Carbon Monoxide Poisoning in Enclosed and Semi-Enclosed Worksites

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Construction Safety Association of Ontario

Summary Statement

Carbon monoxide given off by fuel powered equipment can be deadly; enclosed spaces with such equipment in use must be vented to the outside.

Hazard Summary:

A worker at a large, enclosed construction site died of carbon monoxide poisoning after he and six other workers were exposed to high levels of the gas. Ventilation on the site was inadequate, and three machines were giving off carbon monoxide: a portable mixer and a trowel, both powered by gasoline, and a forklift powered by propane.

Description and Health Effects

Carbon monoxide is often called "the silent killer ,"because it gives no clear warning to its victims. It is an invisible gas with no taste or smell and it will not cause any unusual feeling in the nose, mouth or throat as it is breathed in. The first effects that can actually be noticed are headache and faintness. A worker who does not know about carbon monoxide poisoning may ignore these early symptoms or think a minor illness is coming on. But continuing exposure can cause confusion, loss of consciousness, and even death.

Unless workers know about the dangers of carbon monoxide, they may never even think about moving away from air that is poisoning them.

Sources and Patterns of Spread

All fuel-burning equipment emits some carbon monoxide. This includes car, truck and forklift engines, construction equipment powered by propane or gasoline, and non-electric heaters. Poorly maintained equipment usually emits more carbon monoxide: a poorly tuned gas engine may give off up to 12 times as much carbon monoxide as a well tuned one.

Carbon monoxide tends to build up in pockets in the poorly ventilated areas of workplaces. This can happen even in semi-enclosed workplaces,where tarpaulins and plastic sheeting are used for shelter.

Recommended Precautions:

  • All areas where fuel-burning equipment is being used must be ventilated by mechanical means to the outside.
  • Tune and maintain engines and other equipment regularly.
  • Where practical, install air-monitoring devices. The alarms should be set below 35 ppm (see 'Measuring Exposure' below).
  • Employers and workers should be made aware of the danger of carbon monoxide poisoning from fuel-burning equipment. They should know the warning signs of carbon monoxide exposure— headache, faintness, dizziness, confusion, nausea, and irregular heartbeat—and should NEVER ignore them when working where fuel-burning equipment is being used.
  • In emergency situations, use supplied-air or self-contained respirators(SCBA) to enter areas where carbon monoxide may be present.
  • Workers who have been exposed to carbon monoxide need immediate medical attention.

Measuring Exposure:


  • detector tubes (e.g., Drager, Gastec);
  • dosimeter tubes, badges, and direct-reading meters.

Time-Weighted Average Exposure Value (TWAEV):

  • 35 PPM (i.e. maximum allowable long-term exposure in parts per million)

Short-term Exposure Value (STEV):

  • 400 PPM maximum allowable short-term exposure)

Immediately Dangerous To Life and Health (IDLH):

  • 1500 PPM