Even moderate levels of air pollution have now been linked to increased risk of heart attacks in people with heart disease.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/photographer-george-freybloomberg-even-moderate-air-pollution-could-trigger-severe-heart
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/bloomberg
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/heart-diseases
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/united-states-america-us