The Air Quality Index in Delhi dropped to 999 over the weekend, making it the most polluted capital city in the world. While the safe limit set by the World Health Organisation is 25, anything ...

This measure, based on data from the Environmental Protection Agency, evaluates states on the number of days the Air Quality Index in counties was not rated “good.” U.S. News calculated a ...

It comprises two measures: the number of days with an air quality index below a certain threshold and the number of violation points against drinking water systems in each state. Learn more about ...