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.
Medical masks (surgical/procedure masks) are loose fitting masks that cover the mouth and nose. A respirator is designed to reduce exposure to airborne contaminants. Known airborne pathogens include TB, SARS, Anthrax, and Hanta virus.