The world’s largest airports saw year-over-year passenger growth of 5.5 percent in 2016, but one in four of the world’s smallest airports — defined as those with fewer than a million passengers a year — experienced a decline.

The numbers come from new research released by the industry group Airports Council International.

“The downside of airline capacity shifts is that a proportion of smaller regional airports have either stagnated or experienced a reduction in nonstop destinations between cities,” said Angela Gittens, director general, ACI World. “The reason smaller airports remain in operation hinges on the fact that they contribute to the local, social and economic development of their surrounding communities. Strategies to ensure their sustainability are important if we are to have a robust aviation system.”

The report also notes Hartsfield-Jackson Atlanta International Airport is the busiest domestic airport in the word, seeing 92.7 million passengers annually (up 2.7 percent). It is followed by Beijing (PEK) and Chicago O’Hare (ORD). The busiest international airports were Dubai (DXB), London Heathrow (LHR) and Hong Kong (HKG).

Worldwide, airport passenger traffic was up 6.5 percent in 2016 to almost 7.7 billion, with all regions seeing growth except Africa.