For comfort, the human body requires a relative humidity of 40% to 60% — Page 134, Lesson 207

I want to walk you through the stress factors that come from the cabin environment, starting with low humidity. In a modern aircraft, the air conditioning system draws air from outside via the engine’s compressor. At airline operating altitudes, the outside temperature can be as low as -30°C to -55°C. At those temperatures, the air is very dry, with a relative humidity as low as 5%. For comfort, the human body requires a relative humidity of 40% to 60%. So you can see the cabin air is far drier than what we’re used to on the ground. At low humidity, you become uncomfortable because the mucous membranes of your nose and throat dry out. Your eyes become sore as tears evaporate rapidly and the tear ducts dry. Water vapour is added to the cabin environment during respiration—every time you breathe out—but because the air is continually replaced, this has only a limited effect in raising the cabin humidity. It would be possible to add water to the cabin air, but the weight penalty of carrying extra water is not considered commercially viable. So airlines don’t do it. Now, a practical point: it is not advisable to take on a lot of extra fluids in these circumstances. Instead, drink only enough to maintain comfort. The note in the book tells us that humidity in the cockpit typically varies between 5% and 15%. So flight crew should drink sufficient fluids in flight to avoid dehydration. During flight, the relative humidity in the cabin is similar to a dry summer climate or to being indoors in the wintertime. Be aware that caffeine and alcoholic beverages actually contribute to dehydration. So if you’re drinking coffee or alcohol, you’re making the problem worse. Dry air may cause irritation of the eyes, especially if you wear contact lenses, and those may have to be removed. Dry air can also aggravate allergies or asthma. Some aircraft have a humidity control system that may alleviate this problem, but not all do. In general, human performance is poor in an environment that is humid, regardless of the ambient temperature. Surroundings that are both dry and warm are most conducive to high performance. Now let’s move to extreme temperature stress factors. The human body is extremely sensitive to heat and cold and functions efficiently only over a remarkably small temperature range. Normal oral temperature is considered to be between 36.1°C (97°F) and 37.2°C (99°F). Physical and mental performance starts to become significantly impaired at an internal body temperature of about 38°C. That’s only about a degree above normal. Apart from the skin and fat, which both act as insulators, the body has mechanisms controlled by the autonomic nervous system—the ANS—that try to cope with changes in body temperature and maintain equilibrium. Sweating and shivering are examples. But when the body is exposed to extreme temperatures that these internal homeostatic mechanisms cannot cope with, it reacts violently. Let’s look at extreme heat stress specifically. Once the blood temperature rises to approximately 41°C (106°F), the self-regulatory systems of the body can no longer cope. The effects of extreme heat include: excessive sweating, which leads to fast depletion of body fluids and electrolytes. That dehydration causes a further rise in body temperature, making the situation worse. Typical symptoms are muscle cramps, giddiness, and fatigue. There is also a rapid increase in heart rate and an associated need for more oxygen. And, of course, thirst. So to summarise: low humidity in the cabin dries out your mucous membranes and eyes, and you need to manage your fluid intake carefully—avoiding caffeine and alcohol. Extreme heat, once your internal temperature hits about 41°C, overwhelms the body’s cooling systems and leads to dehydration, cramping, fatigue, and increased heart rate.