air lock, compartment connecting two different environments, usually at different pressures, that enables personnel to transfer from one environment to the other. Space capsules have air locks to enable astronauts to move between the pressurized cabin and the near vacuum of space. A more common example is the air lock between the outer atmosphere and the working chamber of a caisson. By its means access can be gained to the working chamber without loss of pressure. It is also used at the head of tunnel excavations underwater. There is a door at each end. When the outer door of the air lock is opened, men or material may be admitted into the compartment. After the outer door is closed, compressed air is admitted to raise pressure in the air lock to the level of the pressure in the working chamber, and the inner door can be opened. The reverse of this procedure takes place on leaving the working chamber. Great care must be exercised in passing workmen through an air lock, so that the change of atmospheric pressure takes place gradually. Too sudden a change of pressure may cause decompression sickness.