that is why. When you are at the surface, you are at 1 atmosphere of pressure. At about 10 meters deep, you have doubled the pressure (1 atmosphere of pressure from the air and one from water). Ascending from 10 meter to the surface would reduce your pressure by half. But what if you ascend from 20 meters to 10 meters? You are not reducing the pressure by half. You are going from 3 atmospheres to 2 atmospheres.
The point is - holding your breath is even worse at shallow depths. But either way, just don't hold your breath just to be safe.
