A hazardous atmosphere can quickly overcome anyone who enters a confined space. This may result in injury or death.

An atmosphere can be hazardous when:

  • it has too much or too little oxygen
  • there is accumulation of flammable, combustible or explosive agents
  • the accumulation of atmospheric contaminants like gases, vapours, fumes, dusts or mists can pose an immediate threat to life or interfere with a person's ability to escape unaided from a confined space

A confined space is a fully or partially enclosed space that is not both designed and built for continuous human occupancy and which may contain a hazardous atmosphere because of its construction, location, contents or the work done in it. It may be part of a structure or may be mobile or portable, such as a manure spreader tank.

There are many examples of potentially dangerous confined spaces on a farm, including:

  • silos
  • grain bins
  • manure pits and spreaders
  • mixing or holding tanks
  • valve pits
  • cisterns and pump houses

Even a few seconds of oxygen depletion can impair brain cell function. This may result in confusion and poor judgment and may compromise a worker's ability to exit the space.

Legal responsibilities

As an employer it is your responsibility under section 25 of the Occupational Health and Safety Act to provide information, instruction and supervision to protect the health or safety of  your workers. On a farming operation, this would include information, instruction and supervision on:

  • potentially hazardous confined spaces
  • work areas that may contain hazardous gases
  • the appropriate procedures when working in these areas

For each area where a hazardous atmosphere or confined space may exist, you should develop a written plan to protect workers and you must communicate the plan to workers.

When a worker is required to enter a controlled atmosphere storage warehouse that cannot be purged, you must make sure the worker is properly equipped with a breathing apparatus that allows safe entry.

Your supervisors must make sure that workers:

  • use or wear any personal protective equipment you require them to wear
  • follow any written plans and procedures you developed

Your workers should not enter:

  • confined spaces or hazardous atmospheres when there are no written plans or procedures for working in these areas
  • or remain in a building or structure adjoining a liquid manure tank while the manure is being agitated
  • any adjoining building or structure following the agitation of manure until it has been thoroughly ventilated
  • a grain bin or silo where grain is stored while the grain is being unloaded due to the risk of death from becoming trapped in flowing grain

Manure storage and silos

Manure storage entry procedures

People have died entering manure storage areas without proper safety precautions. Even small amounts of manure can produce toxic gases. A pit may not be safe even if it has only one foot (30 cm) of liquid.

If you must enter a manure storage area, wear a self-contained air supply like those used by firefighters. Dust masks or other cartridge respirators will not filter out the toxic gases nor will they provide the oxygen requirement to work in confined spaces such as manure pits.

Protecting people and animals

Remove all people and animals from the area if possible. If this is not possible, maximize ventilation and agitate slurry very slowly at first. Monitor the condition of the animals. If the animals are restless, agitated or behaving abnormally, stop the agitation immediately and ventilate the area.

Always keep at least one foot of space between the highest manure level and the slats. This protects animals that lie on the slats and inhale the gases that accumulate at the surface of the pit. The greater the space between the surface of the manure and the slats, the lower the risk of animals inhaling the gases.

Working safely in and around silos

Silo gas is heavier than air and will displace oxygen. The greatest danger from nitrogen dioxide gas from silage is during the first 12 to 60 hours after filling. Gases can form in the silo for up to 6 weeks after filling.

Anyone working in or around silos should:

  • be alert for silo gas odours and/or yellowish-brown or reddish fumes in or near the silo
  • take care to avoid possible exposure for 10 days after filling the silo and when opening the silo for feeding
  • not enter the silo for up to six weeks after filling stops, if gases are detected
  • not enter a silo without a self-contained breathing apparatus and a safety harness attached to a life-line, especially during the danger period when gases may still be forming

If anyone has to enter a silo, they should have another person who is an attendant outside to help if needed. Keep a hatch door open near the level of the silage within the silo.

You should post all appropriate warning signs. Oxygen-limiting silos require a sign that warns people of the absence of oxygen.

What you should know about gases

Characteristics of manure gases

The four main gases produced from decomposing manure are:

  • hydrogen sulfide
  • methane
  • ammonia
  • carbon dioxide

In high concentrations, each of these gases may pose an immediate threat to life or health of humans and livestock.

In animal housing facilities, where the manure pit is often located below the facility floor, manure gases may always be present in low concentrations.

The primary hazards of these gases are:

  • toxic or poisonous reactions in people or animals
  • oxygen depletion that can result in asphyxiation
  • explosions that can occur when oxygen mixes with gases such as methane

Hydrogen sulfide

Considered the most dangerous of the by-products of manure decomposition, hydrogen sulfide has a distinct rotten egg smell.

At low concentrations, hydrogen sulfide irritates the eyes and respiratory tract. At moderate levels, it causes headache, nausea and dizziness. At high concentrations, hydrogen sulfide paralyzes the nerve cells of the nose to the point where the person can no longer smell the gas.

Hydrogen sulfide is heavier than air and will tend to settle to the lower areas of a storage facility. It can remain in high concentrations even after ventilation.


Ammonia has a distinct, sharp, penetrating odour detectable at very low concentrations. It is heavier than air and can irritate the eyes and respiratory tract at moderate concentrations. At high concentrations, it can cause ulceration of the eyes and severe irritation of the respiratory tract.

Carbon dioxide

Carbon dioxide is a by-product of manure decomposition and livestock respiration. It is heavier than air and difficult to detect. It replaces oxygen in air and can act as an asphyxiant (cause suffocation). At moderate concentrations, it causes shortness of breath and dizziness.

It is a major contributing factor to animal deaths by suffocation when animals stay in buildings with faulty ventilation.


Methane is odorless and lighter than air, so it tends to accumulate at the top of covered manure pits. It is considered to be an asphyxiant at extremely high concentrations. Another key hazard of methane is its flammable, explosive nature.

Methane is extremely difficult to detect without gas detection instruments. Expect that methane may be present in all manure storage areas.

Characteristics of silo gases

Silo gas forms during the natural fermentation of chopped silage shortly after it goes into the silo. This process releases various gases.

The type of silo in which the forage is stored is important in determining which gas will be predominant. For instance, sealed silos create both nitrogen dioxide and carbon dioxide but produce carbon dioxide in far greater amounts. This is desirable because high carbon dioxide levels help to maintain high quality silage.

At the same time, this odorless and colorless gas is dangerous. This gas replaces the silo's oxygen and, in high concentrations, it gives a person little warning that he or she is about to be overcome. Because of this hazard, sealed silos are designed in such a way that entering them is unnecessary.

Nitrogen dioxide

Various gases also form in conventional or open-top silos but nitrogen dioxide is more abundant. This highly toxic gas has a strong bleach-like odour and low-lying yellow, red or dark brown fumes.

Unlike carbon dioxide, nitrogen dioxide levels reach a peak about three days after harvesting and rapidly begin to decrease thereafter, particularly in ventilated silos. After two weeks, it is unlikely to produce more gas, although some hazard remains if the gas has not been able to escape the silo.

Nitrogen dioxide is harmful because it causes severe irritation to the nose and throat and may lead to inflammation of the lungs. What makes this gas especially dangerous is that low-level exposure to it may cause only a little immediate pain or discomfort, yet death can occur immediately.

A person might breathe the gas without noticing any serious ill effects and then die in their sleep hours later from fluid collecting in the lungs.

In addition, many victims suffer relapses with symptoms similar to pneumonia two to six weeks after the initial exposure. For these reasons, it is extremely important for anyone exposed to this gas, even for a short time, to seek immediate medical attention.

Like carbon dioxide, nitrogen dioxide is heavier than air. As it forms, it tends to settle on top of the silage, or flow down the silo chute and collect in the adjoining feed rooms or other low-lying areas near the base of the silo.

Gas may even flow into the barn itself and become trapped in corners, under feed bunks or lie low against the floor. The threat that this poses to livestock is a serious one.