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.
1 A mask or other device that fits over your nose or your nose and mouth. 2  Straps keep the mask in place while you're wearing it. 3  A tube that connects the mask to the machine's motor. 4  A motor that blows air into the tube.