Identifying and Limiting Dangerous Gases on the Farm

Silo Gas

Silo Gas Development

As silage and high-moisture shelled corn are stored for later use, gases are produced during the fermentation process. The primary gases produced during this process are:

• Carbon dioxide (CO₂)
• Nitrogen dioxide (NO₂)
• Methane (CH₄)

Gas formation begins within hours after the forages are sealed inside the silo, bunker, or bags, with concentrations typically peaking about three days after harvest.

Identification and consequences

Carbon dioxide, nitrogen dioxide, and methane pose serious hazards to farm workers, making it essential to understand how to identify and prevent these types of injuries from occurring.

• Nitrogen dioxide is easier to identify out of the two gases. Itmay appear as a very faint yellow-red haze with a bleach-like odor (Figure 1). It can cause severe irritation to the nose and throat, lung inflammation, permanent lung damage, or rapid death even at low concentrations.
• Carbon dioxide, though odorless and colorless, can displace oxygen and cause suffocation when CO₂ levels reach 30% or higher. High concentrations can cause fainting, relapses resembling pneumonia, or death within minutes.
• Methane is odorless and colorless, it is flammable at lower levels and can cause suffocation at higher concentrations.

Hazard Prevention

There are several safety precautions to follow, and these are applicable to both carbon dioxideand nitrogen dioxide:

• Avoid entering silos during peak gas-production periods, typically 12 to 72 hours after filling.
• Ventilate the silo by running the blower for at least 30 minutes before entry (only if there is 15 feet or less of headspace) and by opening all available doors and vents. Ventilation works by diluting and removing accumulated gases while restoring oxygen levels.
• Use lockout–tagout procedures to ensure that no power is supplied to the silo unloader while someone is inside the silo. See the lockout–tagout article for more information on how it works.
• Post warning signs around the base of the silo and at all entry points. Signs should clearly state "Danger: Toxic Gases," restrict unauthorized entry, and indicate required personal protective equipment (PPE).
• Use a multi-gas or single gas monitor (Figure 2.) to check for oxygen deficiency and to detect nitrogen dioxide when working around a recently filled silo. For more information about gas monitors, see the Confined Space, Manure Gas Monitoring article.
• If an individual is unresponsive or injured while inside the silo, do not enter the silo and call emergency services right away. Make sure to tell them that silo gas is a concern so that they bring the appropriate equipment.

Examples and Properties of Silo Gases

Manure Gases

Manure Gas Development

There are four gases to be worried about when dealing with manure:

• Hydrogen sulfide c forms during the anaerobic breakdown of manure, particularly when manure is agitated.
• Methane (CH₄) is produced as manure decomposes in storage.
• Carbon dioxide (CO₂) is released from both animal respiration and manure decomposition.
• Ammonia (NH₃) forms in the breakdown of manure, especially in manure pits and enclosed animal housing.

Gas concentrations can increase rapidly, particularly during agitation, pumping, or warm weather. Because many of these gases are heavier than air, they tend to collect at the bottom of pits or low-lying areas, displacing oxygen and creating an immediate risk of asphyxiation.

Identification and Consequences

Manure gases vary in how easily they can be detected and in the severity of their health effects. The most effective way to detect gas is to use a meter (see more below).

• Hydrogen sulfide smells like rotten eggs at low levels, but at concentrations of 100 ppm or higher it overpowers the sense of smell, making it especially dangerous. Exposure can cause eye and nose irritation, headaches, nausea, unconsciousness, and death. Concentrations of 1,000 ppm can be fatal within minutes.
• Carbon dioxide is colorless and odorless, making it impossible to detect without monitoring equipment. High levels reduce oxygen availability and can cause headaches, drowsiness, unconsciousness, and death. Concentrations above 40,000 ppm are immediately dangerous to life and health.
• Ammonia has a sharp odor similar to kitchen cleaners and irritates the eyes and respiratory system. High concentrations can cause asphyxiation and may be fatal. Levels above 300 ppm are considered immediately dangerous.
• Methane is colorless and odorless and is primarily an explosion hazard. While it can cause headaches and oxygen displacement at high concentrations, its greatest danger is its flammability. Levels at or above the lower explosive limit (50,000 ppm) can ignite from sparks, flames, or welding.

Hazard Prevention

Preventing manure gas incidents requires a combination of built-in safety controls, monitoring, and safe work practices.

• Workers should avoid entering manure pits or confined manure storage whenever possible. If entry is necessary, proper confined-space precautions must be followed, including the use of supplied-air and safety harness. This is the same guidance as for silo gases. The Confined Space Manure Storage Hazards article provides more information on working around manure storage.
• Mechanical ventilation systems (Figure 3) are critical and should be used to force fresh air into enclosed manure storage areas, increasing oxygen levels and reducing toxic and explosive gases before and during any work. The Confined Space Manure Storage Ventilation Systems article provides more information about manure ventilation.
• Gas monitoring equipment, such as gas detection badges, detection tubes (Figure 4), single or multi-gas monitors (Figure 2), and fixed monitoring systems, should be used to identify hazardous conditions before entry. Single gas monitors are the most commonly used devices for their ease of use and cheaper price (around $150). The Confined Space Manure Gas Monitoring article provides more information about gas meters.
• Be aware that gypsum bedding adds sulfur to the manure when agitated, which can contribute to dangerous levels of hydrogen sulfide.
• Ignition sources, such as welding, grinding, and smoking, should be controlled, especially near anaerobic digesters. Smoking and other ignition source areas should be located at least 500 feet away, and clear safety signage should be posted.
• Trainings with everyone coming near manure storage need to communicate information about the dangers of manure gases, how to recognize symptoms of exposure, and know when to call emergency responders if an individual is unresponsive or injured in areas with high amounts of gas.

Examples and Properties of Manure Gases

Exhaust Gas

Development of Exhaust Gases

Exhaust gases develop when internal combustion engines operate in or near confined or poorly ventilated spaces. Common sources include tractors, skid steers, forklifts, pressure washers, generators, and other gasoline, diesel, propane, or natural gas–powered equipment. When these engines run, they produce combustion byproducts such as carbon monoxide (CO), nitrogen oxides (NOx), carbon dioxide (CO₂), and unburned hydrocarbons.

In open areas, these gases typically disperse into the atmosphere. However, in confined spaces such as barns, machine sheds, and enclosed workshops, exhaust gases can accumulate. Poor ventilation, sealed doors and curtains, or idling equipment significantly increase the rate of buildup. Even brief operation of an engine can result in dangerous gas concentrations, particularly when exhaust systems are poorly maintained or engines are malfunctioning.

Potential Hazards of Exhaust Gases

Exhaust gases pose serious and often life-threatening hazards to farm workers.

• Carbon monoxide is especially dangerous because it is colorless and odorless, making it difficult to detect without monitoring equipment. CO can lead to symptoms such as headache, dizziness, nausea, confusion, loss of consciousness, and, at high concentrations, death.
• Nitrogen oxides can irritate the eyes and respiratory system and may cause delayed lung injury after exposure.
• Carbon dioxide, while not toxic at low levels, can displace oxygen in confined spaces, leading to an increased risk of asphyxiation.

The combined presence of these gases can quickly overwhelm workers, often without warning, especially during physically demanding tasks. Because symptoms may resemble fatigue or illness, workers may not recognize the danger until exposure becomes severe.

Prevention of Exhaust Gas Hazards

Preventing exhaust gas exposure in confined farm spaces requires a combination of built in safety controls, safe work practices, and workplace procedures

• Whenever possible, internal combustion engines should not be operated in enclosed or poorly ventilated areas. Electric-powered equipment is a safer alternative for indoor work.
• Good ventilation is critical. Natural or mechanical ventilation systems should be used to ensure fresh air circulation and removal of exhaust gases. Doors, windows, and vents should be opened before equipment is started, and ventilation should continue during and after engine operation. Exhaust systems and engines must be properly maintained to reduce emissions and prevent leaks.
• Portable or fixed gas monitors capable of detecting carbon monoxide and oxygen deficiency should be used in confined spaces where exhaust exposure is possible. The guidance is similar to what was discussed above.
• Anyone operating equipment and machinery should be trained to recognize the symptoms of exhaust gas exposure and understand confined space hazards.
• Establishing policies that prohibit idling equipment indoors and implementing confined space entry procedures can further reduce the risk of injury or death.

Examples and Properties of Exhaust Gases

Further information related to gas safety

Penn State Extension: Silo Gases - the Hidden Danger.

Penn State Extension: Safety Risk from Manure Storages of Dairy Cows Bedded with Gypsum.

Ag-Safety Extension: Dangers of Silo Gases.

Ag-Safety Extension: Confined Spaces: Hazards of Manure Gases.

Cornell University: Carbon Monoxide: The Silent Killer | Cornell Agricultural Workforce Development

References

Canzonier, J. Carbon Monoxide: The Silent Killer. Cornell Agricultural Workforce Development, 2025.

Cornell University Environment, Health and Safety. Confined Spaces in Agriculture Toolbox Talk. n.d.

Farm and Ranch Extension in Safety and Health (FReSH) Community of Practice. Confined Spaces: Hazards of Manure Gases. 2019.

Farm and Ranch Extension in Safety and Health (FReSH) Community of Practice. Dangers of Silo Gases. 2019.

Hile, Michael, Eileen E. Fabian-Wheeler, and Dan Hofstetter. Safety Risk from Manure Storages of Dairy Cows Bedded with Gypsum. Penn State Extension, 22 Mar. 2023.

Lovarelli, Daniela, and Jacopo Bacenetti. "Exhaust Gases Emissions from Agricultural Tractors: State of the Art and Future Perspectives for Machinery Operators." Biosystems Engineering, vol. 186, 2019

Murphy, D. (2013) Silo gases—the hidden danger. Penn State Extension.