Permeability is the tendency of gas to pass or diffuse through the elastomer. This should not be confused with leakage which is the tendency of a fluid to go around the seal. Permeability may be of prime importance in vacuum service and some few pneumatic applications involving extended storage but is seldom consequential in other applications. It should be understood that permeability increases as temperatures rise, that different gases have different permeability rates, and that the more a seal is compressed, the greater its resistance to permeability. Refer to Section III, O-Ring Applications, for additional information on permeability and vacuum service.