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A group of Australian infantry wearing Small Box Respirators (SBRs) at the Third Battle of Ypres in September 1917. After the introduction of poison gas in World War I, countermeasures were developed. SBRs represented the pinnacle of gas mask development during the war, a mouthpiece connected via a hose to a box filter (hanging around the wearer's neck in this picture), which in turn contained granules of chemicals that neutralised the gas. The SBR was the prized possession of the ordinary infantryman; when the British were forced to retreat during the German Spring Offensive of 1918, it was found that while some troops had discarded their rifles, hardly any had left behind their respirators.Photo credit: Frank Hurley
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