What is a gas mask?

   A gas mask is an airtight device used to protect the wearer from harmful gases by chemically purifying the air as it passes through the apparatus to the lungs. When the wearer exhales, the air passes out through a one-way outlet valve in the mask. In warfare, poison gas was first used by the Germans in World War I. Since then, when war is imminent, gas masks are provided for troops and civilians alike.
   The gas mask consists of a con­tainer carried from the shoulders and a flexible hose running from the container to the mask. The mask fits snugly over the face, as part of a helmet, and has very strong glass eyepieces.
   Gas masks are also used by mine rescue crews to protect them from smoke and fumes following mine explosions and fires. These masks, tubes, and canisters are more substantial than the military masks and are similar to masks used by members of fire departments.
   After the development of the atomic bomb the gas mask was adapted for protection against nu­clear radiation. The new masks have the canister attached directly to the mask without the use of a hose. Special diaphragms are built in to permit talking.