Wash Post Washington Post this article most polluted city most polluted city impressive reduction most polluted city Mexico City drastically reduced air pollutants since 1990s Mexico City drastically reduced air pollutants since 1990s Mexico City drastically reduced air pollutants since 1990s Wash Post Mexico City drastically reduced air pollutants since 1990s this article most polluted city Mexico City drastically reduced air pollutants since 1990s