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Working within safe limits in a confined space


According to Statistics Canada, there were 4,990 deaths associated with carbon monoxide (CO) poisoning between 2000 and 2013. Although this number includes exposure incidents in residences, many CO-exposure cases occur in workplaces.

Carbon monoxide is one of the most widespread and dangerous industrial hazards, WorkSafeBC notes. Potentially lethal at concentrations as low as 1000 parts per million (ppm), it is also the most common cause of occupational gas poisoning leading to death. As there is no way to detect CO by odour, colour or irritation, special detection methods like the following must be used:

  • Gas detector tubes: These clear tubes are about the size and shape of a ballpoint pen and can be read much like a thermometer. The tube contains a material that may change colour when it reacts with air drawn through it by a small hand pump. The amount of colour change depends on the CO level.
  • Electronic detectors: These range from small personal samplers to large, stationary monitors with a display screen that shows the gas level. When CO levels exceed a set limit, these detectors sound an alarm, flash a light or vibrate.
  • Blood samples: If there is particular concern about the effects of CO in a workplace, blood samples can be taken from workers to determine the amount of Carboxyhemoglobin, a stable complex of carbon monoxide that forms in red blood cells when carbon monoxide is inhaled.

For those who work in confined spaces, using instrumentation devices to conduct atmospheric testing is a must. The employer must appoint a qualified person with adequate knowledge, training and experience to perform adequate tests. Testing is required as often as necessary before and while a worker is in a confined space to ensure that acceptable atmospheric levels are maintained, according to Ontario’s Ministry of Labour.

When conducting representative sampling, the following needs to be taken into consideration:

  • The presence of stratified atmospheres and pockets of contaminated air within the confined space;
  • The selection of testing equipment will depend on the circumstances and the nature of work in the confined space and knowledge of possible atmospheric hazards;
  • Whenever practical, continuous monitoring should be considered; and
  • Equipment performance characteristics to be considered include the principle of detection of the hazards of concern, specificity, interferences, detection concentration range, response time, calibration requirements, and intrinsically safe equipment for spaces with potential accumulation of flammable hazards.

To ensure that a gas-detection device is functioning as it should, it is important to maintain and calibrate an instrument so that it can detect the presence of gases accurately. Full calibration involves adjusting the instrument’s reading to match the actual concentration of a certified standard test gas, while a “bump test” verifies that the instrument is working properly by exposing it to a known concentration of gas, WorkSafeBC notes.

With regards to alarm settings, a qualified person should set alarm limits based on the hazard assessment of the confined space, the length of time that workers will be in the confined space, work activities taking place within the confined space and the exposure limits for the contaminants of concern.

Jean Lian is editor of OHS Canada.