A facial breathing mask suitably for regulating the carbon dioxide concentration in the inspired air, the facial mask comprising an air chamber with an opening through which a user can breathe. At least a part of the air chamber is flexible and/or collapsible and comprises one or more of semi-permeable membranes separating the air chamber and a surrounding atmosphere. In use, the invention can raise the carbon dioxide tension in the inspired air and/or influence the depth and rate of breathing of the user.
