For 40 days after a piece of equipment critical to the refinery's operation broke down, a total of 538,000 pounds of toxic chemicals, including the carcinogen benzene, poured out of the refinery.
Rather than taking the costly step of shutting down the refinery to make repairs, the engineers at the plant diverted gases to a smokestack and tried to burn them off, but hundreds of thousands of pounds still escaped into the air, according to state environmental officials.
Neither the state nor the oil company informed neighbors or local officials about the pollutants until two weeks after the release ended, and angry residents of Texas City have signed up in droves to join a $10 billion class-action lawsuit against BP. The state attorney general, Greg Abbott, has also sued the company, seeking fines of about $600,000.
BP maintains three air monitors along the fence around the plant and two in the surrounding community, and they did not show a rise in pollution during April and May, the company said. "BP does not believe there is any basis to pay claims in connection with this event," said Michael Marr, a spokesman for the company.
But scores of Texas City residents said they experienced respiratory problems this spring, and environmentalists said the release of toxic gases ranked as one of the largest in the state's history.
Neil Carman of the Lone Star Sierra Club said the release was probably even larger than BP had acknowledged, because the company estimated that more than 98 percent of the pollution was burned off by a flare, an overly optimistic figure in the eyes of many environmental scientists.
He also said there were too few air monitors to accurately assess what had happened. "There are huge gaps in the monitoring network," Mr. Carman said.
Dionne Ramirez, 29, who lives about a mile from the refinery, said she had little doubt that elevated pollution harmed her family. Not only have both she and her husband had coughs, but all three of their young sons have suffered from severe chest congestion, sore throats and endless coughing since April. Her 4-year-old had to be hospitalized for two nights because he could not stop coughing, she said.
When the news of the pollution was made public on June 4, Ms. Ramirez was irate. "I didn't know why they were getting sick or what was going on," she said. "They are healthy little kids."
Her experience was echoed by other families living in the shadow of the jumbled smokestacks, pipelines, cylindrical tanks and giant globes of the refinery. Nearly every household on one block of First Avenue, just a half-mile from the BP complex, had someone fall ill during May, residents there said.
"We all became real sick - throwing up, diarrhea, couldn't keep anything down - and we just thought it was something that was going around," said Khristina Kelley, who lives with her husband and four children on the street. "But then everybody around here got it."
Ms. Kelley said the release of chemicals was less troubling to her than the company's silence. "I'm worried that one day I'll take my kids to the doctor and something that could have been prevented wasn't prevented because we didn't know to the last moment," she said.
Officials in Texas City said they were not informed of the scale of the release until it was over. BP said it met the requirements of state law by informing state officials of the release in writing on April 7, then filing a final report on June 4, after the equipment was fixed.
That final report said the release of chemicals had gone on for 959 hours, until May 16. Among other pollutants, the plant had released 17,000 pounds of benzene; 37,000 pounds of nitrogen oxides, which can cause respiratory problems; and 186,000 pounds of carbon monoxide. Another 262,000 pounds of various volatile organic compounds also escaped.
"The state's investigation shows that BP's failure to properly maintain its equipment caused the malfunction and could have been prevented," the attorney general's office said in a statement.
Mr. Marr, the BP spokesman, declined to comment on those accusations.
The trouble started when a fire broke out on the seal of a hydrogen compressor, which traps noxious chemicals and returns them to be used as fuel in other parts of the plant. The compressor was part of the refinery's "ultracracker unit," which can process 65,000 barrels of oil per day and mostly produces high-octane blending components for gasoline. The company sent the gases to a flare at the end of a smokestack, 300 feet in the air, hoping to burn off the hazardous chemicals. But a monitor at the top of the stack showed that the emissions were far higher than permitted.
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Many residents of Texas City have joined a class-action suit.
The attorney general's office alleges in its complaint against BP that the fire started because workers had allowed iron sulfide to build up on the seal of the compressor.
Violations are nothing new at the plant, federal and state officials say. In 2005, an explosion at the refinery killed 15 people and injured more than 170, and BP was fined $87 million by the federal Occupational Safety and Health Administration for safety lapses that led to that blast. This month, BP agreed to pay $50.6 million, a record.
On air pollution, the refinery has a similarly checkered history, a pattern of breaking limits on air pollution and being slow to report those events, state officials claim in legal complaints. In 2009, Mr. Abbott, the attorney general, sued BP for violating clean-air standards 72 times in the previous five years.
Still, the refinery is a major employer in Texas City, a town with about 45,000 residents, modest frame houses, fast-food restaurants and dollar stores on the coastal plains across a channel from Galveston. The refineries dwarf the clapboard abodes of workers here, thrusting up into tropical skies in utilitarian ugliness and painting the azure with smoke. Those smokestacks mean jobs, and many people are skeptical about those claiming they have gotten sick.
"This is just money-hungry money grubbers is all it is," said Pete Fernandez, a longtime resident. He called the lawsuits "frivolous - completely, totally frivolous."
Yet some longtime refinery workers are among those suing. Robert L. Sukiennik, 45, has worked at a refinery operated by Valero here for two decades. In early May, he started to cough and felt weak. He finally saw a doctor in mid-July, who became alarmed at his white blood cell count. A CT scan a week later revealed abnormal spots on his kidneys, and he was referred to an oncologist for more tests. Leukemia was a possibility, he was told.
It is impossible to know for certain if Mr. Sukiennik's sudden decline in health is connected to the emissions from BP, but he says that the refinery has had so many troubles over the years, he is filled with suspicion that it might be the root of his problems.
"Every day they have some problem over there," he said. "I don't think BP itself really cares about the community. They are not trying for safety; all they care about is the big bucks."