A Russian air force technician reports to a pilot that a bomber is ready for a combat mission at Hemeimeem air base in Syria Wednesday January 20, 2016. Russian warplanes have flown over 5,700 combat missions since Moscow launched its air campaign in Syria on September 30, 2015. (Photo by Vladimir Isachenkov/AP Photo)

People leave a giant air purifier, which its inventor calls a “super tree”, in Lima's district of Jesus Maria November 24, 2014. In Peru's notoriously polluted capital Lima, local inventor Jorge Gutierrez, a retired naval engineer, is deploying the giant air purifiers that double as billboards to suck up carbon dioxide and dangerous levels of smog. (Photo by Mariana Bazo/Reuters)

North Koreans wave as Hughes MD-500 helicopters fly past during an aerial display on Saturday, September 24, 2016, in Wonsan, North Korea. North Korea on Saturday opened an air festival featuring sky diving, demonstrations by its air force and lots of beer to promote a newly renovated and upgraded commercial airport in the coastal city of Wonsan that it hopes will draw for foreign tourists. (Photo by Wong Maye-E/AP Photo)

A Boeing F/A-18 takes off to perform during the Farnborough International Airshow in Farnborough, Britain, 12 July 2016. The Farnborough International Air Show runs from 11-17 July. (Photo by Hannah Mckay/EPA)

A F-16 fighter jet from The Thai Royal Air Force performs during a media demonstration at the Korat Royal Thai Air Force Base, Nakhon Ratchasima province, Thailand, November 24, 2015. Thailand's military held an air show with China on Tuesday ahead of joint manoeuvres in a sign of warming ties, but Thailand said it was not distancing itself from the United States which downgraded the military relationship following a 2014 coup. (Photo by Athit Perawongmetha/Reuters)

Festivalgoers watch two actors of the Wasteland Warriors movement fight in a cage at the world's largest heavy metal festival, the Wacken Open Air 2019, in Wacken, Germany on August 3, 2019. Wacken is a village in northern Germany with a population of 1,800 that has hosted the annual festival, which attracts heavy metal fans from around the world, since 1990. (Photo by Wolfgang Rattay/Reuters)

People try to board a goverment bus during the second day of a suspension of public transport services in Apopa, El Salvador July 28, 2015. Violent gangs in El Salvador ordered bus drivers to strike on Monday in a conflict that has killed six people and left thousands of commuters stranded on the streets of the Central American capital, police and bus company officials said. (Photo by Jose Cabezas/Reuters)

Pedestrians cover their face as they walk along the dusty road in Kathmandu, Nepal February 27, 2017. Nepal has forced 2,500 old vehicles off roads in its capital city of Kathmandu, part of a fight against alarming air pollution levels that have hit nine times World Health Organisation (WHO) limits. Air pollution has been a chronic problem in rapidly growing Kathmandu, which sits in a Himalayan valley and is home to more than 3mn people. (Photo by Navesh Chitrakar/Reuters)