(outside) pressure — Page 173, Lesson 272

I want to walk you through the effects of decompression on the human body, starting with the distinction between a slow decompression and a rapid or explosive decompression. These are two very different events, and the sensations a passenger experiences are completely different in each case. Let’s begin with a slow decompression. This is a gradual loss of cabin pressure. In this scenario, passengers will notice three specific things. First, the cabin temperature will fall as the pressure reduces. That’s a direct physical relationship — as the air expands and thins out, it cools down, so the cabin gets noticeably colder. Second, body cavities — and I mean your sinuses, your ears, any air-filled space inside you — will become pressurized. Because the outside pressure is dropping, the air trapped inside those cavities is now at a higher relative pressure. This can cause discomfort, and the passenger may require assistance in venting that trapped air — for example, by swallowing, yawning, or using the Valsalva manoeuvre to equalize the pressure in the ears. Third, as soon as the pressure falls past a certain threshold, the oxygen drop-out system will operate. That’s the automatic deployment of oxygen masks from the overhead panels. So in a slow decompression, the masks will drop and deploy automatically when the cabin altitude triggers the system. Now, the fourth option in the list says "no contra effects, as slow depressurization has no effect on the human body." That is incorrect — we’ve just listed three definite effects, so that statement is false. Now let’s move to the other side — a rapid or explosive decompression. This is a sudden, violent loss of cabin pressure. Here, the effects on passengers are much more severe. First, there is gross discomfort — that means intense, widespread physical distress. There is breathlessness — a feeling of not being able to get enough air. And there is possible damage to sinus cavities, because the pressure change is so fast that the body cannot equalize quickly enough, and the tissue can be injured. Second, passengers may notice tunnel vision and sensorial depletion. Tunnel vision is a narrowing of the visual field — you lose peripheral vision and see only what’s straight ahead. Sensorial depletion means a general dulling of the senses — things feel muffled, distant, or numb. Third, there is a marked reduction in night vision. This is a specific and important effect — even if the cabin lights are on, the ability to see in low-light conditions is significantly impaired. This is because the retina, particularly the rods responsible for night vision, is very sensitive to oxygen deprivation. Fourth, passengers may experience pain in muscles and joints caused by the bends. The bends is the common term for decompression sickness — the same condition divers get if they surface too quickly. Nitrogen that was dissolved in the body tissues comes out of solution and forms bubbles, causing pain in the joints and muscles. So to summarize: a slow decompression gives you a cold cabin, pressurized sinuses and ears that need help venting, and automatic mask drop. A rapid or explosive decompression gives you gross discomfort, breathlessness, possible sinus damage, tunnel vision, sensorial depletion, reduced night vision, and joint and muscle pain from the bends.