A seal for a gas treatment hood assembly is disclosed. The hood assembly is placeable over a user's head for treating a user with a gas such as oxygen. The assembly may have a neck ring around a user's neck with an elastomeric, tubular neck seal with one end of the neck seal connected to the neck ring and the other end configured to fit around the user's neck. The neck seal may have a double sealing element construction and may also have variable thickness along a sidewall for enhanced structural properties and improved patient comfort. The seal and hood assembly may be used in providing oxygen to a patient in a hyperbaric chamber.