A toxic chemical leak Saturday at a Dupont plant in La Porte, Texas, 20 miles from Houston, killed four workers instantly and sent a fifth worker to the hospital where he is expected to recover. The victims were from the crop protection unit, which was reportedly responding to the leak when they were overcome by methyl mercaptan, a chemical used in insecticides.

It was not until 12 hours after the still unexplained leak occurred that DuPont confirmed four workers had been killed. Plant manager Randall Clements gave no details about the victims except that their tenure with the company ranged from 40 years to just eight months. Though not all the workers have yet been publicly identified, family members have confirmed that two of them were brothers Robert and Gibby Tisnado, 39 and 48. Robert leaves a wife and three children.

In addition to its use in insecticides, methyl mercaptan is used in natural gas, ironically as a safety precaution to provide a rotten-egg smell to unburned gas. Dupont’s La Porte facility stored 122,000 pounds of the chemical. It took two hours for crews to stem the discharge before the hazardous gas was dispersed into the surrounding area over a 40-mile radius, causing a noxious odor across the Houston metropolitan region.

DuPont company spokesman Aaron Woods insisted that there was no danger to the public. In an on-air apology, Woods referred to the “strong odor” as merely a “nuisance.”

The Environmental Protection Agency (EPA) describes methyl mercaptan as a colorless gas with a strong odor used in the manufacture of pesticides, jet fuels and plastics.

The chemical is highly toxic. Its odor is detectable at 0.0016 parts per million (ppm). The Occupational Health and Safety Administration (OSHA) has set the legal acceptable airborne limit at 10 ppm. The National Institute for Occupational Safety and Health (NIOSH), part of the federal agency Centers for Disease Control and Prevention, sets the recommended airborne exposure limit at .5 ppm, not to exceed a 15-minute work period. Exposure to quantities in the hundreds of ppm can result in unconsciousness and death within minutes. High exposure causes pulmonary edema and damages the liver and kidneys.

The cause of the leak is being investigated. The federal agency responsible for the investigation of such accidents, the US Chemical Safety Board, announced it was sending a seven-person team to the plant. DuPont said it would conduct its own investigation. News reports indicated that the leak was stopped at 6 a.m. Saturday with the replacement of a broken valve.

DuPont has a long history of industrial accidents and questionable employee safety practices.

In 2011, the Chemical Safety Board uncovered that "a series of preventable safety shortcomings" were responsible for the death of a worker from a 2010 phosgene gas release at a DuPont facility in Belle, West Virginia. That same year, a welding explosion killed another worker at a DuPont plant outside of Buffalo, New York. That accident was attributed by the Board to DuPont’s failure to monitor flammable gas levels in a storage tank.

As one of the world’s largest and oldest corporations, DuPont, formerly known as E.I. du Pont de Nemours and Company, developed an anti-working class reputation. Founded in 1802 as a gunpowder and explosives manufacturer, it earned the moniker the “Merchant of Death.”

Texas officials, from both the Republican and Democratic parties, are notorious for anti-regulatory and pro-business policies and the state regularly has one of the highest industrial death rates in the United States.

Last year, 15 workers, including 12 emergency personnel, were killed and nearly 200 wounded in an ammonium nitrate explosion at a fertilizer plant in West Texas. Prior to the explosion the plant had not been inspected for 28 years. At the time OSHA found five “serious” violations, “involving improper storage and handling of anhydrous ammonia and improper respiratory protection for workers.” For these violations, the company received a $30 fine.

On March 23, 2005, a major explosion ripped through BP’s Texas City refinery, killing 15 workers and injuring 170 others. The company has been repeatedly cited for “willful violations” of federal safety standards.