Coal workings can hold methane, which is odourless and explosive at 5-15% in air and collects at the roof, carbon monoxide, which poisons without warning, and carbon dioxide or oxygen-poor air, which sinks to the floor. None can be reliably smelled or seen, and a naked flame is not a safe detector: the defence is through-ventilation, no ignition sources underground, and leaving at the first symptom.
Part of Coal Surface Mining
The gases found in coal workings, where each collects, what each does, and the controls that do not depend on a detector.
Four Gases, Four Behaviours
Gas density is set by molar mass against air (average about 29), which decides where each collects:
| Gas | Density against air | Collects | What it does |
|---|---|---|---|
| Methane (CH₄) | 16 ÷ 29 = 0.55 | Roof, high pockets, blind ends | Explodes at 5-15% in air; at high concentrations displaces oxygen |
| Carbon monoxide (CO) | 28 ÷ 29 = 0.97 | Mixes through the whole space | Poisons; colourless, odourless; from smouldering or burning coal |
| Carbon dioxide (CO₂) | 44 ÷ 29 = 1.52 | Floor, sumps, low ground | Displaces oxygen; headache, panting, collapse |
| Hydrogen sulphide (H₂S) | 34 ÷ 29 = 1.17 | Low spots | Smells of rotten eggs at low levels, then dulls the sense of smell |
Air is about 21% oxygen. Below 19.5% a person is already impaired, and a flame goes out near 16% (Safety Protocol sets these out for wells). So a person is disabled well before a candle shows anything.
The Parent Article’s Candle Test Should Not Be Used
The parent article suggests holding a candle on a pole at the roof, watching for an elongated bluish flame to show methane. Do not do that. If the roof holds 5-15% methane, a candle ignites it. The flame-cap test belongs to a sealed, gauze-protected safety lamp used by trained men, in the nineteenth century, with damaged gauzes and draughts still causing explosions. An improvised candle is a gas ignitor, not a gas detector. The same goes for the floor-level candle for carbon dioxide: by the time it dims, a person is already in trouble.
What To Do Instead
Step 1: Ventilate every time. Do not test; assume bad air. Keep two openings at different heights and keep air moving through the working. Natural draught depends on the temperature difference between inside and outside and can reverse with the seasons, so do not assume it blows the right way.
Step 2: Remove ignition sources. No naked flames, no smoking, no steel striking rock on a face that may hold methane, no stoves or fires at the mouth that could draw gas toward them.
Step 3: Use a salvaged detector if you have one. An electronic combustible-gas detector or carbon monoxide meter is useful. Keep it as an addition to ventilation, never a substitute.
Step 4: Watch for heating. A tarry or petrol smell, a haze, warm or sweating walls and warm coal mean the coal is oxidising and may be producing carbon monoxide. Leave.
Step 5: Know the symptoms. Headache, drowsiness, nausea, confusion, weak legs. Treat any of these as an order to walk out, in company, and not to sit down.
Step 6: Small animals. A caged canary or mouse was the historical early warning for carbon monoxide because it succumbs sooner than a person. It gives no warning for methane and a late one for oxygen-poor air; it does not replace ventilation.
Common Mistakes
- Going back for a collapsed colleague. The rescuer breathes the same air.
- Thinking a mine “has no gas” because nothing has happened. New workings open fresh coal, and a fall in barometric pressure can release gas that was held back.
- Lighting a fire at the shaft foot to draw air. In a gassy seam a fire is an ignition source.
Safety
Gas kills without warning and explosions kill everyone inside
Methane at 5% by volume, one litre in every twenty of air, is the lower explosive limit. Carbon monoxide is lethal and odourless. Never enter a stalled or abandoned working, and never alone. If anyone is overcome, ventilate first and rescue from outside on a rope, with the rescuers breathing fresh air, before anyone goes in. See Carbon Monoxide Risks for the poisoning.
What’s Next
- Learn next: Dust & Respiratory Protection — the third respiratory hazard
- Combine with: Roof Collapse Prevention — the other underground killer