What is hydrogen sulfide? Toxic gas eyed in Colorado dairy deaths is infrequent but dangerous feature of agricultural work.

The toxic gas eyed as a possible culprit in the deaths of six dairy workers in Weld County last week is an infrequent but potentially dangerous feature of this type of agricultural work, while experts say the number of victims has little precedent in modern American history.

First responders reported the fatalities at the Prospect Valley Dairy outside Keenesburg on Aug. 20 were possibly associated with exposure to high levels of hydrogen sulfide, known as H2S, a colorless, flammable and corrosive gas that smells like rotten eggs.

Hydrogen sulfide naturally occurs in sewers, manure pits, well water, oil and gas wells and volcanoes. It’s also produced in a number of industries, including mining, tanning, oil and gas refining and certain types of agriculture.

Its effects depend on how much of it a person breathes in and for how long.

Low levels of H2S can produce headaches or eye, nose and throat irritation. But higher levels can cause nausea, vomiting, shortness of breath and loss of consciousness. At extreme levels, hydrogen sulfide can kill a person in just a few breaths. There is no known antidote.

“Hydrogen sulfide is definitely (a) killer,” said Dan Neenan, director of the National Education Center for Agricultural Safety.

Weld County authorities have released few details surrounding the deaths of the six workers, which included four family members, one of whom was a local high school student. The coroner’s office said the deaths are being investigated as a possible gas exposure within a confined space. At the scene, first responders noted finding men with possible high levels of hydrogen sulfide, according to archived radio transmissions.

Experts said the fact that the incident occurred in a confined space lends itself to more dangerous conditions. The Occupational Safety and Health Administration, which is also investigating the deadly incident, has special rules for confined spaces, but the agriculture industry is exempt from those regulations.

Though H2S initially has a pungent, rotten-egg odor, prolonged exposure knocks out your sense of smell, leading to a false sense of security, experts said.

Neenan said dairy workers and other people working with hydrogen sulfide should wear monitors that can detect the toxic gas and alert them when the fumes reach certain levels. It’s not clear whether the Prospect Valley Dairy laborers had these monitors.

His organization runs training sessions and classes around the country for both agricultural workers and first responders. They do an hour of classwork, followed by rescue simulations that mimic the conditions of a manure pit.

One of the most important rules when dealing with H2S, Neenan said: Don’t try to rescue people yourself.

What often happens is that one worker falls unconscious from the fumes and their coworkers or family members rush in to try and help them. But a person cannot hold their breath long enough to extract people from a manure pit, Neenan warned.

“I know it’s counterintuitive,” he said. “If it’s a family member, I want to help. But if I go in and pass out, the fire department is not sure which pit to go to, and I’m not helping at all. I’m even making it worse.”

Hydrogen sulfide deaths around the country

This exact situation played out in Fort Morgan last year.

In May 2024, one worker died and four others were hospitalized due to hydrogen sulfide exposure in an underground pump house at the Western Sugar Cooperative. One worker became overwhelmed by the H2S gas, while another employee was also overexposed while attempting a rescue, OSHA found in its investigation. That individual also lost consciousness and was later found unresponsive.

Data shows incidents involving hydrogen sulfide are relatively uncommon — but can often be deadly when they do occur.

There were at least 389 incidents involving 459 individuals between 1975 and 2019 involving livestock waste, according to a 2022 study in the National Library of Medicine. Fifty-nine percent of these were fatal. Forty-nine rescue incidents involved a total of 119 secondary victims, meaning more than a quarter of the victims were secondary victims, including first responders, the study found.

Purdue University in 2024 tracked at least 51 cases that year involving agricultural confined spaces, including 22 fatal and 29 non-fatal incidents. That represented a 38.6% decrease over the 83 cases in 2022, and a 7% decrease since 2023.

In 2016, a Wisconsin farmer died due to exposure to hydrogen sulfide from a manure pit. Five years later, three adult brothers in western Ohio died while performing maintenance in a manure pit. Last year, two young men died after they fell into a manure tanker and were overwhelmed by the gases at a farm in New York.

‘There can never be enough training’

But two agricultural safety experts told The Denver Post that in their decades of work, they had never seen six people die from hydrogen sulfide in the same exposure incident.

This shocking and tragic story should spur legislators to take action to protect farmworkers from similar dangers, said Hunter Knapp, development director with Project Protect Food Systems Workers, a Colorado organization dedicated to advancing food worker justice issues.