DuPont reported regular malfunctions with a multimillion-dollar exhaust and ventilation system inside its La Porte pesticide plant that exposed workers to potentially dangerous fumes for years before a catastrophic release killed four there on Nov. 15, public records obtained by the Houston Chronicle reveal.

The rate of methyl mercaptan released on the day of the accident - 23,000 pounds in a single morning - was far above anything DuPont predicted for routine maintenance activities.

But it appears that no one ever alerted officials with the Occupational Health and Safety Administration about smaller but still significant levels of the toxic gas that DuPont reported releasing during troubleshooting activities for the last six years, according to reports filed with the Texas Commission on Environmental Quality.

Those amounts indicate workers involved in maintenance at the pesticide plant since 2008 could have been exposed to levels of methyl mercaptan far above those that OSHA has deemed unsafe, based on information about the plant, procedures and DuPont's own disclosures.

Sam Mannan, director of Texas A&M University's Mary Kay O'Connor Process Safety Center, said a regular exchange of information about toxic releases could have saved lives.

"These maintenance emissions that you're pointing to … if we looked at them earlier, we might have been able to find the problem and fix the problem before four had to die," he said.

TCEQ has cited DuPont for dozens of emissions violations at the La Porte plant, records show. Terry Clawson, a spokesman for TCEQ, said if agency officials identify "a potential concern that is beyond our jurisdiction, we contact the appropriate local, state or federal agency, such as OSHA."

But there's no record that TCEQ or anyone else filed a complaint that prompted an OSHA inspection. Prior to the deaths, OSHA had not inspected DuPont's plant since 2007, records show. Both agencies are investigating the deaths, along with the Environmental Protection Agency and the Chemical Safety Board.

DuPont refused comment on its workers' potential exposure to methyl mercaptan during maintenance activities prior to the deaths.

"Considering individual issues in isolation or speculating can lead to inaccurate conclusions," DuPont spokesman Aaron Woods said. "DuPont is committed to maintaining the integrity of ongoing investigations. To that end, it is premature for us to comment or provide additional information outside of these processes."

Three bodies were found on the third floor of the Lannate unit in La Porte, an area where DuPont employees were regularly assigned to physically open up pipes to clear gelatinous or liquid clogs to keep the insecticide manufacturing process and its pollution-cleaning systems functioning properly.

Maintenance work was ongoing the day of the accident, and liquid-clearing activities were typically done inside the building without supplied air or respirators, according to written testimony U.S. Chemical Safety Board Chairman Rafael Moure-Eraso submitted to a congressional committee last month.

U.S. Rep. Gene Green, whose district includes much of the Houston Ship Channel and has received briefings on the ongoing investigations, told the Chronicle that it appears to him that the company's line-clearing procedures likely violated OSHA's process safety management standard. Federal work safety standards include developing protocols to protect workers during maintenance procedures involving hazardous chemicals.

Even at extremely low and non-harmful levels, methyl mercaptan stinks, and workers may have gotten used to that smell, often compared to rotten cabbage.

Potentially lethal

Methyl mercaptan can kill humans at levels over 1,000 parts per million, medical experts have found, though studies are based on research involving mostly rats and mice. DuPont performed one of the most recent animal studies, an EPA-funded scientific report published in 2013 shows.

At far lower levels, the toxic can cause eye irritation and unconsciousness. There have been isolated reports of organ damage, comas and deaths of workers in industrial accidents involving methyl mercaptan, which is used in paper-making, pesticides, jet fuel and some West Texas oil gas wells.

But the La Porte culture did not emphasize those potential dangers, according to Gilbert Tisnado, a former DuPont employee who lost two sons in the accident. Killed were Gilbert and Robert Tisnado, Wade Baker and Crystle Wise.

Laurence Pearlman, another process safety expert and consultant who is a partner in Oliver Wyman in Chicago, told the Chronicle that, in general, company officials sometimes fail to adequately prepare for such a large release because they see it as the equivalent of a 100-year flood, something unlikely to occur in their lifetimes.

"If you think it can happen, you tend to be more rigorous," he said.

Lannate, a pesticide, is made inside a towering, ill-lit building at the La Porte plant, where reactors and tanks are enclosed and toxic waste gases are supposed to be routed to an incinerator and scrubber on the roof to protect the environment.

But repeatedly, the pipes and vents inside the unit have gotten clogged over the years with unwanted liquids. During line-clearing activities, DuPont's plant managers have reported that as much as 3 pounds of methyl mercaptan leaks out in an hour, according to documents filed with state environmental regulators in 2009 and 2010.

At that rate, in an hour, as much as 600 ppm of that toxic gas could have filled a room where workers generally cleared clogs - if the room was poorly ventilated, given the enclosed design of the antiquated plant and the estimated size of the third-floor rooms where maintenance often occurred.

OSHA guidelines say workers should not be exposed to an average of more than 10 ppm of that toxic during a workday.

The National Institutes of Occupational Safety and Health recommends workers be exposed to no more than .5 ppm.

The actual level of exposure to workers during DuPont's maintenance activities, however, would vary depending on the location of the clog, the size of the room and how quickly the gases released were vented to the atmosphere, said Peter Dooley, one of two industrial hygienists who reviewed the estimates and the related documents at the Chronicle's request.

Public records do not indicate how maintenance emissions were monitored.

Workers had complained for months that the ventilation fans atop the Lannate unit where the employees died were out-of -service, according to preliminary information the CSB has released to Congress.

Chronic problems

DuPont's own maintenance emission reports, filed with the TCEQ, reveal chronic problems that should have prompted additional monitoring, protection and training for workers long before the accident occurred, Dooley and Mannan both said.

Mannan, the Texas A&M expert, said that DuPont should have, at the least, required more monitoring and respirators to protect workers given the toxic releases associated with maintenance activities but also should have considered engineering solutions to eliminate risks.

DuPont's records show clogs persisted even after the company reported to the TCEQ that it spent $18 million on upgrades to the unit, a project that, according to public records, included new ducts, pipes and a scrubber in 2010-2011.

Dooley, a safety and health project consultant for the National Council on Safety and Health, said it appears DuPont's actions fell short.

"Every indication points to that the right protections were not in place and that the facility wasn't prepared to deal with possible exposures from maintenance operations," he said.