The “strong odor” in an air traffic control facility for Washington D.C.-area airports that caused flights to halt in the region on Friday night came from an overheated circuit board, officials said.

Transportation Secretary Sean Duffy said in a post that the circuit board causing the smell was replaced and that there was no danger to the air traffic controllers.

Flights at the region’s largest airports — Ronald Reagan Washington National Airport, Washington Dulles International Airport and Baltimore-Washington International Thurgood Marshall Airport — resumed flights around 8 p.m. after a three-hour ground stop.

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Inside the air traffic control facility in Warrentown, Virginia, called the Potomac Consolidated Terminal RADAR Approach Control, people reported a “strong chemical smell,” according to the Federal Aviation Administration, which owns and operates the facility. Ground stops were issued just before 5 p.m.

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The FAA personnel felt lightheaded and nauseated from the odor, said Dale Kidwell, a spokesperson for Fauquier County Fire Rescue. Over 30 FAA workers were evaluated, but none of them required medical transport, according to fire officials.

Battalion Chief Robert Smith with Fauquier County Fire Rescue said the circuit breaker that overheated was inside a faulty building monitor being worked on by a contractor.

While there were nearly 100 delays at BWI on Saturday afternoon, there were few cancellations, according to FlightAware, a flight tracking website. Reagan had just under 100 delays and cancellations each, while Dulles had about 50 delays and five cancellations on Saturday afternoon.

The Potomac Consolidated TRACON has been in operation since December 2002. It was built to manage flight operations at the major D.C.-area airports and Andrews Air Force Base. Similar facilities have been built in regions where there are multiple large airports or busy airways to consolidate communications among air traffic controllers.