People in Kathmandu joke that the face mask has now become part of the national dress – an indispensable accessory in an effort to protect themselves from the dust. Nepal has the worst air quality in the world, according to the Environmental Performance Index. (Photo by Pete Pattisson/The Guardian)

Zamora, 55, self-employed, poses for a photograph in front of the Cuban and U.S. flags after buying a pineapple in Havana, March 25, 2016. Regarding Obama's historic visit to the island, Zamora said “It's good for the Cubans that he came and re-established relationships between the two countries”. (Photo by Ueslei Marcelino/Reuters)

It is already one of the world’s most iconic cities, but this incredible collection shows how much Paris has changed over the course of the past century. The collection, by French photographer Julien Knez, shows Paris in the 1940s against a backdrop of how the same places look today. Knez says he put the striking collection together to celebrate the 71st anniversary of the city’s liberation from Nazi control in August 1944. Here: Soldiers outside the Hotel De Ville in the 1940s. (Photo by Julien Knez/Caters News)

Two women lighting cigarettes on a tennis court in Essex, England circa 1930's. (Photo by Keystone View/FPG/Getty Images)

A man reacts over dead bodies prior to burial in a graveyard after shelling in the rebel held besieged town of Douma, eastern Ghouta in Damascus, Syria, October 24, 2016. (Photo by Bassam Khabieh/Reuters)

Donald Ganslmeier, operator of the Motodrom, rides a motorbike inside the Motodrom during a press tour at Theresienwiese on the premises of the 182nd annual Oktoberfest in Munich, Germany, September 17, 2015. (Photo by Matthias Balk/EPA)

As a social experiment, journalist Esther Honig contacted Photoshop artists across the world and told them to make her beautiful, and they did. Honigh said she was surprised by how her image was altered, saying the process has greatly changed her perception of beauty. Photo: “Before & After” project: Greece. (Photo by Esther Honig)

Annual celebration of Dinagyang festival of Iloilo, Philippines. (Photo by Raniel Jose Castañeda/Sony World Photography Awards)