(Updates with residents returning to homes)
By Kevin Murphy
Feb 20 (Reuters) - Authorities allowed residents of a small Iowa town to return to their homes on Thursday hours after they were told to evacuate because of a fire involving sulfuric acid at a local airport.
The entire population of about 2,000 people living in Northwood were ordered to leave town after the fire broke out around 7:30 a.m. CT in a storage shed for sulfuric acid and other chemicals at the airport, said Sergeant Scott Bright of the Iowa State Patrol. Residents were told to go Kensett, Iowa, about 6 miles (9.7 km) south of Northwood.
Officials lifted the evacuation order around 3:00 p.m. CT after conducting environmental tests, Bright said.
Four people living in the area went to a hospital with respiratory concerns, Bright said. He added the cause of the fire remains unknown.
"I could see the black smoke billowing up," Northwood resident Susan Mega said by telephone from Kensett. She said an odor similar to rotten eggs filled the air.
(Reporting By Kevin Murphy in Kansas City, Missouri; Editing by David Bailey, Grant McCool and Cynthia Osterman)