Great Smoky Mountains National Park is considered the most polluted national park according to a 2004 report by the National Parks Conservation Association. From 1999 to 2003, the park recorded approximately 150 unhealthy air days, the equivalent of about one month of unhealthy air days per year. In 2013, Colorado State University reported that, due to the passing of the United States Clean Air Act in 1970 and the subsequent implementation of the Acid Rain Program there was a significant improvement to the air quality in the Great Smoky Mountains from 1990 to 2010.