Abstract: A hypobaric chamber is adapted to receive the affected body part to be treated through an entry port formed in one wall of the chamber. An aperture is formed at the entry port for creating a loose, non-occlusive seal around the body part of the patient, the aperture comprising an adjustable iris formed from a pliable material attached to the edges of the entry port and having a center opening to which are attached the inner ends of a plurality of iris leaves or slides. The pliable material is stretched between the edges of the entry port and the inner ends of the slides to form a diaphragm-like structure. In the preferred embodiment, the pliable material is latex or some other rubber-like material. The seal is adjustable by moving the slides radially to adjust the portion of the port that is covered by the pliable material. The inner edges of the slides are positioned so that a small gap exists between the skin and the edge of the seal, allowing unrestricted air flow-through.