Access to clean water is critical worldwide — but a study from Michigan State University finds parts of the U.S. are facing problems with water affordability.

In Philadelphia, for example, four out of 10 water accounts are past due.

The methodology used is complex, but basically, the researchers based their numbers on the EPA’s affordability criteria that a water bill should not exceed 4.5 percent of a median household income.

Elizabeth Mack is an economic geographer at MSU, and the paper’s lead author.

The paper also found pockets of water poverty in downtown Phoenix, as well as other cities.

Kathryn Sorensen is the director or Phoenix Water Services.