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.
A procedure mask is used for performing patient procedures, or when patients are in isolation to protect them from potential contaminants. They are used for 'respiratory etiquette' to prevent people from spreading germs via talking, coughing, or sneezing.