Hydrogen sulfide and ammonia in confinement: what the gases do to animals, people, and equipment

Four gases come off stored manure. Hydrogen sulfide is the killer: heavier than air, it pools in the pit and knocks people down during agitation. Ammonia damages airways at levels you can live in. Methane burns. Carbon dioxide displaces oxygen. Ventilate hard, monitor, and never enter a pit.

Where do these gases come from?

Stored manure is a digester whether you meant to build one or not. Bacteria break down organic matter without oxygen. Sulfur in the feed becomes hydrogen sulfide, protein nitrogen becomes ammonia, and carbon becomes methane and carbon dioxide. You cannot stop that. What you control is how much accumulates and whether it comes out steadily or all at once. Gas coming off an open liquid surface behaves nothing like gas that has built under a crust for six months and then releases in a slug when a prop hits the mat.

What are the four gases and where does each one sit?

Density is the practical part of that table. Hydrogen sulfide and carbon dioxide sink, which is why the pit, the bottom of a pump-out, and the low corner of a barn are the dangerous places, and why a man who steps into a pit goes head-first into the worst concentration in the building.

Why does hydrogen sulfide kill people who felt fine a minute earlier?

Two things beyond the density.

Your nose quits. At low concentrations it smells like rotten eggs, well below any level that would hurt you. That is where people get their confidence. At high concentrations it paralyzes the olfactory nerve and the smell goes away. A man who notices the rotten-egg smell cleared up is not in a safer place. His warning system just shut off.

The curve is steep. This is not a gas where twice the concentration is twice as bad.

Between the fourth row and the sixth there is not much time and there is no smell. That is the whole story of manure pit fatalities.

The rescuer problem. Read the incident reports and one pattern repeats: one man goes down, a second goes in after him without protection, and both die. Sometimes it is three or four. NIOSH has documented this for decades. It is not carelessness. It is the natural response to watching family collapse, which is why the rule has to be settled in advance, in daylight, when nobody is lying on the floor.

The rule is that nobody goes in after them. Call 911, get a fan on the opening, and pull from outside with a rope and harness if a retrieval line was rigged beforehand. Anyone entering an untested pit is likely to become the second body.

When is the risk highest?

Agitation, without close competition.

Hydrogen sulfide dissolves in the liquid. While the manure sits still, much of it stays in solution or capped under the crust. Agitation drives dissolved gas out of solution and breaks the mat holding the rest. Barn concentrations that were survivable all winter can spike to lethal levels within minutes of starting the prop, and a crusted pit is the worse case because the crust has been a lid on a reservoir.

Foam is its own problem, covered in manure pit foam and flash fire hazard.

What does ammonia do before it ever gets dangerous?

Ammonia rarely kills anybody on a livestock operation. It costs money every day instead.

It is water soluble, so it goes to work on the wettest tissue it can find: eyes and the lining of the upper airway. At sustained levels well below any acute hazard it damages the respiratory epithelium and impairs the mucociliary escalator, the mechanism that clears inhaled particles and pathogens out of the airway. Animals in an ammonia-loaded room run with their primary respiratory defense degraded, which is why the research treats elevated ammonia as a contributing factor to respiratory disease in swine and poultry, not just an irritant. Work on swine growth also links it to reduced feed intake and reduced daily gain. Animals do not have to be sick for it to cost you. Figures vary by study and by how the barn is run, so treat them as directional.

Then there is the heat bill. The answer to ammonia is ventilation, and ventilation in January means paying to heat air you throw out. Every producer on minimum ventilation in cold weather makes the same trade: gas concentration against propane. Ammonia is also corrosive to exposed metal, wiring, and fan components in a warm humid barn. A barn that runs high on ammonia ages faster.

What about methane and carbon dioxide?

Methane does not poison you. It burns. The flammable range in air runs roughly 5 to 15 percent. It is lighter than air, so it collects at the ceiling of an enclosed pit space and under any cap holding it, crust or foam. Ignition sources in a barn are ordinary: a heater, a light fixture, a grinder. The gas is odorless. The only warning is the meter.

Carbon dioxide gives no warning either, and it is heavier than air. It comes from the manure and from the animals. Properly ventilated, it is a non-issue. In a pit or a low space with stalled ventilation it displaces oxygen and asphyxiates. A barn where ventilation failed overnight can have oxygen-deficient low areas before anybody opens the door.

What should the rules be on your operation?

Post these where the agitation equipment is stored.

1. Never enter a manure pit. Not for a plugged pump, not for a dropped tool, not for a rescue. Pits are permit-required confined spaces. Entry means atmospheric testing, forced ventilation, supplied air, a harness with retrieval line, and a trained attendant outside who does not enter. Without all of it you do not have an entry plan, you have a plan to lose somebody. See OSHA's confined spaces material. Many farms fall outside OSHA enforcement; the rule may not apply to you, but the gas does not know that.
2. Own a personal H₂S monitor and turn it on. A four-gas meter is cheap next to what it protects. Bump test it and replace sensors on schedule. A meter with a dead sensor is a decoration.
3. Maximize ventilation before the prop starts, not when somebody starts coughing, and keep it running.
4. Get people out during agitation. If somebody has to be inside, he wears a monitor and somebody outside knows it.
5. Move animals if you can. If not, agitate under maximum ventilation and watch them. Animals going down is a late warning.
6. Work in pairs, from outside. No solo agitation, no solo pump-out.
7. Assume the pit is worse than your reading. Your meter reads where your meter is.

None of this becomes optional because you treated the pit or because you have done it this way for thirty years. Manure gas incidents are not a gradual slope. They are a long stretch of nothing followed by one very bad afternoon.

Does biological treatment change any of this?

It changes the pattern of gas release. It does not change any of the rules above.

Here is the problem, plainly. A pit stalls when the fiber coming in every day, undigested feed and bedding, stops breaking down. That fiber accumulates as a floating crust on top and a hardpan on the bottom, with gas trapped between. Helping break down that material weakens the crust and makes solids easier to suspend, so gas is not building under a cap. That is how PitCharger works, and what Manure Digester, the flagship monthly treatment, is for. A pit with solids in suspension also agitates faster than one where you are chopping a mat, which is covered in cutting agitation and pumping time. For odor specifically, N-Loc addresses odor-producing compounds in the system rather than masking them.

On odor, a year-long Iowa State/Farmland study, as reported by PitCharger, showed approximately a 50% reduction in hydrogen sulfide, ammonia, and overall odor compared with untreated buildings, and some producers have noticed a substantial change within the first 24 hours. Producers can often notice a significant change in odor within the first week. Those figures are about odor, not about acute hydrogen sulfide during agitation. Treat them accordingly.

What treatment does not do:

• It does not make a pit safe to enter. Nothing makes a pit safe to enter.
• It does not replace ventilation, monitoring, or an agitation plan.
• It is not an overnight fix. Most producers should notice a difference within the first month, with continued improvement through the second and third months. Significant results are normally measured after about six months. That is not a minimum program; it is the point where broader changes in the manure system are measured.

It does convert organic nitrogen to plant-available ammonium nitrogen, raising the fertilizer value of what you haul. See organic versus ammonium nitrogen in manure.

Hog operations commonly start around one gallon per month per 1,000 head, about four gallons a month for a 4,000-head finishing barn. Cattle and dairy commonly start around one gallon per 75 animals. Lagoon rates may be adjusted based on the manure already in the lagoon and the amount expected to be added each month. PitCharger ships by UPS, Spee-Dee Delivery, and LTL freight, depending on the size and destination of the order, and Auto-Ship is available for scheduled monthly delivery. To have it worked against your pit volume and head count, get a treatment plan or call 833-258-7631. No public price list, no cart. Different problems require different treatment approaches.

Frequently asked questions

If I can smell the pit, am I in danger?

Not necessarily, and the reverse is not safe either. Hydrogen sulfide is detectable by smell far below hazardous concentrations, so a strong rotten-egg odor is common in barns where nobody is being harmed. The dangerous case is the opposite: at high concentrations the gas paralyzes your sense of smell, so an odor that disappears during agitation is a warning, not an all-clear. Use a meter, not your nose.

What is the most dangerous time on a manure system?

Agitation, and specifically the first minutes of it in a pit that has crusted over. The mat has been holding gas underneath, and breaking it puts that reservoir into the barn fast. Ventilate at maximum before you start, clear the building, and monitor.

Can I go in the pit if I hold my breath, it will only take a second?

No. This is how a large share of manure pit deaths happen. Hydrogen sulfide at pit concentrations can drop a man in a breath or two, and oxygen deficiency gives no warning at all. Once you are down you are lying in the highest concentration in the building. Nobody who died in a pit planned to be in there more than a second.

Does ammonia hurt production even when the barn seems fine?

It can. Ammonia damages respiratory tissue and impairs airway clearance well below any level that feels alarming, which is why research links it to respiratory disease and reduced gain in swine and poultry. The trade you make every winter is more ventilation against more propane. See hog operation applications.

References: the NIOSH Pocket Guide to Chemical Hazards, OSHA's hydrogen sulfide page, and air quality material from University of Minnesota Extension and Iowa State University Extension. Terms are defined in the glossary.