The Armstrong limit, often called Armstrong's line is a level of altitude that water boils at 37ºC, normal body temperature. It is named after Harry Armstrong, founder of the U.S air forces department of space medicine in 1947. An atmospheric pressure of 6.3kPa, experienced at very high-altitude flights is capable of inducing boiling at normal body temperature, as a result exposed liquid such as saliva, tears and sweat literally boil off exposed skin. Due to the low amounts of oxygen and the effect of boiling experienced at these altitudes, pilots are required to wear pressure suits when flying at or above the Armstrong limit.