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Hazardous Gases: Carbon Dioxide

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What is Carbon Dioxide?

Carbon dioxide (CO2) is a colourless and odourless gas, existing in Earth’s atmosphere as a trace gas at a concentration of about 400ppm (0.04%) by volume. Natural sources of CO2 include volcanoes and hot springs, and being soluble in water, it occurs naturally in groundwater, rivers, lakes, ice caps, glaciers, and seawater. It is also present in deposits of petroleum and natural gas.

Carbon dioxide is produced by all aerobic organisms when they metabolise carbohydrate and lipids to produce energy by respiration — expelled to the air via the lungs of air-breathing land animals, including humans. Carbon dioxide is also produced during the processes of decay of organic materials and by the combustion of wood, carbohydrates and fossil fuels such as coal, peat, petroleum and natural gas.

Safety Hazards of Carbon Dioxide


Carbon dioxide is not classified as toxic or harmful. However, it is an asphyxiate gas which can reduce or displace the average oxygen concentration in breathing air. The breathing of oxygen-depleted air can lead to death by asphyxiation (suffocation).

In concentrations above 1%, it can cause drowsiness and slight shortness of breath in some people. Levels between 7 and 10% can cause suffocation, even in the presence of sufficient oxygen, exhibited as dizziness, headache, visual and hearing dysfunction and unconsciousness within a few minutes to an hour. The physiological effects of acute carbon dioxide exposure are grouped under the term hypercapnia, a subset of asphyxiation. Occupational exposure levels are set at 0.5% for eight hours.

Humans exhale CO2 at a rate of approximately 30 Litres per hour. A group of people in a confined, sealed space like a refuge chamber will quickly contaminate the breathable air with CO2 in far less time than it would take to deplete the oxygen.

Exposure Standard Details

Standard NameCarbon Dioxide
Cas Number124-38-9
Time Weighted Average (parts per million)5000
Time Weighted Average (mg/m3)9000
Short Term Exposure Limit (parts per million)30000
Short Term Exposure Limit (mg/m3)54000


As the gas is fully oxidised, carbon dioxide is non-flammable and does not support combustion. It is often used within some fire extinguishers as a suppressant.

Nevertheless, electrical discharges and extremely high temperatures may decay carbon dioxide into toxic carbon monoxide and flammable oxygen.

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 Dioxide Levels

Carbon Dioxide can be measured using a non-dispersive infrared (NDIR) sensor. Measurement is based on the physical property that CO2 molecules absorb infrared light or particular wavelengths. By shining light through the target gas and using suitable optical filters, the light detector will give an output that can be converted into a CO2 concentration value.

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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