Airplanes naturally have less oxygen in the air than that of the ground air. Unfortunately, some people are susceptible to hypoxia when exposed to environments with low oxygen levels. To protect against hypoxia-related fainting, commercial airlines maintain cool cabins.

Planes are kept cool for a combination of passenger health, safety, and technical reasons. One of the primary reasons is to prevent hypoxia-related fainting. Scientific studies have shown that passengers are more likely to faint on a plane due to high cabin pressure and warm temperatures; keeping the air crisp helps maintain consciousness and reduces the feeling of "stale" air. Additionally, a packed cabin generates an immense amount of "body heat" and heat from the onboard electronics and lighting. If the air conditioning weren't set to a low temperature, the cabin would quickly become stifling and uncomfortable. The air you breathe is a 50/50 mix of recirculated air and fresh air bled from the engines (compressed and then cooled), which is naturally very dry. Because dry air makes you feel cooler than humid air at the same temperature, the low humidity on a plane exacerbates the "chilly" feeling. Airlines generally provide blankets on long-haul flights because it is easier for a cold passenger to warm up with a layer than it is for a hot, fainting passenger to cool down in a confined, pressurized space.