The air in the United States is much cleaner than even a decade ago.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/study-shows-why-eastern-us-air-pollution-levels-are-more-stagnant-winter
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/phys
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-quality-monitoring
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/united-states-america-us
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/asthma
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/lung-disease
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects