7. Houston – The Woodlands, TX
This area is showing very poor results when it comes to particle-pollution. The cities in this area moved from the 23rd place in 2016 to 16th in 2017. As for ozone pollution, the situation is similar since this area moved from 15h to 12th place during the same period. Something needs to be done soon, as 6.85 million people are at constant risk of developing various diseases.