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.
Citation: C N Trueman Gas masks in World War One. historylearningsite.co.uk. The History Learning Site, 31 Mar 2015. 16 Aug 2016. Gas masks used in World War One were made as a result of poison gas attacks that took the Allies in the trenches on the Western Front by surprise. Early gas masks were crude as would be expected as no-one had thought that poison gas would ever be used in warfare as the mere thought seemed too shocking.