Breathing… we do it all day long, every day, and yet we never even put a thought to it. But so many respirator users do not understand the basics of how we get the air (more importantly, the oxygen in the air) into our lungs so that it can enter our bloodstream for survival. As I said, breathing is an involuntary act – meaning we are not sucking air into our lungs. In the simplest terms, we use the 14.7 psi atmospheric pressure to push the air into our lungs, which have relaxed and created a vacuum; it is the pressure differential that moves the air into our lungs – NOT us sucking it in. Here is some of the best (simplest) medical language explaining this I have ever seen. The author is unknown, but I am pretty certain this came out of a medical textbook…
Alveoli act like balloons. The lungs are suspended in the thoracic cavity at a slight negative pressure. When the diaphragm is lowered, that pressure becomes more negative and the lungs expand into the cavity. Atmospheric air moves into the resulting partial vacuum and inflates the alveoli. One is aware of the effort. Once the alveoli are fully inflated, exhalation can be accomplished by merely relaxing the diaphragm since the wall tension in all the tiny alveoli will act to force the air out of them. By forcing the diaphragm upward, we can exhale forcefully by adding the diaphragm effort to the recoil of the elastic alveoli.
So here is the problem, when we don a tight-fitting negative pressure respirator, we actually have to work much harder to get the air into our lungs, as the respirator acts as a barrier between the atmospheric pressure and the vacuum inside our lungs. Breathing is still an involuntary act, but the amount of work necessary increases considerably just by the mere presence of the respirator on our face. Add in some heavy labor and we have a serious demand on our respiratory system as well as our circulatory system (e.g. respirations and heart rate increase).
The medical evaluation intends to ensure that the user’s systems can tolerate this increased demand to just get the air around us into our lungs.