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 drastically lowering pollution levels. By 2014 carbon monoxide pollution had dropped drastically, while sulfur dioxide and nitrogen dioxide were at levels about a third of those in 1992. The levels of signature pollutants in Mexico City are similar to those of Los Angeles. 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.