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.