Method and apparatus for providing clean air to an environment. Incoming air, which may be contaminated with chemical, nuclear or biological contamination is initially cooled and water is removed from the cooled air by a first cooler, a first water separator, a second cooler and a second water separator. The cooled air is passed through a regenerative pressure swing absorption system which removes the contaminants. The resulting, cleaned, air is expanded by an expander. The expanded, cleaned air is provided to the environment, after being used in the second cooler to clean the incoming air.