On this day in labor history, the year was 2014.

That was the day four workers were killed and a fifth injured during a chemical leak at a DuPont insecticide plant near Houston.

The plant used methyl mercaptan in its production of insecticides.

24,000 pounds of the deadly chemical were released through two valves in a poorly ventilated building onsite.

The U.S. Chemical Safety Board found numerous flawed safety procedures, design problems and inadequate planning.

Days earlier, liquid methyl mercaptan had solidified in piping, causing a blockage.

Workers attempted to clear it by spraying the pipes with hot water.

They didn’t realize they had cleared the blockage, which then created high-pressure buildup of the chemical in other piping.

When two workers went to drain those pipes in a routine procedure, they were overcome by toxic vapor.

Another two workers answering the subsequent distress call were also killed.

DuPont blamed workers for the release of the toxic gas.

But the CSB found a number of violations.

The building where the release occurred had an inadequate toxic gas detection system, ventilation fans were not working and workers were not required to wear additional breathing protection for tasks they performed there.

Line-clearing procedures were faulty, routinely exposing workers to toxic fumes.

The Board also found that DuPont worked to conceal from environmental regulators, as many as four major releases of methyl mercaptan two days before workers were killed.

The CSB asserted that design flaws prompted months of clogs before the deadly incident.

More generally, they noted the design of the building that housed the pesticide unit inherently increased the threat of exposure to workers and the public.

DuPont opted to close the plant in 2016 rather than meet recommendations of federal regulators.