A shirt-sleeve environment is a term used in aircraft and spacecraft design to refer to an environment which requires no special clothing be worn for survival or moderate comfort. In terms of aircraft design, the term gained significance from the fact that early aircraft were not pressurized (or even enclosed), and the pilots and passengers were required to wear thick clothing for warmth and, in the case of stratospheric aircraft, special breathing apparatus as well. In regards to spacecraft design, the primary concern in creating a shirt-sleeve environment is maintaining proper pressure and protection from cosmic radiation.