About 130 people have sought treatment since ammonia spilled Monday at Millard Refrigerated Services in Theodore, creating a vapor cloud that also caused respiratory problems for people working nearby.
Officials say four people were in ICU at the University of South Alabama Medical Center on Tuesday, and three more were in intensive care at Infirmary West.
Hospitals say people are still showing up with complaints.
The leak was reported about 9:25 a.m. CDT at Millard, which uses ammonia in the refrigeration process for chickens that are frozen and shipped around the world.
Between 400 and 800 gallons of ammonia leaked at the site located on the Theodore Industrial Canal, south of Mobile.
The leak also forced workers to evacuate one of BP PLC's main staging areas for the oil spill cleanup. BP said dozens of cleanup workers from its Theodore site were among those taken to hospitals.
Injuries to most of those admitted were not believed to be life-threatening, and others were treated and released, mostly for breathing difficulties.
Most reported minor breathing problems, scratchy throats or dizziness, said Capt. Shaun Hicks with the Mobile Fire-Rescue Department.
"They were walking wounded, but they were still having respiratory problems," Hicks said.
"They basically put us on oxygen," one man told CBS Affiliate WKRG. "They said, you know, if you don't feel better after the oxygen let us know, and 15, 20 minutes of oxygen, and we feel good."
Nearby residents hid in their homes while warning sirens blared.
"It was scary not knowing what the chemical was," said Teri Anderson, who lives near the plant and was first alerted by one of the sirens. She was told by emergency officials to seal her windows and doors, and shut off her air conditioner.
Hospital officials and authorities said more than 120 people were treated, including at least 52 transported by emergency services. Many went by private vehicle.
BP said 65 of its workers were taken to hospitals from the complex that's used for storage, boom decontamination and repair along the canal. The site, which was visited by President Obama in June and where about 500 people still work, was evacuated but will reopen as early as the night shift, the company said.
Emergency officials made telephone calls to alert area residents within one mile of the leak to remain inside and turn off their air conditioners, and children were kept inside an elementary school that was within the danger zone.
The leak was stopped by plant workers, Hicks said, and residents were told it was again safe to go outside by early afternoon.
Hicks said it was unclear what caused the leak.
Millard Refrigerated Services hasn't commented. A spokesman at company headquarters in Omaha, Neb., did not return phone messages.
"It was very stressful for all of us," said Sabrina Greene, whose nephew is in first grade at a school where children had to shelter in place.
She said she had been having respiratory problems, and her brother said his eyes have been stinging since the leak, but neither sought medical help. They were alerted by one of the sirens.
Chris Booth, who works nights at an oil operation, was awakened when his wife phoned him to warn of the leak. He said he turned off the air conditioner, despite the heat index - a measure combining heat and humidity - hitting 105.
"It was plenty warm," he said with a laugh.