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H2S hazard lethal and unpredictable
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H2S hazard lethal and unpredictable

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If storing, hauling or applying liquid manure is on your list this fall, protecting personnel and livestock must be top priority.

The hazard is hydrogen sulfide (H2S) — a heavy, colorless gas with no warning properties at unsafe concentrations. H2S is produced by bacteria that digest manure and organic matter.

In undisturbed slurry, H2S remains dissolved in the liquid, with concentrations in the surrounding air typically undetectable or a few parts per million (ppm). It has a characteristic rotten-egg odor at low concentrations.

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But when manure is disturbed or transferred, H2S breaks through the slurry surface into the air (like when you shake a bottle of your carbonated beverage before opening it). During agitation and pumping, H2S is released in bursts. In a matter of seconds, airborne concentrations can increase a hundred- to a thousand-fold, with deadly consequences.

‘Knock-down’ effect

H2S concentrations over 5 ppm cause increasingly severe health symptoms and outcomes: from headache and eye irritation, to disorientation, to collapse, loss of consciousness, and death. At 100 ppm, the concentration deemed Immediate Danger to Life and Health, H2S is not detectable by smell.

At 500 to 2000 ppm — levels that have been documented during agitation and pumping — the effects can be lethal to people and livestock nearby.

H2S is heavier than air, accumulating in low areas like pits and along floors. Over the past 40 years, many H2S-related human and livestock deaths involved deep pits in swine or dairy buildings.

Common scenarios have included entering pits to perform maintenance work or retrieve objects that fell through slats. Scores of incidents have involved multiple fatalities of unprotected, would-be rescuers — often family members — who didn’t recognize the hazard and were also overcome.

More recently, H2S-caused deaths and close calls have occurred during stirring and pumping at open lagoons, and in or atop liquid manure tanks used in field application.

What do these calamities have in common? The imminent risk was unrecognized, and individuals were unprotected.

Risk factors

Poor-quality well water and high-sulfur-containing dietary ingredients — e.g., distiller’s grains, gluten and syrup — can increase sulfur excreted from animals, resulting in greater H2S content in manure.

Environmental factors, including agitation and stirring practices, ambient temperature, wind currents and supplemental ventilation, affect the release rate and dispersion of H2S.

Despite these known contributing factors, there is no formula to accurately predict risk vs. no-risk. And building configuration or past experience do not predict future safety.

In fact, most accounts of animals down or lost and human fatalities occurred after many successfully uneventful experiences. “Never had a problem before” may be a reason people mistakenly presume that risk doesn’t exist at their own operation.

Gas monitors

The only way to confirm dangerous conditions is to use direct-reading gas monitors specific for H2S. Options range from low-cost, single-gas personal monitors that clip to the collar to multi-gas data-logging monitors that read H2S, oxygen, explosive gases and carbon monoxide. Monitors provide continuous detection with audible, vibrating and visual alarms that alert the wearer to leave the area.

The easiest-to-use single-gas H2S models have a 2-year sensor life and are virtually maintenance-free. They display “service life remaining” (in months) but lack real-time concentration readouts.

For manure handling activities, a better option are monitors with visual and audible alarms that display real-time H2S ppm concentrations when gas is present. Real-time ppm display is important because concentrations change rapidly during pumping and with distance from the source. Users can observe display readings and factor them into their decision-making. When no H2S is present, the monitors display service life remaining.

Two examples are GasClip Technologies’ Single Gas Clip Plus Hydrogen Sulfide (SGC-P-H) and Honeywell BW Clip Real Time H2S. They alarm at preset levels, display concentration up to 100 ppm, have 2-3-year service life, and can be “hibernated” off-season. Costs are under $200.

Will the monitor respond when you need it?

Most manufacturers advise “bump testing” before each use; this involves applying gas with a known H2S concentration to the monitor to confirm that alarms function and the sensor readout is accurate. Doing a bump test is like switching on your flashlight to make sure the bulb and batteries work before taking it into the dark basement during an outage!

H2S monitors and accessories for bump testing are available through local and online safety supply stores and some local manure handling equipment dealers.

Safe manure handling starts with recognizing and respecting this hazard. It follows with best practices accounting for environmental conditions, agitation techniques, communication, gas-detecting monitors and knowing that the only respiratory protection for H2S is an atmosphere-supplying (SCBA) respirator.

Stephanie Leonard is an industrial hygienist at the University of Iowa. She can be reached at

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