Millions of vacationers escape to national parks each summer to take in the fresh air and scenic vistas.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/%E2%80%98f%E2%80%99-air-quality-california-national-parks-top-list-worst-pollution
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/washington-times-usa
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/national-parks
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/california
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/united-states-america-us
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/ozone
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/climate-change