Iowa State Patrol spokesman Scott Bright said the fertilizer used for crop dusting contains sulfuric acid, and that the evacuation is necessary to prevent residents from breathing in that chemical.
"We're in the process of evacuating the town. We're trying to get everybody out of the town and moving them to the Kensett Community Center about six miles south of Northwood," he said. "The number one concern is to get everybody out of the town until the fire is put out."
The fire broke out Thursday morning at the plant at Northwood Municipal Airport, about a mile east of the city.
Mercy Medical Center-North Iowa in nearby Mason City said four Northwood residents walked into the hospital seeking treatment.
"Some of the general symptoms that we are expecting include vomiting, nausea and respiratory conditions," said hospital spokeswoman Amanda Franzen.
She declined to provide details about the conditions of those already at the hospital. "It's not necessarily what the patients that have come in are experiencing," Franzen said.
The Mason City Globe Gazette reported several residents heard explosions when the fire broke out. Highway 105, a main artery in Northwood, was blocked off near the fire.
The Iowa State Patrol, Northwood police and the sheriff's department were assisting at the scene.
Northwood, with some 2,000 residents, is located on the Minnesota state line, about 130 miles north of Des Moines.
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