 In recent years , the expansion of the southern suburbs of Washington , D.C. into Northern Virginia has introduced an urban heat island primarily caused by increased absorption of solar radiation in more densely populated areas . In the American Lung Association 's 2011 report , 11 counties received failing grades for air quality , with Fairfax County having the worst in the state , due to automobile pollution . Haze in the mountains is caused in part by coal power plants . 
