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Entering an unventilated and untested confined space may pose a hazard that is immediately dangerous to life or health.
Specific requirements for confined space entry exist that depend upon the hazard level of the atmosphere in the confined space. These requirements can be found at Regulation and Guidelines for Confined Spaces.
The Occupational Health and Safety Regulation defines the hazard levels of atmospheres in a confined space as follows:
"High Hazard Atmosphere" means an atmosphere that may expose a worker to risk of death, incapacitation, injury, acute illness or otherwise impair the ability of the worker to escape unaided from a confined space, in the event of a failure of the ventilation system or respirator.
"Moderate Hazard Atmosphere" means an atmosphere that is not clean respirable air but is not likely to impair the ability of the worker to escape unaided from a confined space, in the event of a failure of the ventilation system or respirator.
"Low Hazard Atmosphere" means an atmosphere which is shown by pre-entry testing or otherwise known to contain clean respirable air immediately prior to entry to a confined space and which is not likely to change during the work activity, as determined by a qualified person after consideration of the design, construction and use of the confined space, the work activities to be performed, and all engineering controls required by this Regulation.