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. The mask is your window to the underwater world. It provides an air pocket so your eyes can focus and you can see clearly underwater. Your nose must be within the eye pocket so you can adjust for pressure changes (this is why swim goggles cannot be used for snorkeling).