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An evacuation zone enforced around a Harvey-wrecked chemical plant outside of Houston was lifted Monday, the company said, after it ignited and burned the six remaining trailers containing highly flammable materials on site.

The controlled burn at Arkema Inc. was ordered Sunday to allow crews to stop waiting and get to work assessing the threat at the flooded facility, authorities said.

Arkema officials told reporters Monday that 100 families were helped to find housing since the evacuations began Tuesday.

"We know people are frustrated, and that's our focus at the moment — is dealing with that frustration," said company executive Richard Rennard.

Three of the nine trailers, which each house a half-ton of organic peroxide chemicals, had already ignited at the plant in Crosby, about 20 miles northeast of Houston, sending choking black smoke high into the sky. Two of those three trailers exploded.

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Authorities have warned since last week that further blasts were likely, saying there was nothing they could do about it.

On Sunday, Arkema and the Harris County Fire Marshal's Office said the six remaining trailers were degrading but had "failed to ignite completely," forcing hazard assessment teams to stay out of the plant.

Related: Harvey Danger: How Toxic Is the Air in Texas Chemical Plant Explosion?

To get it over with, the company said, "a decision was made by Arkema Inc. in coordination with unified command to take proactive measures to initiate ignition of the remaining trailers through controlled means."

"The measures taken did not pose any additional risk to the community," the fire marshal's office said Sunday night. A 1½-mile evacuation zone had been installed around the site since Tuesday as a precaution, it said.

The smoke that has poured from the facility has irritated the eyes and throats of more than a dozen law enforcement officers, Harris County Sheriff Ed Gonzalez said. Some of them went to the hospital but were released the same day.

Smoke rises from the Arkema chemical plant Friday in Crosby, Texas, after Hurricane Harvey's floodwaters knocked out equipment used to cool the plant's volatile chemicals. Joe Raedle / Getty Images

"We're continuing to maintain air quality testing in the 1½-mile radius," Rennard said Monday. "We're gonna continue to maintain air quality testing for some time to come."

Harvey's waters have also flooded at least 13 of the 41 Superfund toxic waste sites in the region, the Environmental Protection Agency said Saturday based on aerial imagery, raising alarms about long-term pollution and contamination.

The announcement confirmed an Associated Press report that the EPA hadn't yet been able to physically visit the Houston-area sites, most of which Mayor Sylvester Turner said are just outside the city.

"It certainly would be important" to have the EPA on the ground at the sites to contain potential contamination, Turner said Sunday on NBC's "Meet the Press."

Texas Gov. Greg Abbott said Sunday that does expect the EPA "to get on top" of the threat.

"The EPA is monitoring that," Abbott said on "Fox News Sunday." "We are working with the EPA to make sure that we contain any of these chemicals harming anybody in the greater Houston area or any other place."