What is a heifer cow? A heifer is a mature female cow that has not calved, or given birth to a calf, yet. As soon as a heifer gives birth, she becomes a cow. A cow refers to a female of any age that has given birth.
When most people think about gas masks or respirators, what they usually envision is a tight-fitting plastic or rubber face mask with some sort of filter cartridge. The mask covers the nose and mouth. These are called half-mask air-purifying respirators.
The single most important point: For a mask to work properly, it has to seal the area of nose and mouth from the external environment. The inhaled air has to pass through the filter material in order to remove pollution by physically trapping or adsorbing (different from absorption) gas or vapour molecules.