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

Soldiers use electronic sensors to search for missing firefighters under a line of fire trucks caught in massive gas explosions in Kaohsiung, Taiwan, Friday, August 1, 2014. A series of underground explosions about midnight Thursday and early Friday ripped through Taiwan's second-largest city, killing scores of people, Taiwan's National Fire Agency said Friday. (Photo by Wally Santana/AP Photo)

Women carry gas cylinders to fill them at a distribution point in Cairo January 19, 2015. Egypt is going through its worst energy crisis in decades and is seeking fresh sources of natural gas, which powers most of its homes and factories. (Photo by Mohamed Abd El Ghany/Reuters)

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

Members of the Moranbong Band from North Korea wait inside the lobby of a hotel in central Beijing, China, December 11, 2015. North Korea's premier pop group, the all-girl Moranbong band formed by leader Kim Jong-Un, is electrifying audiences in China in shows aimed at harmonising out-of-tune ties between the traditional allies, reports and the venue said on December 9 – but tickets are not available to the public. (Photo by Reuters/Stringer)

Members of the Cyclophonica band play instruments as they make a stop during a ride in Rio de Janeiro May 17, 2015. The Cyclophonica band was formed by a group of musicians riding bicycles around the city, mixing sport and culture and entertaining people on the streets, according to its founder Leonardo Fuks. (Photo by Pilar Olivares/Reuters)

A woman wearing a bikini stands next to fuel pumps as she waits to top up fuel at a petrol station in Kiev, Ukraine, September 26, 2015. One of the largest networks of petrol stations in Ukraine organized a campaign in which anyone who showed up in swimwear at a particular gas station during specified hours would receive free gasoline, according to representatives of the company. (Photo by Valentyn Ogirenko/Reuters)