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

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 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)

Angele Blank, right, and Virginia Promeyrat wait for the start of the Greenwich Village Halloween Parade, Saturday October 31, 2015, in New York. (Photo by Tina Fineberg/AP Photo)

Mike Branch helps customers shop for a Halloween masks at Fantasy Costumes on October 28, 2011 in Chicago, Illinois. The store, which had long lines at the registers at 4 AM this morning, is open around the clock through Halloween to help keep up with customer demand. Retailers nationwide are expecting record sales for Halloween merchandise this year with shoppers spending close to $7 billion dollars to celebrate the holiday. (Photo by Scott Olson/Getty Images)

Three workers demonstrating the different types of gas masks used in mine rescue work. (Photo by Fox Photos/Getty Images). December 1925