Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 10^-7 pascal or 100 nanopascals (10^-9 mbar, ~10^-9 torr). UHV requires the use of unusual materials for equipment, and heating of the entire system above 100 °C for many hours ("baking") to remove water and other trace gases which adsorb on the surfaces of the chamber. At these low pressures the mean free path of a gas molecule is approximately 40 km, so gas molecules will collide with the chamber walls many times before colliding with each other. Almost all interactions therefore take place on various surfaces in the chamber.