Residents were urged to shelter in place for several hours after an explosion and fire broke out Wednesday, July 31 at a massive ExxonMobil plant in Texas.

(CNN) -- Residents were urged to shelter in place for several hours after an explosion and fire broke out Wednesday, July 31 at a massive ExxonMobil plant in Texas, officials said.

City officials said via Twitter Wednesday afternoon that the advisory had been lifted after monitoring failed to detect "any levels of concern" in the air.

Officials said 66 employees and contractors were examined at a health clinic, with some receiving first aid. All were released.

The fire had been contained by the evening.

"We realize the people who live here in Baytown and our surrounding communities are worried," ExxonMobil spokeswoman Natasha Barrett told reporters. "We understand that and we've been working hour after hour to check on things, to monitor air quality."

The fire occurred at the company's Baytown Olefins Plant, ExxonMobil spokeswoman Sarah Nordin said. The company website describes the complex as one of the largest refining and petrochemical complexes in the world. It's located about 25 miles east of Houston.

Barrett said the fire began about 11:07 a.m. local time and the precautionary shelter-in-place was issued about 10 minutes later.

The blaze is in a unit that contains polypropylene material and Exxon asked that the shelter-in-place order be issued west of the plant and south of the Texas Spur 330 freeway "out of an abundance of caution," the city of Baytown said via Twitter.

Duncan said the fire broke out at a polypropylene recovery unit, where the plastic is purified for production. He said crews were working to shut down the units to isolate the fire source.

Duncan said the company is conducting air quality monitoring at the site and fence line, and cooperating with regulatory agencies. He said no adverse environmental effects had been detected.

Barrett said city and county officials were also monitoring the air.