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.
Nasal cannulas and simple face masks are typically used to deliver low levels of oxygen. Another type of mask, the Venturi mask, delivers oxygen at higher levels. Sometimes nasal cannulas are also used to deliver high levels of oxygen.