CARBON MONOXIDE POISONING RISK FROM EQUIPMENT IN ENCLOSED CONSTRUCTION SPACES

← Back to Blog

Carbon monoxide (CO) is an invisible, odourless gas that kills construction workers every year in Ontario and across Canada. It is produced by the incomplete combustion of fossil fuels — gasoline, diesel, propane, and natural gas — and it accumulates quickly in enclosed or poorly ventilated spaces. What makes CO particularly dangerous is that you cannot see it, smell it, or taste it. By the time symptoms appear, you may already be too impaired to save yourself.

Winter construction amplifies the risk dramatically. Workers close up building openings to retain heat, run gas-powered generators and heaters inside partially enclosed structures, and seal off ventilation paths that would normally disperse exhaust. Every winter, Ontario emergency rooms treat construction workers for CO poisoning — and some cases end in death.

How Construction Equipment Produces CO

Any piece of equipment that burns fuel produces carbon monoxide as a byproduct. On construction sites, the primary sources include:

• Gas-powered generators — portable generators are the leading source of CO poisoning on construction sites. A single 5,500-watt generator can produce CO concentrations lethal within minutes in a poorly ventilated space. Generators are often brought indoors during winter to power tools, lighting, and temporary heating — a practice that kills workers every year.
• Concrete saws and cutters — gas-powered cut-off saws (commonly called quick-cut or partner saws) produce significant CO emissions. When used in basements, tunnels, stairwells, or partially enclosed buildings, CO can accumulate rapidly.
• Propane-fired heaters — forced-air propane heaters and radiant heaters are widely used for temporary heating during winter construction. While propane burns cleaner than gasoline, it still produces CO, especially when heaters are poorly maintained or operated in inadequately ventilated spaces.
• Gasoline-powered compressors and pumps — air compressors, water pumps, and power trowels with gasoline engines all produce CO exhaust. These are often used in basements or lower levels of buildings under construction.
• Vehicles and heavy equipment — diesel and gasoline engines on vehicles, forklifts, and heavy equipment produce CO. Idling vehicles inside or near building openings can push exhaust into enclosed work areas.

CO Symptoms at Different PPM Levels

Carbon monoxide exposure is measured in parts per million (ppm). Understanding the symptoms at different concentration levels can help workers recognize exposure early — before it becomes incapacitating or fatal.

• 35 ppm — Ontario's occupational exposure limit (OEL) for an 8-hour time-weighted average. At this level, most healthy adults will not experience symptoms, but prolonged exposure can cause headaches in sensitive individuals.
• 200 ppm — mild headache, fatigue, and dizziness after 2 to 3 hours of exposure. These symptoms are often mistaken for the flu, a hangover, or simple tiredness — which is why CO poisoning goes unrecognized so frequently.
• 400 ppm — frontal headache within 1 to 2 hours, life-threatening after 3 hours. Nausea, confusion, and impaired coordination begin to develop.
• 800 ppm — dizziness, nausea, and convulsions within 45 minutes. Loss of consciousness within 2 hours. Fatal within 2 to 3 hours.
• 1,600 ppm — headache, dizziness, and nausea within 20 minutes. Death within 1 hour.
• 6,400 ppm and above — headache and dizziness within 1 to 2 minutes. Loss of consciousness and death within 15 to 30 minutes.

A small gasoline generator running in a closed garage can produce CO levels above 1,000 ppm within minutes. On a construction site, a partially enclosed basement or a sealed-up floor can reach dangerous concentrations just as quickly.

Ventilation Requirements

The primary control for CO exposure is ventilation — ensuring that enough fresh air moves through the workspace to dilute and remove CO before it reaches hazardous concentrations.

• Never operate gas-powered equipment indoors — this is the most important rule. Generators, compressors, and concrete saws should be operated outdoors, away from building openings, windows, and air intakes. If a generator must be used to power indoor work, keep the generator outside and run extension cords in.
• Mechanical ventilation for propane heaters — when propane heaters are used inside a structure, mechanical ventilation must be provided to supply fresh air and exhaust contaminated air. The ventilation rate must be sufficient to maintain CO levels below the OEL of 35 ppm.
• Do not seal all openings — in winter, there is a strong temptation to close every window, door, and opening to retain heat. Resist this temptation in any space where fuel-burning equipment is operating. Maintain at least two openings on opposite sides of the space to allow cross-ventilation.
• Direct exhaust outdoors — if equipment must operate near an enclosed space, use flexible ducting to direct exhaust gases away from the work area and outside the building. Ensure the exhaust discharge point is located away from air intakes and building openings.

CO Detector Placement

CO detectors are a critical layer of protection — but only if they are properly selected, placed, and maintained.

• Use industrial-grade detectors — consumer-grade home CO detectors are not suitable for construction sites. They alarm at higher thresholds and respond more slowly than industrial units. Use detectors designed for occupational settings that alarm at 35 ppm (TWA) and 100 ppm (ceiling).
• Place at breathing zone height — CO mixes readily with air and distributes relatively evenly in a space, but detectors should be placed at breathing zone height (approximately 1.2 to 1.5 metres above floor level) near work areas where workers are present.
• Place near sources and in work areas — position detectors between the CO source (heater, equipment) and the workers. Also place detectors in areas where CO could accumulate, such as low-lying areas, dead-end corridors, and spaces with limited air movement.
• Calibrate regularly — industrial CO detectors require regular calibration (typically every 6 months or per manufacturer's recommendations) using calibration gas. An uncalibrated detector is unreliable and may fail to alarm when CO levels are dangerous.
• Personal CO monitors — clip-on personal gas monitors that workers wear on their collar or lapel provide individual-level protection. These monitors alarm when the worker's personal exposure exceeds safe limits, regardless of where they move within the site.

Winter Construction Risk Factors

Winter is the peak season for construction CO poisoning. Several factors combine to create elevated risk:

• Sealed buildings — plastic sheeting, temporary walls, and closed windows and doors reduce natural ventilation dramatically. Spaces that were well-ventilated in summer become traps in winter.
• Increased heater use — propane heaters running for extended periods produce a continuous supply of CO. As the shift progresses, CO levels can creep upward even if initial levels were safe.
• Generator placement — snow, ice, and cold weather sometimes lead workers to move generators closer to buildings or even inside to keep them running. This is the single most common factor in fatal construction CO incidents.
• Reduced awareness — early CO symptoms (headache, fatigue, dizziness) mimic the effects of cold weather, dehydration, and physical exertion. Workers may not recognize CO exposure until it becomes severe.

Emergency Response for CO Exposure

If a worker shows signs of CO exposure — or if a CO alarm sounds — time is critical. Every minute of continued exposure increases the severity of injury.

• Evacuate immediately — move all workers out of the affected area to fresh air. Do not re-enter the space until CO levels have been confirmed safe by monitoring.
• Shut down the source — if it can be done safely without re-entering the contaminated space, shut off the equipment producing CO.
• Call 911 — CO poisoning requires medical evaluation even if symptoms appear mild. CO binds to haemoglobin in the blood and can cause delayed neurological effects hours or days after exposure. Workers with significant exposure may require hyperbaric oxygen therapy.
• Administer oxygen — if supplemental oxygen is available on-site, administer 100 percent oxygen via a non-rebreather mask to the affected worker while waiting for paramedics.
• Do not allow affected workers to drive — CO impairs judgment, coordination, and reaction time. Workers who have been exposed should not drive themselves to the hospital.
• Investigate before re-entry — before anyone re-enters the affected space, identify the source of CO, ventilate the area thoroughly, and confirm with monitoring equipment that CO levels are below 35 ppm.

Carbon monoxide does not announce itself. It does not smell, and it does not look like anything. The only defences are proper ventilation, proper equipment placement, continuous monitoring, and worker education. Take CO seriously — it earns its reputation as the silent killer every single winter.