the September 11 attacks. Preparedness lists, such as those presented by Ready.Gov, often recommend smoke hoods, although some lists use alternate names such as "fume hoods," "respirator hoods," or "self-rescue hoods." As most modern construction contains materials that produce toxic smoke or fumes when burned, smoke hoods can allow people to make a safe escape from buildings when it might not otherwise be possible. Positive-pressure hoods Smoke hoods present on aircraft, also called protective breathing equipment (or PBEs), typically generate oxygen for anywhere from 30 seconds to 15 minutes. The oxygen is kept in a closed circuit, usually thanks to a tight neck seal. A scrubber system may be present to reduce the levels of carbon dioxide, and is breathable for around 20 minutes. When the oxygen supply ends, the hood will begin deflating and must be removed to avoid suffocation. These devices represent a subgroup of smoke hoods called positive-pressure respirators, which prevent the ingress of smoke or toxic gases by maintaining a higher air pressure inside the mask than outside. Consequently, any leak will cause fresh air to leak out of the mask, rather than toxic air to leak in. References Aviation safety Smoke Gas masks Protective gear