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Section 7: Hazardous atmospheres and confined spaces : Occupational Health and Safety Guidelines for Farming Operations in Ontario


The Occupational Health and Safety Guidelines for Farming Operations in Ontario were developed to highlight specific, and sometimes unique and unusual hazards on farms. They were jointly prepared by representatives of the farming community, the Farm Safety Association (now Workplace Safety and Prevention Services), the Ministry of Agriculture, Food and Rural Affairs and the Ministry of Labour.

The purpose of the guidelines is to help employers, supervisors and workers on farms recognize hazards and determine the ways they may best comply with their obligations under the Occupational Health and Safety Act (OHSA), and the relevant regulations. The guidelines provide general information to those in the workplace to help them identify specific hazards and dangerous situations. The guidelines may also provide the workplace parties with suggestions to consider in determining how to protect worker health and safety and to prevent injuries.

It is important to understand that the guidelines do not replace the laws that are in place. Employers, supervisors and workers on farms have responsibilities and rights under the Occupational Health and Safety Act and the following three regulations under the Act: Regulation for Farming Operations, O. Reg. 414/05, Critical Injury Defined, R.R.O. 1990, Reg. 834 and Occupational Health and Safety Awareness and Training, O. Reg. 297/13. The requirements in the OHSA and these three regulations must be complied with.

Employers have a legal obligation to take every precaution reasonable in the circumstances for the protection of workers; and, supervisors and workers must take appropriate steps to identify and address all workplace hazards. The guidelines are a starting point for the workplace parties to think about how to fulfill their obligations under the OHSA. Following the recommendations suggested in these guidelines does not relieve the workplace parties of their obligations to comply with the OHSA.

This is the first edition of the guidelines. They will be reviewed and updated on an ongoing basis, as needed, and expanded as new production methods and technologies emerge.

There are many examples of potentially dangerous confined spaces on a farm, including silos, grain bins, manure pits and spreaders, mixing or holding tanks, cisterns, valve pits and pump houses. Farmers and workers who enter a confined space, and those who attempt a rescue in an emergency, could quickly be overcome by a hazardous atmosphere, resulting in injury or death. For example, even a few seconds of oxygen depletion can impair brain cell function. This in turn may result in confusion and poor judgement and may compromise a worker's ability to exit the space.

This section covers the following topics:


Hazardous Atmosphere:
An atmosphere is hazardous when:
  • it has too much or too little oxygen; or,
  • it contains flammable, combustible or explosive agents; or,
  • it contains contaminants (for example, fumes, dusts, mists) that could pose an immediate threat to life or interfere with a person's ability to escape unaided from a confined space.
Confined Space:
A fully or partially enclosed space that is not designed and not built for continuous human occupancy and, which may contain a hazardous atmosphere because of its construction, location, contents, or because of work that is done in it. Entry to, and exit from a confined space may be difficult or restricted. A confined space may be part of a structure or may be mobile or portable such as a manure spreader tank.

General responsibilities

  1. The employer shall provide information and instruction to workers on potentially hazardous confined spaces and work areas that may contain hazardous gases, and the appropriate procedures when working in these areas.
  2. For each area where a hazardous atmosphere or confined space may exist, the employer should develop a written plan to protect workers and shall communicate the plan to workers.
  3. The supervisor shall ensure that a worker uses or wears any personal protective equipment that the employer requires to be worn; and, that workers follow any written plans and procedures developed by the employer.
  4. Workers should not enter confined spaces or hazardous atmospheres when there are no written plans or procedures for working in these areas.
  5. A worker should not enter or remain in a building or structure adjoining a liquid manure tank while the manure is being agitated. Following agitation of the manure, any adjoining building or structure should be thoroughly ventilated before a worker is permitted to re-enter it.
  6. A worker should not enter a grain bin or silo where grain is stored while the grain is being unloaded. The greatest danger of death in this situation is from becoming trapped in flowing grain. By locking out the unloader before entering a grain bin or silo with grain, the hazard of entrapment is greatly reduced.
  7. When a worker is required to enter a controlled atmosphere storage warehouse that cannot be purged, the worker must be informed of the risk of entering such a facility without respiratory protection and be properly equipped with a breathing apparatus that allows safe entry.

Factors to consider when working around manure storage and silos

Manure Storage Entry Procedures

  1. Avoid entering manure storage areas if at all possible. Many deaths have occurred when people entered 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 a foot of liquid in it.
  2. If you must enter a manure storage area, wear a self-contained air supply like those fire fighters use. (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.)
  3. Never enter a manure pit during or just after agitation because there is always the possibility of deadly concentration of gases. Plumbing and pumping equipment should be installed so that it can be easily removed for repairs.
  4. Before agitation, workers and animals should be a safe distance away.
  5. Remove all people and animals if possible. If animals cannot be removed, 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.
  6. 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

  1. Be alert for silo gas odours and/or yellowish-brown or reddish fumes in or near the silo. Silo gas is heavier than air and will displace oxygen.
  2. The greatest danger from nitrogen dioxide gas from silage is during the first 12 to 60 hours after filling. However, take care to avoid possible exposure for 10 days after filling the silo and when opening the silo for feeding.
  3. If gases are detected, do not enter the silo for up to 6 weeks after filling stops.
  4. Workers should 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. Gases can form in the silo for up to 6 weeks after filling.
  5. If a worker has to enter a silo, there should be another person/attendant outside to help if needed. Keep a hatch door open near the level of the silage within the silo.
  6. Post all appropriate warning signs. Oxygen-limiting silos require a sign that warns people of the absence of oxygen. People need to be told to stay away from these areas and to only enter them with appropriate training and personal protective equipment.


1. Characteristics of Manure Gases

The four main gases produced from decomposing manure are hydrogen sulfide, methane, ammonia and 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. When pits are agitated for pumping, some or all of these gases are rapidly released from the manure and may reach toxic levels or displace oxygen, increasing the risk to humans and livestock.

The primary hazards of these gases are toxic or poisonous reactions in people or animals, oxygen depletion that can result in asphyxiation, and explosions that can occur when oxygen mixes with gases such as methane.

Hydrogen Sulfide

Hydrogen sulfide is considered the most dangerous of the by-products of manure decomposition. It has a distinct rotten egg smell.

At low concentrations, hydrogen sulfide irritates the eyes and respiratory tract while 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 asphyxiate. At moderate concentrations, it causes shortness of breath and dizziness.

It is a major contributing factor to animal deaths by asphyxiation when animals are housed 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 asphyxiate at extremely high concentrations. Another key hazard associated with methane is its flammable, explosive nature. Methane is extremely difficult to detect without gas detection instruments but it should be anticipated as being present in all manure storage areas.

2. Characteristics of Silo Gases

Silo gas is formed by the natural fermentation of chopped silage shortly after it is placed in the silo.

Though a variety of gases are released during this process, the type of silo in which the forage is stored is important in determining which gas will be predominant. For instance, in sealed silos both nitrogen dioxide and carbon dioxide are created but carbon dioxide is produced in far greater amounts. This is desirable because high carbon dioxide levels help to maintain high quality silage.

At the same time, however, 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

A variety of gases are also formed in conventional or open-top silos but nitrogen dioxide is found more abundantly. This highly toxic gas is characterized by 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 if the silo is ventilated. After two weeks, it is unlikely that more gas will be produced, 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 is accompanied by only a little immediate pain or discomfort, yet death can occur immediately.

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

Also, 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 who is 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 is produced, it tends to settle right 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.

For more information about the health effects of manure and silo gases, visit the Canadian Centre for Occupational Health and Safety (CCOHS) web site at

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