Hydrogen Sulfide Gas Exposure During the Animal Rendering Process

Hydrogen sulfide gas is a hidden but deadly hazard in animal rendering, where workers risk exposure during decomposition and processing—highlighting the need for strict monitoring, protective controls, and stronger safety practices.

• By Bernard Fontaine
• Nov 20, 2025

Introduction

Hydrogen sulfide (H₂S) is a colorless gas with a distinct rotten egg odor. It occurs naturally in many industries, including manure storage, oil and gas operations, sewage treatment, tank cleaning, and animal waste management. Rendering, the process of converting waste animal tissue into usable materials, has been practiced for centuries and continues to be a vital part of modern agriculture and food production. Instead of landfilling millions of pounds of animal by-products, rendering transforms them into valuable fats, oils, proteins, and other materials. This not only supports the economy but also advances sustainability goals by reducing waste and conserving resources.

The Rendering Process

Each year in the U.S. and Canada, more than 62 billion pounds of renderable materials are collected from farms, feedlots, and slaughter facilities. About 16 million tons of rendered products are produced, supplying ingredients for biofuels, pet food, fertilizers, personal care products, and even pharmaceutical capsules. Rendering involves thermal and chemical processing of animal by-products—blood, offal, bones, fat, feathers, hides—into proteins, fats, gelatin, and fertilizers.

While rendering supports sustainability, decomposition of biological materials such as poultry feathers produces hydrogen sulfide gas. Workers may be exposed during various points in the process, especially when materials are stored or processed under heat and pressure. In October 2024, employees dumping offal at a rendering plant were exposed to levels of H₂S far above OSHA’s ceiling limit of 20 ppm.

Sources of Hydrogen Sulfide Exposure

Hydrogen sulfide is generated during decomposition of sulfur-containing amino acids and in lagoons, digesters, and waste pits. In rendering facilities, high-pressure steam hydrolyzers used to convert feathers to meal are a major risk point. Workers may also encounter other hazardous byproducts, including ammonia, volatile organic compounds, and bioaerosols.

The dangers are well documented. In 2009, Tyson Foods was fined $500,000 after a worker was killed and two others injured from H₂S exposure at a rendering plant in Arkansas. Similar incidents have been reported in Texas and abroad, underscoring the ongoing hazards of this process.

Toxicity of Hydrogen Sulfide Exposure

H₂S exposure occurs primarily through inhalation, with rapid absorption into the body. Even at low levels, it can cause eye, nose, and throat irritation, headaches, and nausea. At higher concentrations, symptoms escalate to coughing, loss of smell, dizziness, drowsiness, and airway problems. Above 100 ppm, exposures are considered Immediately Dangerous to Life and Health (IDLH), with unconsciousness and death possible within minutes.

Hydrogen sulfide disrupts cellular respiration by inhibiting cytochrome oxidase in mitochondria, impairing oxygen use in tissues with high demand, such as the brain and heart. This can lead to collapse, respiratory arrest, and death.

Occupational Exposure Limits

OSHA sets a ceiling limit of 20 ppm for H₂S in general industry, with a peak limit of 50 ppm for up to 10 minutes. NIOSH recommends a 10 ppm limit over the same duration. Despite these limits, workers in rendering plants have suffered overexposures. Case studies highlight employees collapsing in enclosed waste intake areas, workers knocked unconscious by sudden gas releases, and fatalities in biodigesters where H₂S was present.

Case Studies

• Animal Rendering Plant: Two workers collapsed in a waste intake area, one remaining unconscious. Urine samples confirmed elevated thiosulfate levels in one worker, indicating acute H₂S exposure.
• Chicken Waste Rendering Facility: A blocked condenser caused an H₂S release, hospitalizing three workers; one required intensive care.
• Biodigester Fatality: A worker was killed after exposure, with postmortem samples showing thiosulfate levels consistent with fatal H₂S poisoning.

Conclusion

Hydrogen sulfide exposure remains a serious occupational hazard in animal rendering. While real-time gas detection and alarms can prevent acute, high-level exposures, monitoring strategies for chronic low-level exposure are less developed. Case studies show that biological monitoring of blood or urine thiosulfate may be useful in determining exposure levels after incidents, but routine monitoring is not common.

Protecting workers requires rigorous hazard assessments, strict adherence to exposure limits, continuous air monitoring, and effective training for process operators, maintenance staff, and emergency responders. Rendering may be vital to sustainability, but the risks posed by hydrogen sulfide demand constant vigilance to prevent injury and loss of life.

This article originally appeared in the November/December 2025 issue of Occupational Health & Safety.