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 mask (or nasal interface) is probably the most important piece of CPAP equipment that can ultimately determine whether or not you can benefit from your machine. The mask forms an air-tight seal around your nose or nose and mouth, and is held in place by various types of headgear. The mask is also attached to a soft flexible hose that connects to your machine. Notice that I said air-tight, not tightly fitting.