Hold your breath. After a little while, you just have to breathe. It's as if every cell in your body is screaming, "Breathe!" That's because you need oxygen. Cells use it to create energy. Muscle cells use it to contract. Nerve cells use it to send signals...and so on.
What would happen if you were locked in an airtight room?
In order to answer this question, we need to figure out how much oxygen you actually use in a normal day. First, let's assume that you're sitting in a chair, just hanging out. Maybe you're watching TV. You are not panicking, and you are not exercising. You might take 10 to 12 breaths per minute. That means that you will inhale and exhale seven to eight liters of air (about one-fourth of a cubic foot) every minute.
You could actually measure this yourself by holding a garbage bag in your hand and exhaling each breath into it. A typical white garbage bag that you find in the kitchen holds 50 liters or so. It would take you five to ten minutes to fill it full of air, depending on how big you are, and what you are doing. If you do the math, you can see that in a day you breathe something like 11,000 liters of air. That's roughly 200 garbage bags full of air, or 388 cubic feet of air.
Now, what if someone were to completely seal your bedroom with plastic and lock you inside. How long would you last? A typical bedroom is roughly 12 feet by 12 feet by 8 feet. That is 1,152 cubic feet or 32,621 liters. It would take you three days to inhale and exhale 1,152 cubic feet of air. However, that does not mean that you would actually be able to live in your sealed room for three days.
If you could somehow use your room as a giant "air tank" -- so you only breathed the air in it once -- that would be one thing. This is what people do when they go scuba diving. They breathe air out of their tank and then exhale it into the water. But you're not just breathing in - you're breathing out into the room, too.
Since you are sitting inside a sealed room, you have two problems. The first problem comes because you are breathing out carbon dioxide with every breath. You are "polluting" the room with carbon dioxide. The other problem is that, with each breath, you are consuming oxygen. So the amount of carbon dioxide in the room is rising with every breath, and the amount of oxygen is falling.
When the person first locks you in the room, about 21% of the air in your room is oxygen. The rest is nitrogen. There is hardly any carbon dioxide at all in the room (0.05% or less). Now you take a breath. What goes in is 21% oxygen and 0% carbon dioxide. What comes out is 16% oxygen and 5% carbon dioxide. Once you get to the point where the room's total oxygen falls to 19% and the carbon dioxide level reaches 2%, you're in trouble. Not only are you getting less oxygen, but you're now also taking in carbon dioxide, which is actually a poison.
Therefore, in reality, you will only last a day and a half or so. Then your body will begin having problems. So, you had better start working on finding a way out of that room!
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