By the 1990s Mexico City had become infamous as one of the world's most polluted cities; however the city has become a model for dramatically lowering pollution levels. By 2014 carbon monoxide pollution had dropped dramatically, while levels of sulfur dioxide and nitrogen dioxide were nearly three times lower than in 1992. The levels of signature pollutants in Mexico City are similar to those of Los Angeles.[citation needed] Despite the cleanup, the metropolitan area is still the most ozone-polluted part of the country, with ozone levels 2.5 times beyond WHO-defined safe limits. COUNT 5
QSN1 How bad is the ozone in Mexico City? ANS1 2.5 times beyond WHO-defined safe limits QSN2 Carbon Monoxide levels are about 3 times lower now than in what year? ANS2 1992 QSN3 What city is similar to Mexico City in terms of pollution? ANS3 Los Angeles QSN4 When Mexico one of the worst cities in terms of pollution?  ANS4 1990s QSN5 Who decides safe levels of o-zone in cities? ANS5 WHO