While the provided documents don't explicitly list specific Permissible Exposure Limits (PELs) for various substances within a confined space, they do emphasize the importance of atmospheric testing and maintaining a safe atmosphere 1, 2, 4, 17. Here's a breakdown of key requirements:

• Atmospheric Testing: Before entry, the atmosphere in a confined space must be tested by a competent person to verify oxygen levels (19.5% - 23.0%) and identify any toxic, flammable, or explosive substances [1]. Continuous monitoring may be required if conditions could change unpredictably 1, 17.
• Ventilation: If a hazardous atmosphere is present or likely to develop, the confined space must be ventilated or purged before entry 1, 4. Continuous ventilation is needed to maintain a safe atmosphere during occupancy [2].
• PEL Exceedance: If airborne concentrations of substances meet or exceed permissible contamination limits, respiratory protective devices that meet specific requirements must be used [2].
• Action Levels: Concentrations of chemical substances should not exceed their occupational exposure limit under Part 2: Occupational Health, of the Workplace Health and Safety Regulations or 50% of its lower explosive limit [3].
• Hazardous Atmosphere Definition: A hazardous atmosphere can include flammable gases exceeding 10% of their lower flammable limit, combustible dust at or exceeding its lower explosive limit, oxygen levels outside 19.5%-23.5%, or any substance exceeding its dose or exposure limit 6, 8, 13.

It's crucial to consult specific safety regulations and guidelines to determine the exact PELs for particular substances relevant to your confined space. If a safe atmosphere cannot be maintained through ventilation, work should only proceed with respiratory protection and continuous atmospheric monitoring [2].