CONFINED SPACE HAZARDS: WHAT ONTARIO WORKERS NEED TO KNOW

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Confined spaces are among the most dangerous environments workers enter. They do not look threatening — a manhole, a storage tank, a utility vault, a sewer line — but the hazards inside are invisible, fast-acting, and frequently fatal. According to the Workplace Safety and Insurance Board (WSIB), confined space incidents in Ontario often result in multiple casualties because would-be rescuers rush in without proper equipment and become victims themselves.

In Ontario, confined space entry is governed by Ontario Regulation 632/05 (Confined Spaces) under the Occupational Health and Safety Act. For construction projects, additional requirements exist under Ontario Regulation 213/91. This article explains what qualifies as a confined space, what can kill you inside one, and what the law requires before anyone goes in.

What Is a Confined Space?

Under Ontario Reg. 632/05, a confined space is a fully or partially enclosed space that meets all three of the following criteria:

• Not designed or intended for human occupancy: It was built for a purpose other than having people work inside it — storing material, channelling water, housing equipment, containing a process.
• Has limited or restricted means of entry or exit: You cannot just walk in and out freely. Entry may require climbing through a hatch, crawling through an opening, using a ladder to descend, or removing a cover.
• May become hazardous to a worker entering it: Due to its design, construction, location, atmosphere, the materials or substances in it, or any other conditions. The key word is "may" — a space does not need to be actively hazardous right now to be classified as confined. If conditions could become hazardous, it qualifies.

Common confined spaces on construction sites include: manholes, catch basins, storm and sanitary sewers, water and fuel storage tanks, vaults, caissons, excavations deeper than 1.2 metres with limited access, pipe assemblies, boilers, and silos.

Atmospheric Hazards: The Invisible Killer

The number one cause of death in confined spaces is atmospheric hazards. You cannot see, smell, or taste most of these dangers. By the time you feel symptoms, you may already be unable to escape.

• Oxygen deficiency: Normal air contains 20.9% oxygen. Below 19.5%, it is considered oxygen-deficient. At 16%, you experience impaired judgment and rapid breathing. At 12%, you lose consciousness within minutes with no warning. At 6%, death occurs in minutes. Oxygen is displaced by other gases (methane, carbon dioxide, nitrogen) or consumed by rusting metal, decomposing organic material, or chemical reactions. A space can have normal oxygen levels one hour and lethal levels the next.
• Oxygen enrichment: Above 23% oxygen creates an extreme fire and explosion risk. Leaking oxygen lines, certain chemical reactions, or improperly stored compressed gas can enrich the atmosphere. Materials that would not normally ignite can catch fire or explode in an oxygen-enriched environment.
• Toxic gases: Hydrogen sulphide (H2S) is the most notorious — it smells like rotten eggs at low concentrations but destroys your sense of smell at higher levels, so you stop detecting it just as it becomes lethal. At 100 ppm, you lose your sense of smell. At 500-700 ppm, you collapse within minutes. Carbon monoxide (CO) from fuel-burning equipment, engines, or welding is odourless and colourless. Other toxic gases include nitrogen dioxide, sulphur dioxide, and ammonia, depending on the space.
• Flammable gases and vapours: Methane, propane, gasoline vapours, hydrogen, and other flammable substances can accumulate in confined spaces. If the concentration reaches the lower explosive limit (LEL) and there is an ignition source — a spark from a tool, static electricity, a hot surface — the result is a flash fire or explosion in an enclosed area with nowhere to go.

Other Confined Space Hazards

Atmosphere is not the only danger. Confined spaces present multiple hazards that can act simultaneously:

• Engulfment: Grain, sand, gravel, water, or other materials stored in or flowing through the space can shift suddenly, trapping and suffocating a worker. Engulfment in grain can be fatal in under five minutes.
• Physical hazards: Moving parts (augers, mixers, agitators), extreme temperatures, noise amplified by enclosed walls, poor lighting, slippery surfaces, and electrical equipment.
• Configuration hazards: Spaces that narrow, slope, or have converging walls can trap workers. Floors that slope toward a smaller cross-section can funnel a worker into a position where they become stuck.
• Biological hazards: Sewers and wastewater spaces may contain infectious agents. Decomposing organic material produces dangerous gases and harbours bacteria.

The atmosphere inside a confined space can change in seconds. A space that tested safe 20 minutes ago can become lethal if a chemical reaction occurs, a gas pocket is disturbed, or ventilation fails. Continuous atmospheric monitoring is not optional — it is the only thing standing between a routine entry and a fatality.

The Entry Permit System

Ontario Reg. 632/05, Section 5 requires that before any worker enters a confined space, the employer must develop a written entry permit for that specific space and entry. The permit system is the core of confined space safety — it forces a systematic assessment of hazards and controls before anyone goes in.

A confined space entry permit must include:

• Location and description of the confined space
• Hazards identified in the hazard assessment for that space
• Date and duration of the entry
• Names of every person entering the space
• Names of the attendant(s) and entry supervisor
• Atmospheric test results taken before entry and the schedule for ongoing monitoring
• Control measures in place (ventilation, lockout/tagout, PPE, communication procedures)
• Rescue procedures and equipment available
• Authorization signatures from the entry supervisor

The permit must be available at the entry point for the duration of the work. When the work is complete or conditions change, the permit is closed out. A new entry requires a new permit — you cannot reuse yesterday's permit.

Gas Detection

Atmospheric testing is mandatory before entry and must continue throughout the time workers are in the space. Ontario Reg. 632/05, Section 18 requires that the atmosphere be tested before any worker enters and that adequate testing continue during the entry.

• Four-gas monitors: The standard instrument for confined space entry tests for oxygen (O2), hydrogen sulphide (H2S), carbon monoxide (CO), and lower explosive limit (LEL/combustible gases) simultaneously. Every entrant should have a personal gas monitor, or at minimum the attendant must continuously monitor conditions.
• Test before you enter: Lower the monitor into the space on a line to test the atmosphere at multiple levels — top, middle, and bottom. Gases stratify by weight. H2S and propane are heavier than air and accumulate at the bottom. Methane is lighter and rises to the top. A reading at the opening tells you nothing about conditions at the floor.
• Bump test and calibration: Gas monitors must be bump-tested before each use (expose the sensors to a known concentration of test gas to confirm they respond) and calibrated on the schedule specified by the manufacturer — typically every 6 months at minimum.
• Alarm set points: Know what triggers your monitor's alarm. Standard alarm set points are: O2 below 19.5% or above 23%, H2S at 10 ppm (TWA) and 15 ppm (STEL), CO at 25 ppm (TWA) and 100 ppm (ceiling), and LEL at 10% (action level) and 25% (evacuate immediately).

Ventilation

Mechanical ventilation is required whenever atmospheric hazards are identified or reasonably anticipated. Ventilation serves two purposes: it supplies fresh air to the space and it dilutes or displaces hazardous gases.

• Continuous supply ventilation: The most common method — a blower forces fresh air into the space through flexible ductwork. The air supply must be from a clean source (not near vehicle exhaust or other contamination).
• Run it before entry: Ventilate the space for a sufficient period before anyone enters to establish safe atmospheric conditions. The required purge time depends on the size of the space and the volume of the blower.
• Keep it running: Ventilation must operate continuously while workers are inside. If the ventilation system fails, workers must evacuate immediately.
• Ventilation alone is not enough: Continuous atmospheric monitoring must accompany ventilation. Conditions can change — a pocket of gas can be released, contaminated groundwater can seep in, or a process upstream can introduce hazards.

Roles and Responsibilities

Ontario Reg. 632/05 defines three critical roles for every confined space entry. Each role has specific duties that must be understood and followed:

• Entrant: The worker who physically enters the confined space. Must be trained on the hazards of the specific space, the entry permit conditions, the communication procedures, and what to do if an emergency occurs. Must wear the required PPE and personal gas monitor. Must exit immediately if alarms sound, conditions change, or the attendant orders evacuation.
• Attendant: Remains outside the confined space at the entry point for the entire duration of the entry. The attendant monitors the entrants, maintains communication (visual, voice, radio, or hardwired), tracks who is in the space, monitors atmospheric conditions, and controls access. The attendant must never enter the space for rescue — their job is to summon the rescue team, maintain communication with the entrant, and manage the situation from outside.
• Entry supervisor: Authorizes the entry by signing the permit after verifying that all conditions are met — hazard assessment complete, controls in place, atmospheric testing done, rescue plan ready, personnel trained. The entry supervisor has the authority to cancel the entry or order evacuation at any time.

Rescue Plans

Ontario Reg. 632/05, Section 11 requires a written rescue plan before any confined space entry. This is not optional. The plan must be specific to the space and must be realistic — it must account for the actual conditions, geometry, and hazards of that particular entry.

• Non-entry rescue is preferred: The safest rescue involves pulling the worker out from outside the space using a retrieval system — a body harness with a retrieval line connected to a mechanical retrieval device (tripod and winch) positioned at the entry point. This must be set up before anyone enters.
• Entry rescue: If the space geometry makes non-entry rescue impossible, a trained rescue team with self-contained breathing apparatus (SCBA) and rescue equipment must be immediately available. "Immediately" means on site and ready to go — not 20 minutes away at a fire station.
• Never attempt rescue without proper equipment: This is the single most important rule in confined space safety. The instinct to rush in and help a fallen colleague is powerful, but the atmosphere that incapacitated the first worker will incapacitate you too. More than 60% of confined space fatalities in North America are would-be rescuers.
• Practice the plan: The rescue plan must be practiced before it is needed. Drill the procedure. Test the equipment. Make sure the retrieval system actually works with the entry point and the space geometry.

Real Incidents in Ontario

Confined space tragedies continue to happen in Ontario despite clear regulations. In 2018, a worker died after entering a manhole in Southwestern Ontario without atmospheric testing — H2S levels inside were lethal. In a separate incident, two workers were killed in a sewage pumping station when one entered without a gas monitor and collapsed, and the second rushed in to help and was overcome by the same atmosphere. These are preventable deaths caused by skipping the procedures that the regulation exists to enforce.

Confined space work is high-risk, but the regulation gives you a clear system to manage that risk. Follow it. Every permit, every gas test, every attendant standing watch at the opening — these are the things that bring you home at the end of the day.