“They make everything here,” said Ed Hirs, an energy expert at the University of Houston. “Some of them make comparatively benign things. Other make really nasty things.”

The plants all face challenges now. Most, like the Arkema plant, were shut down protectively before Harvey hit, and have remained closed because of high water. Some have reported damage from the storm, like sinking tank roofs and loss of power, which have led to the release of millions of pounds of hazardous chemicals into the air over the past week.

There will be more challenges in the coming weeks and months as the floodwaters recede and closed facilities are made operational again. Some equipment might not be in safe condition, experts said.

The U.S. Chemical Safety and Hazard Investigation Board, an independent federal agency, has issued a notice detailing precautions that should be taken as plants are restarted. The process is complex, the board said, “because numerous activities are occurring simultaneously and many automatic systems are run under manual control.”

The organic peroxides made at the Arkema plant, which employs about 60 people, are used as catalysts in plastic manufacturing. The Texas A&M study said that the plant held up to half a million pounds of one the chemicals, cumene hydroperoxide.

The explosions occurred at about 2 a.m. local time Thursday in two of the storage trailers, sending black smoke into the air as the material burned. The company said that with no refrigeration in six other trailers, those would likely explode soon as well.

Mr. Rennard, the Arkema executive, said that the smoke produced by the blasts and fires was “noxious” and irritating to the eyes, lungs and possibly skin. Residents within a mile and a half of the plant remained under a mandatory evacuation order.

Mr. Rennard said that workers would not enter the site until the floodwaters had receded significantly.

The E.P.A. said that airborne measurements taken at the scene Thursday showed no immediate health threats.

“E.P.A. has emergency response personnel on the scene and the agency is currently reviewing data received from an aircraft that surveyed the scene,” Scott Pruitt, the agency’s administrator, said in a statement.

The Obama-era rules would not necessarily have prevented the explosions, and peroxide itself is not on the list of chemicals the regulations cover. But the plant previously disclosed that it stores two other chemicals, sulfur dioxide and isobutylene, that are covered by the rules.

In pushing back against them, Arkema said it was worried about security. “We have significant concerns with providing security-sensitive information where disclosure of such information could create a risk to our sites and to the communities surrounding them,” Susan Lee-Martin, an Arkema engineer, wrote in a letter to the Environmental Protection Agency in March 2016.

Those rules — which would require companies to undergo third-party audits of their safety measures, make more information available to emergency responders and study more effective ways to keep hazards in check — could help make future accidents less likely, said Gordon Sommers, an attorney at the environmental group Earthjustice.

One critical upgrade at chemical plants that might have been spurred by the new rules would have been developing better backup power systems that could withstand flooding and other disasters, Mr. Sommers said.

“It’s a very good illustration of why rules are important,” he added. “It’s certainly not the last hurricane Texas is going to face. It’s not the last time there’s going to be accidents at chemical plants.”

Arkema has faced regulatory scrutiny in the past, paying some $1.2 million in fines for workplace safety, health, environmental and other violations since 2010 at its United States facilities, according to Violation Tracker. The plant has been in violation of the Clean Water Act for six out of the past 12 quarters, E.P.A. records show.

The plant has also faced state penalties, including two for improper storage of organic peroxides. In one instance, in 2006, the improper storage led to a fire that released 3,200 pounds of volatile organic compounds and other pollutants into the air.