Gas/vapor protection: Such suits (level A in the US) are gas or vapor-tight, providing total encapsulation and the highest level of protection against direct and airborne chemical contact. They are typically worn with a self-contained breathing apparatus (SCBA) enclosed within the suit. These suits are typically constructed of several layers and, being airtight, include a release valve so the suit does not overinflate from air exhaled by the SCBA. The release valve does retain some air to keep some positive pressure ("overpressure") inside the suit. As noted, such suits are usually limited to just 15-20 minutes of use by their mobile air supply. With each suit described here, there is a manufactured device designed to protect the respiratory system of the wearer while the suit/ensemble is used to protect skin exposed to potential, or actual dermal, hazardous agents. That device is a respirator. A respirator may be something as simple as a headband strap filtering facepiece respirator (FFR), to a headharness negative pressure fullface respirator (air-purifying respirator/APR), to a full face, tight fitting, closed breathing air, or open circuit, self-contained breathing apparatus (CC-SCBA or SCBA)