Residents who evacuated after a train derailment triggered a toxic chemical leak south of Hattiesburg were told to remain out of their homes Friday until officials declare the area safe.

Authorities also asked that people to stay out of the waters of the Leaf River and Myers Creek. Mississippi Department of Environmental Quality biologists are working to neutralize the polluted water in the creek and the process could take several days.

Two people were taken to Forrest General Hospital Thursday morning after the derailment that caused some of its cargo of sodium hydroxide, commonly known as lye, and hydrochloric acid to be released.

Officials said the accident happened around 8:30 a.m. Thursday when a southbound freight train operated by Kansas City Southern Railroad jumped the tracks near U.S. 98 and Ralston Road in Forrest County.

The leak prompted environmental officials to assess the air quality and wind direction and 40 homes were evacuated.

Officials said the situation was under control, but they don’t know when residents will be allowed to return home.

The Church of the Nazarene in Hattiesburg opened its doors as a Red Cross shelter. The Red Cross said it was prepared to provide shelter for as long as needed.

One evacuee said he could smell fumes around his home before being ordered to leave.

“I went halfway down and I didn’t see too much, but I could smell it,” George Lee Parker told WDAM-TV, “and I got sort of dizzy and I came back to the house, and later on the officer came to the house and told me I had to evacuate because the gas was coming my direction.”

Two of eight Kansas City Southern rail cars that left the tracks ruptured, releasing the liquefied sodium hydroxide and hydrochloric acid about a quarter-mile south of U.S. 98 East, officials said.

The train, except for the two leaking cars, was removed from the site by 8 p.m. Thursday.

Hattiesburg city spokesman John Brown said work would begin Friday on sealing and removing the two damaged cars.

Water samples taken in Myers Creek revealed high levels of hydrochloric acid contamination, MDEQ scientist Ernie Shirley said.

Despite fears chemicals would migrate to the nearby Leaf River, MDEQ officials said tests indicated chemicals had not traveled that far. Myers Creek was dammed to prevent the fluids’ spread, and workers dumped more than 1,000 pounds of lime into it to neutralize the hydrochloric acid, officials said.

The acid is highly volatile and can easily become an airborne gas, said Frank Woodruff, chemical safety officer at the University of Southern Mississippi.

“If it’s breathed, it’s an irritant to the eyes, throat and lungs,” he said, adding that sodium hydroxide — a base chemical commonly known as lye — is found in household drain cleaners and can cause skin blisters and burns if touched.

The good news is that the chemicals “should not be too difficult to clean up,” Woodruff said. “They dilute very well with water; if people stay away from it, there shouldn’t be a lot of residual effects.”

Determining what caused the train to jump the tracks in the first place could take several days, said Al Rawls, Kansas City Southern’s assistant vice president of police and freight claims.

The company is uncertain whether the ruptured tank cars involved in the derailment were full or how much material was released, according to a statement from spokeswoman Doniele Kane.

The train was also carrying chlorine, Shirley said, but neither of the ruptured cars contained that chemical.