The “State of the Air 2016” found con nued improvement in air quality in 2012–2014, showing lower levels of year-round par cle pollu on and ozone. S ll, more than half of all Americans—166 million people—live in coun es where they are exposed to unhealthful levels of these pollutants. The “State of the Air 2016” report shows that cleaning up pollu on con nues successfully in much of the na on. In the 25 ci es with the worst pollu on, the majority saw improvements from last year. Many saw their lowest levels ever of year-round par cle pollu on or ozone pollu on. Yet, even as most ci es experienced strong improvement, too many ci es su ered worse episodes of unhealthy air. While most of the na on has much cleaner air quality than even a decade ago, a few ci es reported their worst number of unhealthy days since the report began, including some that experienced extreme weather events. The “State of the Air 2016” report provides evidence that a changing climate will make it harder to protect human health.