Tanya Wagner, public relations spokesman for the FAA, told The News that a project began July 26 to seal the air conditioning unit above the control room where moisture had been dripping on equipment below. The name of the contractor performing the service was not available.

After the sealing began, some employees complained of nausea, eye irritation, and headaches. While nine workers went to a hospital for examination and treatment, none were admitted to the hospital, Wagner said, and two returned to work the same day.

The work area was sealed off and heating units were brought in to cure and dry the sealant, she said. (The News was unable to determine the sealant’s composition.)

Supplemental ventilation, including portable air conditioners and fans, were brought in to clear the area overnight Tuesday, and the only flight delays encountered Wednesday by the center were weather-related.

About 400 people work at the flight control center, located about 35 miles west of O’Hare International.