With more than 20 million people living in its metropolitan area, a geographical bowl ringed by mountains, Mexico City has long struggled with air pollution. At times during the past two years, ozone concentration levels in the city reached such extreme levels that officials issued environmental risk alerts, urging people to stay indoors.
FUENTE CITYLAB: https://www.citylab.com/environment/2018/05/mexico-city-looks-for-new-ways-to-tame-air-pollution/558698/