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Hazardous Gases: Carbon Monoxide

Table of Contents
What is Carbon Monoxide? Also known as the 'silent killer' it is highly toxic. It is present underground, at chemical facilities and can build up within a refuge chamber.
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What is Carbon Monoxide?

Chemical Compound: CO

Carbon monoxide (CO) is a colourless, odourless, and tasteless gas that is slightly less dense than air. It is highly toxic to haemoglobin animals (including humans) when encountered in concentrations above 35ppm.

In the atmosphere, CO is produced from incomplete combustion and is present in exhaust gasses of vehicles and other engines, and from the burning of most fuels. It is also produced in low quantities in normal animal / human metabolic processes and human blood at levels of between 0 and 3% for healthy adults. This level is higher in heavy smokers.

Safety Hazards of Carbon Monoxide


CO poisoning is the most common type of fatal air poisoning, often referred to as the ‘Silent Killer’. Once inhaled, CO combines with haemoglobin to produce carboxyhaemoglobin, which becomes ineffective at delivering oxygen around the body. Oxygen levels are displaced in the blood, causing vital organs to starve, therefore, causing people to suffocate and lose consciousness.

CO molecules will not detach from haemoglobin, leaving them bound to the protein for long periods, often making CO poisoning irreversible. Concentrations as low as 667ppm (0.667%) may cause up to 50% of the body’s haemoglobin to convert to carboxyhaemoglobin. A level of 50% carboxyhaemoglobin may result in seizure, coma, and even fatality.

The most common symptoms of CO poisoning may resemble other types of poisonings and infections, including symptoms such as headache, nausea, vomiting, dizziness, fatigue, and a feeling of weakness. Neurological signs include confusion, disorientation, visual disturbance, syncope (fainting), and seizures. Occupational exposure levels are limited to 50ppm of CO over the long term. Compounded effects of CO occur in an enclosed environment such as a refuge chamber and therefore, must be removed.

Exposure Standard Details

Standard NameCarbon Monoxide
Cas Number630-08-0
Time Weighted Average (parts per million)30
Time Weighted Average (mg/m3)34
Short Term Exposure Limit (parts per million)50


Carbon monoxide is extremely flammable. The vapour can readily form an explosive mixture with air at room temperature.

The minimum and maximum levels of carbon monoxide necessary to support its combustion in the air are listed below, with the concentrations in per cent by volume;

  • 12.5 Lower Explosive Limit (LEL)
  • 74.0 Upper Explosive Limit (UEL)

CO vs. CO2

Carbon Monoxide vs. Carbon Dioxide are often mistaken for one another. Both gases are odourless, colourless, target the cardiovascular system, and can enter the body through inhalation, skin or eyes. They also possess similar symptoms if affected, such as headaches, dizziness, seizures, and hallucination.

Below is a summary list of the differences between the two gases.

Carbon MonoxideCarbon Dioxide
Doesn’t occur naturally in the atmosphereOccurs in the natural atmosphere
Result of oxygen-starved combustion in improperly ventilated fuel-burned equipmentA natural by-product of human and animal
respiration, fermentation, chemical reactions, and combustion fossil fuels/woods
Generated by any gasoline engine WITHOUT a catalytic converterGenerated by any gasoline engine WITH a catalytic converter
A common type of fatal poisoningPoisoning is rare
Flammable gasNon-flammable gas
Symptoms: confusion, nausea, lassitude, syncope, cyanosis, chest pain, abdominal painSymptoms: dyspnoea, sweating, increased heart rate, frostbite, convulsion, panic, memory problems
Target organ: lungs, blood, central nervous systemTarget organ: respiratory system
Based on the Occupational Safety & Health Administration (OSHA) standards, the permissible exposure limit (PEL) is 50 parts per million (ppm)Based on the OSHA standards, the PEL is
5,000 ppm
Based on the National Institute for Occupational Safety and Health (NIOSH) standards, the recommended exposure limit (REL) is 35 ppmBased on the NIOSH standards, the REL is
5,000 ppm

Monitoring Carbon Monoxide Levels

Carbon Monoxide is detected using an electrochemical gas sensor that measures the concentration of CO by oxidising it at an electrode and then measuring the resulting current. To reduce cross-sensitivity to other combustible gases, it has chemical filters for H2S, N02, NO and SO2.

Measuring CO levels aids in the detection of fires and heating underground. Sensors will detect a steady increase in concentrations of CO, without reduction. Although shot firing fumes and diesel exhaust can trigger carbon monoxide alarms, the patterns of rapid increase and decrease are known.

Removing Carbon Monoxide in a Refuge Chamber

Chemical scrubbers, as the name suggests remove ‘scrub’ the air by using a chemical to remove harmful gases. Refuge chambers use dry chemical scrubbers to remove CO and CO2 from the internal environment, especially if compressed air fails or becomes compromised.

Providing breathable air within a refuge chamber is a challenge, along with disposing of the carbon dioxide and carbon monoxide we breathe. As occupants within the sealed environments breathe, CO2 and CO will build up. If these gases are not removed from the internal atmosphere they can build to toxic levels and poison the chambers inhabitants.

Numerous gases associated with mining, tunnelling and underground construction are generalised into combustible, toxic and asphyxiate types. Because of the hazardous nature of these gases and the unique and restrictive structure of underground environments, these gases must be continuously monitored to mitigate risk.

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