Renters are on the rise in America’s biggest cities, but many tenants are scrambling to keep up with growing rent bills and shrinking vacancies, according to a recent study.

From Boston to Miami, New York to Los Angeles, more than half of tenants are paying what experts consider unaffordable rents, says a report by New York University’s Furman Center released on Thursday, which studies real estate and urban policy, and bank Capital One, which is a leading affordable-housing lender and financed the research.