improvements on the eyepiece and eyepiece assembly. Hutson Hurd patented a cup-shaped mask in 1879 which became widespread in industrial use, and Hurd's H.S. Cover Company was still in business in the 1970s. Inventors in Europe included John Stenhouse, a Scottish chemist, who investigated the power of charcoal in its various forms, to capture and hold large volumes of gas. He built one of the first respirators able to remove toxic gases from the air, paving the way for activated charcoal to become the most widely used filter for respirators. Irish physicist John Tyndall took Stenhouse's mask, added a filter of cotton wool saturated with lime, glycerin, and charcoal, and in 1871 invented a 'fireman's respirator', a hood that filtered smoke and gas from air, which he exhibited at a meeting of the Royal Society in London in 1874. Also in 1874, Samuel Barton patented a device that 'permitted respiration in places where the atmosphere is charged with noxious gases, or vapors, smoke, or other impurities.' German Bernhard Loeb patented several inventions to 'purify foul or vitiated air,' and counted the Brooklyn Fire Department among his customers. A predecessor of the N95 was a design by Doctor Lien-teh Wu who was working for the Chinese Imperial Court in the fall of 1910, which was the first that protected users from bacteria in empirical testing. Subsequent respirators were reusable but bulky and uncomfortable. In the 1970s, the Bureau of Mines and NIOSH developed standards for single-use respirators, and the first N95 respirator was developed by 3M and approved in 1972. World War I The first recorded response and defense against chemical attacks using respirators occurred during the Second Battle of Ypres on the Western Front in World War I. It was the first time Germany used chemical weapons on a large scale releasing 168 tons of chlorine gas over a four-mile (6 km) front killing around 6,000 troops within ten minutes through asphyxiation. The gas being denser than air flowed downwards forcing troops to climb out of their trenches. Reserve Canadian troops, who were away from the attack, used urine-soaked cloths as primitive respirators. A Canadian soldier realized that the ammonia in urine would react with the chlorine, neutralizing it, and that the water would dissolve the chlorine, allowing soldiers to breathe through the gas. 21st century China normally makes 10 million masks per day, about half of the world production.