A head enclosing gas hood for treating respiratory ailments is placeable over a patient's head for providing a gas, preferably oxygenated, to said patient. The treatment gas hood has a hood portion which is connected to a hood ring. The hood ring slides over a two-piece neck ring, the two-piece neck ring consisting of an upper neck ring and a lower neck ring retained in adjacent relationship. The hood ring slides over the two-piece neck ring and forms a sealing relationship therewith. The two-piece neck ring has a neck seal retained therein and has a sealing ring on the outer periphery thereof for engagement with the working surface of the hood ring. Ports may be provided for directing a flow of a gas into and from the interior portion of the hood where the individual's head is located. The neck seal provides an adequate seal between the neck ring and the individual's neck such that a pressurized environment may be created in the hood if desired. The hood may also be used to create a local environment different from ambient, such as rich in oxygen for the patient to breathe without pressurizing the hood. The two-piece neck ring may also have a retaining ring for holding the neck seal in place after the upper and lower neck rings are retained together.