South Asian countries in terms of total contribution of travel and tourism to GDP. #6. Vietnam, Total GDP: USD 202.6 billion (2016). Contribution of Travel and Tourism to GDP: 9.1% . Here: A boat in the Thu Bon River, Hoi An, Vietnam. (Photo by Domingo Leiva/Getty Images)

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)

A Mongolian Shamaness or Buu, beats her drum while taking part with others in a fire ritual meant to summon spirits to mark the period of the Summer Solstice in the grasslands on June 22, 2018 outside Ulaanbaatar, Mongolia. Banned for 70 years under Communist rule, shamanism has seen a resurgence in Mongolia since 1992, when the ancient practice became protected by the country's Constitution. Known as Tengrism, in which Shamans channel ancestral spirits, it is widely regarded as Mongolia's national religion and part of its indigenous identity. (Photo by Kevin Frayer/Getty Images)

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

A fireman from the London Fire Brigade, wearing a smoke helmet. (Photo by Topical Press Agency/Getty Images). Circa 1908