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.
"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.
"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.
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).
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.
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.
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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