Extreme environment clothing normally refers to clothing for Arctic or mountainous areas on land, although it is sometimes used for survival suits worn by mariners. The basic approach is to insulate one's body from heat loss, and keep liquid water or ice out of the insulation. The usual clothing is a parka and mittens, gaiters and booties. In high wind-chills, it may be supplemented by a mask, usually of oiled wool. In Arctic areas, the typical modern insulation is very fine hydrophobic polyester fiber batting sewn in laps between a nylon shell. The sewing must not compress or quilt the insulation, because the heat leaks out through the thin spots. Most designers now include a moisture barrier on the inner side to prevent condensation from a body's moisture from condensing and freezing in the insulation. Sometimes the moisture barrier has several layers of aluminized plastic film to reflect infrared back to the body. If plastic film is near the skin, usually some lightweight absorbent cloth is between the skin and the film, for comfort. The feet of booties are usually insulated by a thick layer of flexible closed-cell plastic foam, covered with a boot sole. Traditional tribal insulations are mouth-chewed oiled furs from the winter-killed Arctic animals, with the fur turned toward the body, or in heavy garments, with two layers, the inner turned away from the body, and the outer turned toward the body. The outer layers of skin breaks the wind, and the inner reduces condensation in the fur. Even in modern garments, certain furs (notably Wolverine) are prized for a hood lining, because they do not collect ice crystals from one's breath. For use in wet areas, the insulation is reduced (because the temperature is above freezing), and a barrier that passes water vapor, but not liquid water is sewn into the outer shell. This barrier passes gaseous water from one's body, yet prevents precipitation from soaking the insulation. The most effective modern fluid barrier is trade-named Gore-Tex, and consists of an expanded felt or mesh of polytetrafluoroethylene (trade name Teflon). The basic principle is that the plastic felt does not wet, and the pores are small enough so water's surface tension will resist high wind pressures. At the same time, the pores are large enough to easily pass water vapor. Gore-Tex is mechanically fragile, and is usually bonded to a polyester fabric, and sewn inside a nylon outer shell. Marine survival suits come in two different types, wet and dry. Dry suits are very similar to Arctic clothing, except with a waterproof shell, and closed-cell flexible plastic foam for insulation and flotation. Some have vents that claim to pass air but not water. Wet suits use foam rubber insulation. Water in the foam warms. They remain warm when water gets inside, do not handle the most extreme cold, and are profoundly uncomfortable for extended wear.
