View from Tlanepantla of Mexico City blanketed by smog on March 18, 2016. Mexican officials lifted a four-day air pollution alert in the nation's densely-populated capital after ozone levels dropped, according to them, to acceptable levels. Mexico City authorities issued the first air pollution alert in 14 years due to high ozone levels, restricting traffic, encouraging children to stay indoors and ordering factories to cut emissions. (Ronaldo Schemidt/AFP/Getty Images)
