According to new data by air quality research group Puraka, the EPA's Air Quality Index score, a measure of particulate pollution used to estimate the health impact of different air quality levels, has decreased by 35% over the fifteen year period from 2004 to 2019.

Puraka examined which U.S. cities (combined statistical areas with a population 250k+) have been most effective at improving the quality of the air their residents breathe. Using air quality index (AQI) data, we saw that the median AQI dropped from a score of 2004 score of 46, right around the point where the air quality goes from "good" to "moderate", to a score of 30 in 2019. Additionally, the data highlights the top five cities working the hardest - or possibly the smartest - to increase quality of life and reduce air pollution (see figure below).

The study looked at all U.S. Combined Statistical Areas with a population over 250,000 and air quality index (AQI) records going back every year to 2004. To determine the five cities with the largest reduction in median AQI scores (representing the biggest decrease in air pollution), we relied on data representing the difference between the sum of median AQI scores for years 2004-2008 and years 2014-2018.