TOPSHOT - A group of students wear face masks during a ceremony at Zocalo Square in Mexico City on May 17, 2019. - A thick layer of smog enveloping Mexico City, resulted in an air pollution alert urging people to stay indoors, cancelling school lessons and moving the semi-finals of the first-division football league to another city. (Photo by ALFREDO ESTRELLA / AFP) (Photo credit should read ALFREDO ESTRELLA/AFP/Getty Images)
