If you are preparing for a coal mining exam or just starting out underground, there is one topic you simply cannot skip: the air we breathe, and how it changes once it goes down a mine. This post covers Normal Air and Mine Air in plain language, in the form of the twelve questions that come up again and again.
Normal Air and Mine Air
1. What are the main components of air?
Dry air is a mixture of a few gases:
| Gas | Percentage |
|---|---|
| Oxygen (O₂) | 20.93% |
| Nitrogen (N₂) | 79.04% |
| Carbon dioxide (CO₂) | 0.03% |
| Total | 100% |
For everyday purposes, just remember 21% oxygen and 79% nitrogen. In other words, roughly one-fifth of the air is oxygen.
The makeup of air is almost the same everywhere on Earth. These figures are by volume. By weight the proportions would differ, but you do not need that for gas testing.
Here is how air at the surface compares with air inside a mine:
| Gas | Surface (%) | Mine (%) |
|---|---|---|
| Oxygen (O₂) | 20.93 | 20.28 |
| Nitrogen (N₂) | 78.10 | 78.90 |
| Argon and other inert gases | 0.94 | |
| Carbon dioxide (CO₂) | 0.03 | 0.36 |
| Methane (CH₄) | – | 0.46 |
On the surface, nitrogen and argon together make up 79.04%.
2. What are the main properties of oxygen, nitrogen and carbon dioxide?
Oxygen. Every living thing needs it to breathe, which is why it is also called the "gas of life". It has no colour, smell or taste, and it is about 1.1 times heavier than air. Nothing can burn without it. It is not explosive, but it reacts easily with many substances and gives off heat when it does. That reaction is called oxidation. A small amount dissolves in water, about 3 parts in 100, and that is exactly why fish can live there.
Nitrogen. An inactive gas, also colourless, odourless and tasteless. It is slightly lighter than air (relative density 0.97). It does not burn, explode or poison anyone, and it has no effect on health. It goes in with each breath and comes out unchanged. It dissolves in water only very slightly, about 2 parts in 100.
Carbon dioxide. Its key trait is that it puts out flames. It is about 1.5 times heavier than air (relative density 1.53), colourless and odourless, with a faintly sour taste. It does not form an explosive mixture with air. It dissolves very readily in water, about 100 parts in 100 parts of water.
3. Why do we need oxygen?
Living beings need oxygen every single moment, and nobody survives without it. Wherever people are, they must get a full supply. Fire needs it too, because nothing burns without oxygen.
4. How does the oxygen we breathe get used?
When you breathe in, oxygen passes from the air in your lungs and joins the haemoglobin in red blood cells. Together they form a temporary compound called oxyhaemoglobin. As blood travels around the body, the oxygen is used to repair body tissue, and carbon dioxide is produced as a result.
The air you breathe out holds about 4% carbon dioxide, 16% oxygen and 80% nitrogen. When the air you breathe is short of oxygen, you begin to feel suffocated.
5. What happens when oxygen in the air drops?
- About 21%: normal air.
- Below 19%: health begins to suffer.
- Around 17%: breathing speeds up and you feel tired.
- Lower than that: the ability to think and judge clearly falls.
- Around 2%: a person can black out and die within a minute.
6. How much oxygen should a mine have?
Wherever people work or travel in a mine, oxygen must not fall below 19%. This is a legal requirement under CMR 1957, Regulation 130.
7. How is a shortage of oxygen detected in a mine?
With a safety lamp. Oxygen meters (oximeters) exist today, but the oil safety lamp is still an important tool underground. For every 1% drop in oxygen, the flame loses about 25 to 30% of its brightness, and below 17% oxygen the lamp goes out. An air sample can also be taken and chemically analysed in a laboratory.
8. Why is it dangerous to go where the lamp will not burn?
A lamp that goes out tells you oxygen is below 17%, but not how far below. If it is under 2%, a person can die within a minute without any warning signs. That is why such places must be treated as deadly.
9. What does extra carbon dioxide do to a person?
| CO₂ in air | Effect |
|---|---|
| 0.5% | Breathing becomes a little deeper |
| 3% | Breathing rate doubles |
| 10% | Suffocation and unconsciousness within half an hour |
| 20% | Death within half an hour |
10. What is the legal limit for CO₂ in mine air?
In any part of the mine where people work or pass through, carbon dioxide must not exceed 0.5% (half a percent), as per CMR 1957, Regulation 130. If it stays within this, ventilation is considered satisfactory.
11. What happens if you breathe pure oxygen?
Breathing pure oxygen for up to six hours does no harm. Doctors give it to patients under their care. Even so, the human body is built for ordinary air, and that is what we should breathe.
12. How does mine air become contaminated, and what is done about it?
Clean air starts changing the moment it enters a mine. Breathing, burning lamps, blasting and the oxidation of coal all use up oxygen and add carbon dioxide.
Other things get mixed in as well: methane released from coal and strata, poisonous fumes from blasting explosives, and gases produced when coal oxidises or when timber and similar material rots.
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