A small Iowa city was evacuated after a fire at a fertilizer warehouse Thursday morning. By late afternoon, residents were told that they could return to their homes, the Sheriff's Office said.

The fire, on airport grounds just south of the Minnesota-Iowa border, had prompted authorities to urge all Northwood residents to leave as quickly as possible in what they had called "a dangerous situation."

Four people arrived at Mercy Medical Center-North Iowa in Mason City with health problems such as nausea and respiratory difficulties. "They were all treated and released," said Brian Bechtel, nursing supervisor. "So that's good news."

The fire at the Farmers Feed and Grain Co. warehouse east of Northwood involved sulfuric acid, said Lucinda Robertson, spokeswoman for Iowa Homeland Security and Emergency Management.

Sulfuric acid, used in fertilizer as a drying agent, can burn the skin upon contact. Breathing the vapors can irritate the eyes and respiratory tract. Water can also spread and react with sulfuric acid, a concern as the weather deteriorated Thursday.

An alert issued about 9:10 a.m. by Worth County Emergency Management informed Northwood's 1,900 residents that "this is a dangerous situation. Residents of Northwood are being asked to evacuate immediately."

Airport manager Mike Dierenfeld, who was at his veterinary office three blocks away when the fire erupted about 8 a.m., said the blaze struck a warehouse "pretty full" of herbicides and pesticides. The facility, which was unoccupied at the time of the fire, supplies small planes for application of the chemicals on crops, he said.

"A smoke plume was going up and to the northwest of town, but then the wind switched and it went right over town," Dierenfeld said. "It stinks pretty bad."

He said some people were arriving at the courthouse in town complaining of shortness of breath.

Dierenfeld said the fire had been smoldering as the noon hour approached, but it was "pretty much down"within 90 minutes.

Staff writer Jenna Ross contributed to this report.

Paul Walsh • 612-673-4482