Methane and carbon monoxide get most of the attention, but a mining exam will happily ask you about the less famous gases too: the one that smells like rotten eggs, the brown fumes after blasting, the gas left behind after an explosion. They also matter on the job, because every one of them has been responsible for injuries or deaths underground.
This guide covers six of them in a question-and-answer format that is easy to revise: stinkdamp (hydrogen sulphide), nitrous fumes, gas from mine fires, afterdamp, sulphur dioxide and hydrogen. For each you will see why it has its name, where it comes from, its properties, its effect on the body and how it is detected. There is a quick comparison table and a short practice quiz at the end.
Note on numbers: the concentration limits and relative densities below follow the figures commonly given in Indian coal mining textbooks. Official limits differ between regulations and countries and are revised over time, so for compliance always check the current rules and your mine's own standards.
Quick comparison
| Gas | Formula | Key features | Relative density (air = 1) | Main source in mines |
|---|---|---|---|---|
| Stinkdamp | H2S | Rotten egg smell, very poisonous, soluble in water, flammable | About 1.18 | Old workings and water, spontaneous heating, imperfect blasting |
| Nitrous fumes | Nitrogen oxides (mainly NO2) | Reddish-brown, poisonous, dissolves in water, delayed effects | About 1.59 | Blasting, especially in stone drifts and sinking shafts |
| Gas from mine fire (fire stink) | A mixture | Smell like a brick kiln, poisonous mixture | Varies | Fires and spontaneous heating, found on the return side |
| Afterdamp | A mixture | Left after a methane or coal dust explosion, contains CO and lacks oxygen | Varies | Gas and coal dust explosions |
| Sulphur dioxide | SO2 | Colourless, pungent, very poisonous, very soluble in water | About 2.2 | Blasting, fires, explosions, sulphide ore mines |
| Hydrogen | H2 | Colourless, odourless, lightest gas, flammable | About 0.069 | Fires, explosions, battery charging |
1. Stinkdamp (hydrogen sulphide, H2S)
Why is it called stinkdamp?
Miners named it for its smell. Hydrogen sulphide has the unmistakable odour of rotten eggs (some describe it as rotten onions), which is why it is also called stinkdamp or "sada gas".
How is it formed?
- Rotting of timber and organic matter in water
- Spontaneous heating of coal, particularly where the coal has a high sulphur content
- Imperfect blasting of explosives
Properties
- Colourless gas with a rotten egg smell
- Slightly heavier than air (relative density about 1.18 in the textbook figures)
- Dissolves easily in water, so it collects in old water-filled workings and old sumps. When the water is disturbed, for example during dewatering, it can escape suddenly
- Highly poisonous, in the same league as carbon monoxide
- Flammable. In air it forms an explosive mixture between roughly 4.3 and 45 percent, though such concentrations are never reached in a mine
Where can it be found?
In old sumps, old water-logged workings and old parts of a mine where water has stood for a long time, and it can also come out of a goaf that is on fire.
Effect on the body
Headache comes first, followed by dizziness, tiredness, chest pain and bronchitis (inflammation of the airways). Unconsciousness and death can follow. Textbooks give about 7 ppm (parts per million) as the level beyond which harmful effects begin, and about 6.6 ppm as the level below which there is no harm. The smell can be detected at roughly 1 ppm.
Warning: after a short time in the gas, you lose your sense of smell, so the smell stops warning you at the very moment the danger rises. If you smell it, move out at once, and never assume it is gone because you can no longer smell it.
How is it detected?
- By its smell (only as a first warning)
- With blasting paper dipped in lead acetate solution, which turns black in the presence of H2S
- With a multi-gas detector, which is the reliable method
2. Nitrous fumes
Why is it called "khansi gas" (cough gas)?
The smoke from blasting contains a brownish component called nitrous fumes. Breathing it makes you cough, so miners called it the cough gas.
Where does it come from?
From blasting, especially with nitroglycerine-type explosives, in stone drifts, sinking shafts and at faces. The fumes increase when the explosive does not detonate properly.
Properties and effects
- Reddish-brown gas (mainly nitrogen dioxide)
- Heavier than air (relative density about 1.59 in the textbook figures)
- Dissolves easily in water
- Poisonous: breathing it causes coughing. With longer exposure it can lead to bronchitis and pneumonia, and death in severe cases
- The cough may stop after a short time, so the person thinks no harm was done. Hours later, the condition can suddenly worsen. Anyone exposed should be taken seriously and seen by a doctor
Precautions
- Spray water before and after blasting
- Blast carefully. Poor blasting and poorly broken ground mean more fumes
- Give the blasting area good ventilation
- Do not allow anyone back until the fumes have properly cleared
3. Gas from a mine fire (fire stink)
When a mine fire or spontaneous heating is under way, the burning and heating coal gives off a gas that has a smell like a brick kiln, like the smell of bricks being baked. It is a mixture and can contain blackdamp, carbon monoxide (the killer gas) and stinkdamp.
It is found in goafs, in the fire area and on the return side of the fire or heating. It is poisonous, and the smell is often the first sign that something is burning or heating underground, so it should always be reported and investigated.
4. Afterdamp
What is it?
Afterdamp is the gas found in a mine after a methane (firedamp) explosion or a coal dust explosion. It is a mixture of carbon monoxide, carbon dioxide, nitrogen, some oxygen, hydrogen and unburnt methane.
Typical composition
| Gas (percent) | After a methane explosion | After a coal dust explosion |
|---|---|---|
| Oxygen (O2) | 7.23 | 1.35 |
| Nitrogen (N2) | 76.80 | 73.75 |
| Carbon dioxide (CO2) | 4.54 | 11.25 |
| Carbon monoxide (CO) | 2.48 | 11.25 |
| Methane (CH4) | 8.95 | 2.40 |
Each column adds up to 100 percent. These are illustrative analyses, and the real composition depends on the explosion.
Why it is deadly: afterdamp combines two killers, a high level of carbon monoxide and a shortage of oxygen. People in the area can die within a short time. The coal dust explosion column shows why it is so dangerous: roughly 11 percent carbon monoxide and almost no oxygen.
How is it tested?
After an explosion, only a trained rescue party should go in. They test for carbon monoxide (traditionally with canaries and now with instruments), for low oxygen and for methane (with a flame safety lamp and gas detectors), and they take air samples for laboratory analysis.
5. Sulphur dioxide (SO2)
- Name: it comes from burning sulphur, so it is sometimes called "gandhak gas".
- Properties: colourless, with a sharp pungent smell, about 2.2 times heavier than air, highly poisonous and very soluble in water.
- Sources: blasting, mine fires and explosions (in tiny amounts) and, in larger amounts, sulphide ore mines. It may be found in the blasting area, at the fire and in the return airways, but normally only in very small amounts.
- Effect on the body: it irritates the eyes, throat and lungs. Textbooks say about 20 ppm (0.002 percent) affects the body, and about 7 ppm or less is considered harmless.
- Detection: with a multi-gas detector.
6. Hydrogen (H2)
- Where it is formed: in very small amounts at fires and after explosions, and in underground battery loco charging stations when batteries are charged.
- Properties: the lightest of all gases (relative density about 0.069), with no colour, smell or taste. It is flammable and explosive, with an explosive range of roughly 4 to 75 percent in air.
- Risk in mines: it forms in such small quantities that it seldom causes explosions, but charging stations still need good ventilation and care.
- Detection: air samples are taken from the suspected place and analysed in a laboratory.
Easy ways to remember them
- Stinkdamp = H2S = smell of rotten eggs. If you cannot smell it any more, that is not good news.
- Nitrous fumes = brown, cough, delayed damage to the lungs. They come from blasting.
- Afterdamp = what is left after the explosion. CO plus low oxygen.
- Sulphur dioxide = pungent, heavy (2.2 times air), very soluble.
- Hydrogen = lightest gas, explosive range about 4 to 75 percent, linked with battery charging.
- Rotten egg smell: lead acetate paper turns black.
Practice quiz
- A. Hydrogen
- B. Hydrogen sulphide
- C. Carbon dioxide
- D. Nitrogen
Show Answer
- A. It turns black
- B. It turns blue
- C. It burns
- D. Nothing
Show Answer
- A. Ventilation
- B. Blasting
- C. Winding
- D. Dewatering only
Show Answer
- A. Colourless
- B. Blue
- C. Green
- D. Reddish-brown
Show Answer
- A. Water spray before and after blasting
- B. Entering the area immediately after the blast
- C. Stopping ventilation
- D. Using more explosive
Show Answer
- A. Winding
- B. Dewatering
- C. A methane or coal dust explosion
- D. Drilling
Show Answer
- A. 0.07
- B. 1.2
- C. 1.6
- D. 2.2
Show Answer
- A. Hydrogen
- B. Methane
- C. Oxygen
- D. Carbon dioxide
Show Answer
- A. Shaft bottom only
- B. Underground battery loco charging stations
- C. Offices
- D. Surface stockyards only
Show Answer
- A. It has no smell
- B. It only smells in the daytime
- C. The sense of smell is lost after some time in the gas
- D. It is not poisonous
Show Answer
Frequently asked questions
Which of these gases is most dangerous?
Afterdamp is usually the deadliest in an emergency, because it combines carbon monoxide with a lack of oxygen. Hydrogen sulphide annd nitrous fumes are also extremely dangerous.
Why do nitrous fumes cause delayed symptoms?
They damage the lungs, and the effects can appear hours later. For that reason, anyone exposed must be checked by a doctor even if they feel fine.
Are the textbook limits the legal limits?
Not necessarily. Legal limits and recommended exposure values depend on the regulation, the country and the year. Use the current rules that apply to your mine.
Where can I read about the other mine gases?
See our post on carbon monoxide (white damp) and our Mining Safety MCQs.
Disclaimer: this post is for education and exam preparation. It is not an operating procedure. Gas testing and emergency response must be done by trained, authorised persons following the law and your mine's approved procedures.
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