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.
There are several models of non-rebreather masks available in the market with similar designs. The mask fits over the mouth and nose and elastic straps wrap around the ears or the head. A one-way valve in the mask prevents exhaled air from entering into the tubing or bag containing the oxygen that is to be inhaled.