If you ask an old coal miner which gas he feared most, many will give the same answer: carbon monoxide. Methane gets the headlines because it explodes, but CO has quietly killed more miners than any other gas underground. Here is what every mining student and worker should know about it, in plain language.
Why is carbon monoxide called the "Killer Gas"?
Carbon monoxide (CO) found in mines is called the killer gas because it is so poisonous that it can literally murder a person. Once it gets into the blood, it destroys the blood's ability to do its job.
Good to know: In the old days it was also called White Damp. The name came from the fact that, unlike Black Damp, it does not make the air dark or stuffy, so there is nothing obvious to warn you.
Properties of carbon monoxide
Of all the poisonous gases found in mines, CO is the main life-taker. Here is what makes it what it is:
- It is a little lighter than air (relative density about 0.967).
- It has no colour, no smell and no taste, so you cannot detect it with your senses.
- It does not help other things burn, but it burns itself with a pale blue flame.
- It forms an explosive mixture with air when its concentration is between 12.5% and 74%. In practice, mines almost never reach concentrations that high.
- It is produced by spontaneous heating of coal and by coal fires.
- Large amounts (2% to 8%) are produced after a gas or coal dust explosion.
- It is present in blasting smoke and in the exhaust fumes of diesel locos.
- Compressors working underground can also produce it.
- More deaths in coal mines have been caused by this gas than by any other.
- After any explosion, whether methane or coal dust, you should expect CO to be present.
How does it affect the human body?
CO is a poisonous gas that spoils the blood. Normally, oxygen enters the lungs and attaches to haemoglobin in the blood, forming a temporary compound called oxyhaemoglobin. As the blood travels around the body, tissues pull oxygen out of it to stay alive.
Carbon monoxide ruins this process. It combines with haemoglobin about 300 times faster than oxygen does. Even a small amount reaching the lungs instantly forms carboxyhaemoglobin, and haemoglobin in that state cannot carry oxygen. With every breath, more and more haemoglobin gets locked up, and the body's tissues are starved of oxygen.
In the end, a person either dies from lack of oxygen or, if they survive, is left with permanently damaged lungs.
Effects at different concentrations
The more CO in the air, the less time you have:
| CO in air (%) | Time of exposure | Symptoms and effects |
|---|---|---|
| 0.02 | 2 hours | Slight headache |
| 0.04 | 45 minutes | Headache, vomiting, possible loss of consciousness |
| 0.10 | 30 minutes | Weakness, poor thinking, high chance of fainting |
| 0.20 | 10 minutes | Unconsciousness and death |
| 0.30 | 5 minutes | Unconsciousness and death |
| 1.00 | 1 minute | Breathing stops, death |
What is the safe limit?
A person is not harmed if the mine air contains up to 0.0016% CO. That is 16 parts in a million, or 16 ppm.
How is CO tested for in mines?
There are three ways to check for it:
- Using canary or finch birds
- Using a CO detector instrument
- Collecting an air sample and testing it chemically
1. Canary or finch birds
Warm-blooded animals such as canaries react to CO much sooner than humans do, so they give an early warning before people are in danger. Two or three birds are kept in a cage and carried to the place where gas is suspected. If CO is present, the birds become restless, open their beaks and flap their wings. If the gas is strong or they stay there long enough, they fall to the floor of the cage.
2. CO detector instruments
These instruments contain a chemical that changes colour when it meets the gas, and the colour tells you the percentage present. The main types are:
- Potassium palladium sulphide detector
- Hopcalite detector
- Multi-gas detector
3. Laboratory analysis
Air is collected from the suspected area and sent to a laboratory, where it is tested chemically to find out which gases are present and how much.
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