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13. Ventilation and Purging | Confined Spaces Guideline

Disclaimer: This resource has been prepared to help the workplace parties understand some of their obligations under the Occupational Health and Safety Act (OHSA) and regulations. It is not legal advice. It is not intended to replace the OHSA or the regulations. FOR FURTHER INFORMATION PLEASE SEE FULL DISCLAIMER

Section 20 - Confined Spaces Regulation (O. Reg. 632/05)

  • 20 (1)  This section applies only in respect of atmospheric hazards described in clause (b) or (c) of the definition of “atmospheric hazards” in section 1.
  • (2)  If atmospheric hazards exist or are likely to exist in a confined space, the confined space shall be purged, ventilated or both, before any worker enters it, to ensure that acceptable atmospheric levels are maintained in the confined space while any worker is inside.
  • (3)  If mechanical ventilation is required to maintain acceptable atmospheric levels, an adequate warning system and exit procedure shall also be provided to ensure that workers have adequate warning of ventilation failure and are able to exit the confined space safely.
  • (4)  If compliance with subsection (2) is not practical in the circumstances for technical reasons,
    • (a) compliance with subsection (3) is not required; and
    • (b)  a worker entering the confined space shall use,
      • (i)  adequate respiratory protective equipment,
      • (ii)  adequate equipment to allow persons outside the confined space to locate and rescue the worker if necessary, and
      • (iii)  such other equipment as is necessary to ensure the worker’s safety.
  • (5)  The equipment mentioned in subclauses (4) (b) (i), (ii) and (iii) shall be inspected by a person with adequate knowledge, training and experience, appointed by the employer, and shall be in good working order before the worker enters the confined space.

What is the difference between "purging" and "ventilating"?

"Purging" involves removing contaminants inside the confined space by displacement with air to achieve acceptable atmospheric levels. For example, if a confined space originally contained a toxic gas, air would be blown into the space to reduce the concentration of the toxic gas to below the appropriate atmospheric exposure level.

After the contaminants have been removed ("purged"), the confined space may be ventilated.

"Ventilation" means the continuous provision of fresh air into the confined space by mechanical means to maintain acceptable atmospheric levels. It must be continued while work is being carried out within the space, to maintain an acceptable oxygen concentration, to provide protection in case of accidental release of chemicals, to remove contaminants generated by the work performed, or to cool the enclosure.

Ventilation involves displacing air and diluting it through the introduction of fresh air (forced-air) or the continuous removal of contaminants by local exhaust ventilation for point sources. To ensure adequate ventilation, the points of air supply and exhaust should be separated as far as possible. Openings must be provided for the entry of clean replacement air or to allow the exhaust of air. Pure oxygen must not be used to ventilate a confined space.

What is "inerting"?

"Inerting" is a special form of purging and ventilating. Inerting involves purging oxygen from a confined space using an inert gas (such as nitrogen, carbon dioxide or argon) to remove the hazard of fire or explosion. The concentration of oxygen is decreased to below the level that can support combustion. Following the purging operation the oxygen concentration is continuously monitored and the confined space may be ventilated using the inert gas to ensure that the concentration of oxygen does not increase. The inert gases will create an unsafe atmosphere (oxygen deficiency) and therefore workers entering the confined space should use appropriate supplied air-respirators.

What is an “adequate warning system” to signal failure of mechanical ventilation?

The warning system must be adequate, which is defined in the Confined Spaces Regulation as “sufficient for both its intended and its actual use, and sufficient to protect a worker from occupational illness or occupational injury”. The warning system may signal ventilation failure through audible or visual means, where a warning system is required by the Regulation. The type used must be outlined in the plan and it could be as simple as constant visual observation of flow by the attendant.

If electronic or electrical warning systems for ventilation failure are used, they should be activated by the loss of airflow and be located in the air stream. This is so that the alarm would be activated both when there is ventilation failure due to motor failure, and when there is ventilation failure without motor failure (for example, if the fan belt fails or if the airflow is somehow blocked).

What is an example of "an adequate warning system" to indicate ventilation failure?

A warning system could be an audible or visual alarm, or both, that indicates that the ventilation has failed. The alarm should be activated by a flow or pressure switch in the air stream rather than by electrical failure or other motive power failure. A pressure or flow switch would ensure that if the fan belt fails, for example, or the airflow is somehow blocked, the alarm is activated.

Acceptable Atmospheric Levels

Section 1: Definitions - Confined Spaces Regulation (O. Reg. 632/05)

  • 1. In this Regulation,
    “acceptable atmospheric levels” means that,
    • (a)  the atmospheric concentration of any explosive or flammable gas or vapour is less than,
      • (i)   25 per cent of its lower explosive limit, if paragraph 1 of subsection 19(4) applies,
      • (ii)  10 per cent of its lower explosive limit, if paragraph 2 of subsection 19(4) applies,
      • (iii)  5 per cent of its lower explosive limit, if paragraph 3 of subsection 19(4) applies,
    • (b)  the oxygen content of the atmosphere is at least 19.5 per cent but not more than 23 per cent by volume, and
    • (c)  in the case of a workplace that is not a project, the exposure to atmospheric contaminants does not exceed any applicable level set out in Regulation 833 of the Revised Regulations of Ontario, 1990 (Control of Exposure to Biological or Chemical Agents) made under the Act or Ontario Regulation 490/09 (Designated Substances) made under the Act, and
    • (d)  in the case of a workplace that is a project, if atmospheric contaminants, including gases, vapours, fumes, dusts or mists, are present, their concentrations do not exceed what is reasonable in the circumstances for the protection of the health and safety of workers;

Why is the definition of “acceptable atmospheric levels” for construction projects worded differently from the other workplaces?

Different requirements may apply on a construction project due to the nature of the work. Construction projects are exempt from the Regulation for Control of Exposure to Biological or Chemical Agents (Reg. 833) and the Designated Substances Regulation (O. Reg.490/09). Clause (d) of the definition of “acceptable atmospheric levels” in the Confined Spaces Regulation applies to construction projects and exposures to atmospheric contaminants must not exceed what is reasonable in the circumstances for the protection of the health and safety of workers.

How are acceptable atmospheric levels related to atmospheric hazards?

Atmospheric hazards are considered to determine if a space is a confined space. Acceptable atmospheric levels are those levels that must be maintained when a worker is in a confined space in order to prevent occupational illness or injury.

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