First, a practical point about decompression inside the cabin — Page 55, Lesson 85

I want to walk you through the effects of decompression at altitude and then introduce decompression sickness — what it is, why it happens, and the symptoms you need to recognise as a pilot. First, a practical point about decompression inside the cabin. When the aircraft loses pressure at height, the sudden drop in temperature causes windows and cockpit windshields to mist or fog up. That's a direct consequence of the rapid cooling. Now, the most critical priority in any decompression event: crew protection comes first. I cannot overstate this. If decompression occurs, you as a crew member must individually don your oxygen mask and check that oxygen is flowing — as quickly as possible. Any delay caused by trying to help another crew member or a passenger first could have catastrophic results for everyone on board. Your own mask goes on first, then you help others. Let's move into decompression sickness, which we abbreviate as DCS. You already know that nitrogen makes up the major part of the air we breathe. Under normal conditions, a small amount of nitrogen is dissolved in the blood, and it plays no part in the body's normal processes. The problem arises when that nitrogen comes out of solution as small bubbles. Think of opening a fizzy drink bottle — when you release the pressure, bubbles form. The same principle applies inside the human body. If nitrogen bubbles form in the blood, that process leads directly to decompression sickness. Why does this happen at altitude? Because the body is normally saturated with nitrogen. When ambient pressure drops abruptly — during a decompression — some of that dissolved nitrogen comes out of solution as bubbles. Any ascent above 25 000 feet is normally associated with DCS. However, it is more likely the higher you go and the longer you are exposed to altitudes above 18 000 feet. It is unlikely to occur below 14 000 feet. In severe cases, the individual may collapse. In rare instances, DCS may occur or persist even after descent, and it can go on to cause death. Several factors increase the risk: hypoxia, cold, age, and excess body mass or obesity. Now let's look at the primary symptoms of DCS, which are grouped by where the bubbles form. First, the joints. Bubbles forming in the joints — typically the shoulders, elbows, wrists, knees, and ankles — cause rheumatic-like pains called the bends. In aviation, the shoulder, wrist, knee, and ankle are most commonly affected. Moving or rubbing the affected joint only makes the pain worse, but descending usually resolves the problem. Second, the skin. When nitrogen bubbles are released under the skin, it causes what is called the creeps. The sufferer feels as though a small, compact colony of ants is crawling over or just under the skin. Third, the respiratory system. This symptom is known as the chokes. Nitrogen bubbles may get caught in the capillaries of the lungs, blocking the pulmonary blood flow. This leads to serious shortness of breath, accompanied by a burning, gnawing, and sometimes piercing pain. Fourth, the brain. Bubbles affect the blood supply to the brain and the nervous system. This effect is called the staggers. The sufferer will lose some mental functions and control of movement. In extreme cases, chronic paralysis or even permanent mental disturbances may result. There is also a secondary symptom mentioned, but the excerpt cuts off there — we'll cover that when we continue.