A woman wearing a mask reacts at her garden which locates next to chimneys of coal-fired power plant in Shijiazhuang, Hebei province, China, January 28, 2015. Beijing introduced tougher regulations this year to combat pollution, keen to overhaul China's unwanted image of smog-choked cities, fouled waterways and heavy-metal tainted soil. (Photo by Kim Kyung-Hoon/Reuters)

A cloud of black smoke billows out the exhaust pipe of John Robertson's diesel dragster as he runs down the track during the 2015 NHRA Kansas Nationals, Friday, May 22, 2015, at Heartland Park in Topeka, Kan. (Photo by Chris Neal/The Topeka Capital-Journal via 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 cowgirl participates in the barrel race competition at the Bill Pickett Invitational Rodeo on April 1, 2017 in Memphis, Tennessee. The Bill Pickett Rodeo is the nations only touring black rodeo competition. The rodeo celebrates western heritage and the contributions that black cowboys and cowgirls have made to the sport of rodeo. (Photo by Scott Olson/Getty Images)

A cyclist drives past a flowering rapeseed field not far from the small Bavarian village of Schoengeising, near Munich, during a nice, warm sunny weather day on May 17, 2017. (Photo by Christof Stache/AFP Photo)

A woman dressed up for Halloween travels on a Metro train in São Paulo, Brazil on October 30, 2017. (Photo by Cris Faga/Rex Features/Shutterstock)

People look at a Christmas tree decorated with colourful lights at a residential compound in Beijing on December 8, 2017. (Photo by Nicolas Asfouri/AFP Photo)

People cross Flagstaff Hill as the snow starts to fall, in Schenley Park, on their way to the Carnegie Mellon University campus, Tuesday, March 20, 2018, in the Oakland section of Pittsburgh. Yet another powerful storm bore down on the Northeast on Tuesday, with wind-whipped snow falling in parts of Pennsylvania and New Jersey as people grumbled and complained about a first day of spring that looked an awful lot like the last weeks of winter. (Photo by Darrell Sapp/Pittsburgh Post-Gazette via AP Photo)