A Boeing 777-300 from Swiss International Airlines lands on a foggy morning at Zurich Airport, on Friday, October 13, 2017, in Zurich, Switzerland. The high humidity renders the air vortex visible. (Photo by Christian Merz/Keystone via AP Photo)

A U.S. Army crew chief flying on board a CH-47F Chinook helicopter observes the successful test of flares during a training flight in Afghanistan, March 14, 2018. (Photo by Tech. Sgt. Gregory Brook/Reuters/U.S. Air Force)

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)

Siberian Husky dogs of the Royev Ruchey Park team pull a rig during a practice session for the Karadag Sled Dog Rally on the frozen Mana River, with the air temperature at about minus 21 degrees Celsius (minus 5.8 degrees Fahrenheit), in the Siberian Taiga area outside Krasnoyarsk, Russia February 6, 2018. (Photo by Ilya Naymushin/Reuters)

Mil Mi-8 combat helicopter flies over Moscow, Russia on May 7, 2019 during the dress rehearsal of a Victory Day air show marking the 74th anniversary of the victory over Nazi Germany in the 1941-1945 Great Patriotic War, the Eastern Front of World War II. (Photo by Sergei Savostyanov/TASS)

A Kashmiri man cover his head with plastic bag as it snows in Srinagar, Indian controlled Kashmir, Thursday, November 7, 2019. The region received its first snow on Wednesday, bringing temperatures down drastically and affecting air and vehicular traffic. (Photo by Mukhtar Khan/AP Photo)

Balloons are inflated at the Bristol International balloon fiesta in south west England August 6, 2015. Now in its 37th year, the Bristol International Balloon Fiesta is Europe's largest annual hot air balloon event in the city that is seen by many balloonists as the home of modern ballooning. (Photo by Toby Melville/Reuters)

A sledding dog is covered in ice after spending the night in the open air with his musher at the polar base of Val-Cenis, as they ready for the next stage of the 12th edition of “La Grande Odyssee” sledding race across the Alps on January 17, 2016 in Val Cenis Lanslevillard. (Photo by Jean-Philippe Ksiazek/AFP Photo)