Some of London's telephone operators and engineers, trying out their gas masks at the anti-gas civilian training centre in Gloucester, where they were taught how to deal with gas attacks. (Photo by Harry Todd/Fox Photos/Getty Images). 13th February 1937

A women wearing a mask to protect from extreme smog visits the Tiananmen Gate in Beijing December 8, 2015 as China's capital issues its first ever "red alert" for pollution. (Photo by Damir Sagolj/Reuters)

Sikhs wield swords during their clash inside the complex of the holy Sikh shrine, the Golden Temple, in Amritsar June 6, 2014. Sikhs wielding swords clashed inside the compound of their religion's holiest shrine on Friday, the 30th anniversary of a controversial raid by security forces that flushed out separatist militants holed up in the temple. REUTERS/Munish Sharma

A man standing on the first cables during the construction of the Golden Gate Bridge with the Presidio and San Francisco in the background, San Francisco, California, 1935. (Photo by Underwood Archives/Getty Images)

Masks of Republican presidential candidate Donald Trump (C) and Joaquin "El Chapo" Guzman (2nd L) are seen at Grupo Rev in the Mexican city of Cuernavaca near Mexico City, October 14, 2015. (Photo by Henry Romero/Reuters)

A Tibetan woman displays her Qing Dynasty family dress in Zhailong Village on April 24, 2005 in Danba County of Ganzi Tibetan Autonomous Prefecture, Sichuan Province, China. (Photo by China Photos/Getty Images)