Manure safety – hydrogen sulfide
Manure gasses can be deadly. Hydrogen sulfide (H2S) is one of those gasses. Dangerous H2S levels were measured in barns where deep bedded pack was being removed and around outdoor stored manure basins that were being agitated. Hydrogen sulfide was implicated in incidents of human and animal deaths and injuries on dairy farms in Pennsylvania and Maryland in 2012 and 2013. Human and animal deaths are rare but all deaths are tragic and preventable if you remember that stored manure can generate and give off dangerous levels of H2S.
Not all stored manure generates dangerous H2S concentrations. Moderately priced H2S detectors can be used to measure H2S concentrations to avoid exposure. There are other practices that can help reduce your H2S exposure and risk during bedded pack removal and manure agitation and handling.
Hydrogen sulfide is a colorless gas commonly known for its characteristic rotten egg odor. Physiological effects of H2S exposures are listed in Table 1. Prolonged exposure to H2S at low concentrations reduces your ability to detect its presence so your sense of smell is not a good indicator after even only a few minutes of H2S exposure. The National Institute of Occupational Safety and Health (NIOSH) recommends a 5 ppm short-term exposure limit (10 minutes) concentration for H2S. At 100 ppm, H2S is considered an immediate danger to life and health.
|Effect on humans||Concentration (ppm)|
|Odor threshold||0.01 to 1.5|
|Prolonged exposure may cause nausea, eyes tearing, headaches, sleep loss||2 to 5|
|Possible fatigue, loss of appetite, headache, irritability, poor memory, dizziness||20|
|Eye inflammation and respiratory tract irritation after one hour||50 to 100|
|Coughing, eye irritation, loss of smell after 2 to 15 minutes of exposure||100|
|Marked eye inflammation and respiratory tract irritation after one hour||200 to 300|
|Staggering, collapse in five minutes. Loss of consciousness and possible death in 30 to 60 minutes.||500 to 700|
|Rapid unconsciousness within one to two breaths and death within minutes.||700 to 1,000|
|Nearly instant death||1,000 to 2,000|
Source: OSHA, 2017
Hydrogen sulfide is a by-product of the anaerobic breakdown of sulfur-containing material. Sulfur can come from drinking water, concentrate-based feeds and dried distiller’s grains with solubles (DDGS). Sulfur is also in gypsum (calcium sulfate, CaSO4· 2H2O), which is used as a bedding material and copper sulfate (CuSO4), which is used in footbaths. Waste milk protein contains sulfur too.
Animal manures stored in bedded packs and manure storage basins become anaerobic where sulfate reducing bacteria generate H2S. Hydrogen sulfide emissions depend on manure crusting, pH and temperature. Uncovered manure storage basins that store liquid after solid-liquid separation may not form a crust and tend to have less H2S in the stored liquid manure. Manure storage basins with either natural crusts or a plastic cover would be expected to have higher H2S levels. Deep manure packs may also generate and accumulate H2S. Accumulated H2S is released when manure in basins is agitated prior to removal or the deep bedded pack is broken into for removal. Hydrogen sulfide release can present an immediate danger to people and animals depending on concentration and exposure duration. The greatest risk is during the first hour of agitation.
Hydrogen sulfide gas is heavier than air so it tends to stay close to the ground and collect in low spots if not dispersed by other means. Its dispersion after being released from stored manure depends on many factors including distance from the source, wind speed and direction, nearby obstructions and sunshine. Increasing separation distance allows more time and space for H2S to disperse to less dangerous concentrations. People or animals next to agitated manure or inside buildings may be exposed to dangerous gas levels very quickly. Higher wind speeds help disperse H2S out of doors. People standing upwind of an agitated manure storage will experience lower H2S concentrations compared to people downwind of an agitated source. Nearby buildings and low spots can create dead areas where H2S can remain at dangerous concentrations. Sunny weather warms the ground and causes air to rise which helps disperse emitted gasses.
Human and animal deaths and near-lethal exposures during open-air manure storage agitation led to a Penn State study on manure gas risks associated with gypsum bedding use. Observations and recommendations from Penn State fact sheets include:
- Gypsum bedding adds sulfur to manure that can lead to dangerous levels of H2S gas at agitation; however, not all farms with gypsum bedding have safety problems.
- Keep non-essential people away during agitation, especially children who are at increased risk as H2S is typically at a higher concentration close to the ground. Nearby cattle are also at risk.
- Secure storage from entry; provide rescue and fall protection; gas monitors are recommended.
- Manure moving-mixing-agitation creates highest gas levels for the first hour. Leave the area.
- Crust-free manure and additives seem to allow continuous H2S release lowering agitation risk.
- Gypsum benefits for cow bedding and agronomic values must be balanced against the potential gas hazard.
For more information about the Penn State study and fact sheets about handling manure with gypsum, visit Penn State's gypsum bedding page.
Minnesota has an H2S property line standard of 50 ppb (30 min average) that cannot be exceeded more than two times per year and a 30 ppb (30 min average) that cannot be exceeded more than two times in five consecutive days. These limits are 400 times lower than the concentrations that possibly cause fatigue, appetite loss, headaches and dizziness.