How to reduce hazards, increase safety around manure gas

Rebecca A. Larson
Farm workers should always assume that any confined space used for manure storage is dangerous.

Manure produces gases immediately after excretion. The microorganisms in manure break down the organic matter and produce gas emissions. These emissions are a natural part of the manure cycle.

In many circumstances the manure gases are produced and dissipate into the atmosphere. However, there are circumstances where the manure gases collect in an area and produce conditions that are harmful to people and animals that breathe the gases.

On average, there are a few deaths each year that can be attributed to manure gases, these deaths have most commonly been connected to those working on manure systems and those trying to save others who have been incapacitated by manure gases.

Understanding the gases produced and the conditions that increase gas concentrations can aid in reducing risk to human and animal health.

Manure gases can be numerous, the most common gases produced from manure with potential health risks include ammonia, carbon dioxide, methane, and hydrogen sulfide. 

Carbon dioxide and methane create a health risk as they displace oxygen resulting in conditions that can cause asphyxiation. This is particularly concerning in confined spaces where gases cannot dissipate resulting in oxygen levels below 19.5%. Methane also has the potential to explode at concentrations from 5-15%.

Ammonia and hydrogen sulfide are toxic gases that can have severe health risks at low concentrations. Ammonia can be irritating to the eyes and respiratory tract, and if concentrations increase can lead to death.

In manure systems, the concentrations of ammonia are generally higher in poultry manure as the nitrogen content is higher.

At low concentrations hydrogen sulfide has a rotten egg smell and can irritate the eyes and airways as well as cause nausea and headaches. As the concentrations increase, it can paralyze the olfactory nerve responsible for smell (meaning you may no longer smell the hydrogen sulfide, but it does not mean the concentration is reducing, it could also be increasing causing damage to your sense of smell). As hydrogen sulfide concentrations increase, exposure can lead to death.

Understanding the gases produced and the conditions that increase gas concentrations can aid in reducing risk to human and animal health.

Reducing health risks

To reduce health risks, steps should be taken to reduce the hazard. If the hazard cannot be eliminated completely, then safeguards should be used, such as warnings and education, to reduce the risk.

Finally, personal safety devices should be used when near a hazard that cannot be eliminated. Confined spaces can trap gases at high concentrations where even short exposure times can have significant health risks.

The hazard can be reduced in confined spaces by incorporating ventilation, but as the risk cannot be completely eliminated signs should always warn of the hazard and personal protection devices should always be worn when entering these spaces.

For areas where manure is present that are not a confined space (e.g., manure storages) gases can still increase in concentration and precautions should be used to reduce risk.

Manure gas dispersion in an open manure storage system.

Increased gas concentrations

There are many factors that can lead to increased gas concentrations. As hydrogen sulfide has been linked to many of the deaths recently in these manure spaces, understanding the conditions that lead to increased hydrogen sulfide concentrations is important.

Increasing temperatures (above 64ºF increases hydrogen sulfide production and continues with increasing temperatures), lower pH, and higher concentrations of sulfur in manure (median range is 0.6-3.2 pounds (0.3-1.5 kg) of total sulfur per thousand gallons) can lead to increased hydrogen sulfide concentrations.

Gas dispersion is important to allow the gases to distribute into the atmosphere reducing concentrations. Gas dispersion can be limited by physical barriers (e.g., walls or covers), during low or no wind conditions, and during temperature inversions.

Gas emissions are particularly high when agitating manure storages as trapped gases are released leading to higher gas emission concentrations. Avoiding areas with manure when the conditions above are present can reduce but not eliminate risk.

Monitoring emissions

Monitoring gas emissions and personal protection monitors can alert personnel when concentrations are increasing. Alarms can alert people to move from high-risk areas to lower risk areas to reduce heath risks.

Additional training should be conducted to inform workers of practices when an issue does arise to avoid a potential rescuer also becoming a victim.

Following these safety guidelines can significantly health reduce risks but not eliminate them completely so precautions should always be taken when working around manure. Taking steps to reduce risks will protect yourself, workers, family, visitors as well as livestock.

Additional research is being conducted at this time to further understand the conditions during agitation and manure processing that lead to higher risks, this information will be available in 2022. For more more information, a more detailed fact sheet is available for access free online at  

Rebecca Larson

Larson is Associate Professor of Biological Systems Engineering at the University of Wisconsin-Madison

Extension University of Wisconsin-Madison