A year before its Port Neches plant exploded, Houston-based TPC Group had installed monitors along the facility's fenceline to detect fugitive leaks of 1,3 butadiene — a highly flammable gas produced there.

The four gas chromatographs were one of many requirements in a settlement the company reached with the U.S. Environmental Protection Agency in 2017 after an agency probe found that the plant had spewed butadiene — a known human carcinogen — and other chemicals into the air for more than three years instead of safely burning them off.

Under the settlement agreement, TPC was required to monitor butadiene levels in the air at the periphery of the 215-acre compound for one year and post reports on its website whenever readings hit 25 parts per billion. If readings exceeded that threshold twice in a one-hour period hour, TPC was expected to track down the source of the pollution, take measures to reduce it and explain why those actions did or didn't work.

TPC, which celebrated its 75th year in business last year, implemented a similar monitoring program at its Houston plant in the mid-2000s, which brought about dramatic reductions in butadiene emissions.

But that's not what happened in Port Neches.

The reports TPC posted to its website, which are tucked away on a page that makes no mention of the EPA settlement and doesn't explain what the reports are, show that the facility saw a significant increase last year in both the size of butadiene emissions and the number of times the company exceeded the agreed-upon threshold. In many cases, the emissions surpassed what the state considers safe for short-term human exposure.

Butadiene emissions at TPC Group's Port Neches chemical plant

The uptick emerged in the latter half of 2019 and the emissions became more frequent and larger in the months leading up to the explosions and fire at the plant the day before Thanksgiving that injured multiple workers, blew out windows in scores of nearby homes and prompted widespread evacuations during the holiday week. Despite the actions the company reported taking to address the high readings, the emissions continued to climb.

Experts said it's too soon to know whether the rising emissions are linked to the Nov. 27 incident. That will not be known until state and federal investigators share their findings. The company identified a butadiene processing unit as the source of the initial explosion.

But air quality experts and scientists working for environmental groups who reviewed the reports for the Tribune say the air monitoring data is alarming and should have been a red flag for the EPA, which said in a statement that it had monitored the reports but "not conducted a trend analysis."

"The levels that they were measuring were shockingly high," said Neil Carman, a scientist and former TCEQ pollution inspector who heads the clean air program at the Sierra Club's Lone Star Chapter. "This is a sign that there were some major problems going on at the plant."

While TPC began monitoring for butadiene in February 2018, taking readings every 15 minutes, the company didn't start consistently reporting so-called "exceedances" of the 25 parts per billion threshold on its website until March 2019. A TPC spokesperson said that was because "a gap in the administrative process" led to confusion about posting requirements, adding that the EPA allowed the company to extend the program to make up for the missing time. The most recent reports are from October 2019.

Overall, the eight months of reports from March to October show that butadiene levels surpassed the threshold an average of 4.5 days per month, with an average of eight exceedances per day.

Both the number and size of those exceedances began to creep upward over the summer.

On July 22, TPC reported three exceedances, but they were unusually high — one was 364 parts per billion, the largest 528 parts per billion. That is high enough to cause health impacts in humans — state guidelines say that exposure to more than 500 parts per billion of butadiene over a period of 30 minutes to six hours may results in symptoms that include irritation of the eyes and respiratory system. (The state says exposure to 9.1 parts per billion butadiene over 70 years can increase cancer risk.)

The next month, the company reported 20 exceedances on a single day, Aug. 9, with one reading topping 222 parts per billion. The next week, it reported a measurement of 805 parts per billion. The butadiene emissions peaked on Sept. 27 when the facility reported 10 exceedances — including one that hit 14,846 parts per billion. According to the corresponding report, the spike occurred when a connection on a valve failed, requiring workers to "isolate and purge the line" of butadiene. (Because of the size and cause of the leak, TPC had to report it to the state as well.)

The number and size of the exceedances remained high throughout October, the month before the fire and explosion.

California-based air quality consultant Jim Tarr, a chemical engineer who worked for TCEQ's predecessor agency in the 1970s, said the reports indicate the facility was experiencing an unusual number of fugitive emissions — and that those leaks resulted in an unusual amount of airborne butadiene at the facility's fenceline.

"Clearly there has been mechanical failure after mechanical failure after mechanical failure, which suggests either, one, they're not paying any attention to what's going on in the process unit or, two, it's an old facility that's wearing out," said Tarr, who has been critical of Texas' approach to environmental regulation

While several of the releases were well above federal and state standards for human exposure to butadiene, Tarr said they weren't high enough to pose a risk of explosion — at least at the fenceline. But he added that because butadiene disperses rapidly when released into the air, the levels could have been much higher at the source of the leaks.

What is clear, he said, is that the EPA — or TCEQ, which is primarily responsible for enforcing federal clean air laws in Texas — should have seen the trend as a red flag and followed up.

In a statement, the EPA said it evaluated TPC's compliance with the settlement agreement, including whether it had carried out the fenceline monitoring program, "but EPA has not conducted a trend analysis of the fenceline monitoring results.

"Prior to the November 2019 incident, EPA periodically evaluated individual reports to determine whether they met the terms of the [agreement], specifically whether the reports included identification of a root cause and corrective action," the statement said.

It added that agency "has not identified anything in the reports or corrective actions that has indicated an immediate safety concern at the facility."

The EPA and TCEQ had planned a weeklong inspection at the facility for Dec. 9 but had to postpone it because of the explosions and fire. It will be rescheduled after the facility resumes normal operations.

TCEQ declined comment, citing its pending investigation of the incident "and anticipated litigation relating to the same." It directed further inquiries to the Texas Attorney General's Office, to which it has referred the case.

Asked about the monitoring data and whether it could have been a red flag, TPC spokeswoman Sara Cronin said in a statement that "It's too early in the process to determine cause."

The U.S. Chemical Safety and Hazard Board and the Occupational Safety Health Administration are investigating the incident, she said, and "TPC Group is cooperating fully with all federal and local agencies." The statement added that the company is conducting its own investigation into the cause of the explosion.

A dozen years before the Port Neches incident, TPC installed monitors on the fenceline of its Houston plant after the facility was identified as the source of high butadiene emissions in a nearby city park.

Then known as Texas Petrochemicals, TPC struck a deal with the TCEQ in June 2005 — a year after it emerged from bankruptcy — amid tough media coverage and pressure from city officials to voluntarily reduce butadiene levels at the plant's fenceline to 1 part per billion by the end of 2007 "and zero soon after," according to a Houston Chronicle report.

At the time, the article said, "fence-line concentrations around the facility’s fences, which border some backyards, are typically greater than 3 and less than 10 parts per billion, according to recent data from a nearby monitoring site."

A company presentation posted on the EPA's website shows it was able to reduce butadiene emissions by 78% in the following years and describes the fenceline monitoring system as an "excellent diagnostic tool."

But the fenceline monitoring reports are just one of many indications that something was amiss at TPC's Port Neches facility, said Elena Craft, a toxicologist who works as senior director of Climate and Health at the Environmental Defense Fund.

State records show TPC regularly reports emitting more air pollution than allowed under its government-issued operating permits — that happened more than 50 times over the past 10 years — and that the state has fined it after finding some of the emissions events were avoidable. Federal agencies also fined the company in 2016 for separate air pollution and safety violations, records show.

Craft and others who closely track air quality in the region said they were not aware of TPC's air monitoring reports before the Tribune brought them to their attention. Craft said taken together, those reports indicate "sloppy operations" at the facility.

"To me, their track record suggests that they were at risk of some kind of incident occurring, and to me that's enough that the state agency should have taken action preemptively and they didn't."

Disclosure: The Environmental Defense Fund has been a financial supporter of The Texas Tribune, a nonprofit, nonpartisan news organization that is funded in part by donations from members, foundations and corporate sponsors. Financial supporters play no role in the Tribune’s journalism. Find a complete list of them here.