Four gases cause most chemical-works deaths: chlorine, sulfur dioxide, ammonia and carbon monoxide. Chlorine and sulfur dioxide are more than twice as heavy as air and settle in pits and low rooms, ammonia is lighter and drifts up and downwind, and carbon monoxide has no smell at all. A single kilogram of any of them released in a closed 30 cubic-metre shed gives between about 11,000 and 47,000 parts per million, far above any level a person survives. The defence is ventilation, separation and leaving at the first warning, not a protective mask.
Part of Industrial Chemistry
Where chlorine, sulfur dioxide, ammonia and carbon monoxide come from in a chemical works, how each behaves in a room, and how to respond.
Why This Article Makes The Rest Safe To Read
Every process in this section creates at least one of these gases: electrolysis (Electrolysis Setup) makes chlorine, the contact process (Contact Process) makes SO2, the ammonia and Solvay articles use ammonia, and every combustion step can make carbon monoxide (Fuel Chemistry). The carrier lists the four in its subtitle and says chlorine and ammonia are toxic and ventilation is mandatory.
How Each Gas Behaves
Molar masses (rounded atomic masses Cl 35.5, S 32, O 16, N 14, H 1, C 12) compared with air at about 29 g per mole show where each gas goes.
| Gas | Molar mass | Mass ÷ air (29) | Collects | Warning smell |
|---|---|---|---|---|
| Chlorine, Cl2 | 71 | 2.4 | Pits, cellars, low ground | Sharp, but olfactory fatigue sets in |
| Sulfur dioxide, SO2 | 64 | 2.2 | Pits, cellars, low ground | Sharp, choking |
| Carbon monoxide, CO | 28 | 0.97 | Mixes through the room | None |
| Ammonia, NH3 | 17 | 0.59 | Rises, then drifts | Sharp |
Smell warns of the first three (a general fact, and smell fades with exposure), so treat any whiff as a reason to leave, not a measurement. Carbon monoxide gives no warning. Its main sources are incomplete combustion and gasifiers (Gas Hazards, Wood Gas).
What Each Does To The Body
- Chlorine, SO2 and ammonia dissolve in the wet lining of the eyes, nose and airways and attack it: chlorine and sulfur dioxide form acids, ammonia forms an alkali. The injury can progress for hours after exposure. Thresholds for chlorine (0.5 ppm irritating, 10 ppm severe, 50 ppm lethal within 30-60 minutes, 1,000 ppm immediately fatal) are on Gas Hazards and are not repeated as new figures here.
- Carbon monoxide binds to the blood’s oxygen carrier far more strongly than oxygen does, so the blood stops delivering oxygen. Headache and confusion come first, and the victim may not realise they are in danger.
Worked Example: One Kilogram In A Shed
A shed of 30 m³ is 30,000 litres. Gas molar volume at room temperature about 24 L per mole (recalled). 1% is 10,000 ppm.
| Gas | Moles in 1 kg | Litres of gas | Concentration |
|---|---|---|---|
| Chlorine | 1,000 ÷ 71 = 14.1 | 14.1 × 24 = 338 | 338 ÷ 30,000 = 1.1%, 11,000 ppm |
| Sulfur dioxide | 1,000 ÷ 64 = 15.6 | 15.6 × 24 = 375 | 1.25%, 12,500 ppm |
| Carbon monoxide | 1,000 ÷ 28 = 35.7 | 35.7 × 24 = 857 | 2.9%, 29,000 ppm |
| Ammonia | 1,000 ÷ 17 = 58.8 | 58.8 × 24 = 1,411 | 4.7%, 47,000 ppm |
For scale, the Gas Hazards page calls 1,000 ppm of chlorine immediately fatal, and each release above is more than ten times that concentration. All four gases injure at much lower levels than the table shows. Carbon monoxide is also flammable, but it is a lethal poison long before that matters. Dilution by wind helps a great deal outdoors; indoors nothing does.
What You Need
- Outdoor siting for any gas-making step, with prevailing wind away from people.
- Ventilation at both high and low points of any room.
- A buddy, and an agreed signal, because a collapsing worker cannot call for help.
- Escape routes upwind and uphill, and rope for retrieval from pits.
- Alkaline scrubbing for chlorine and SO2 (Waste Treatment); an acid wash can scrub ammonia.
- Eye protection and, for escape only, respiratory protection if you have it.
Method: Responding To A Release
Step 1: Leave at the first smell, cough or eye sting. Move crosswind or upwind. For the heavy gases, go up, not into a pit or stairwell down.
Step 2: Do not go back in for a person without breathing support and a second person.
Step 3: Shut the source from a distance if it can be done safely.
Step 4: Fresh air and rest for the exposed. Exertion makes lung injury worse. Remove contaminated clothing and flush eyes and skin with water for at least 15 minutes (the same duration the corpus gives for methanol splashes).
Step 5: Get the best medical help available, even if the person seems well. Lung effects can appear hours later. There is no home antidote to give.
Gases Covered Elsewhere
- Carbon dioxide from fermentation and kilns pools in pits and displaces air; see Fermentation.
- Hydrogen sulfide from digesters: Anaerobic Digestion Science and Mixing and Agitation.
- Hydrogen burns invisibly: Haber Process.
Common Mistakes
- Mixing bleach with acid or ammonia, which makes chlorine or toxic chloramine gases.
- Burning sulfur in a closed room.
- Running an engine or gasifier indoors.
- Going down after a collapsed worker and becoming the second casualty.
- Using a flame to look for a flammable gas.
Safety
Heavy gases fill the low spots; carbon monoxide has no smell
Never enter a pit, cellar, tank or closed low room where chlorine, SO2 or carbon dioxide may have collected without ventilation first and a rescue line, and never work alone. Never test an atmosphere with a flame. Outdoors and upwind is the main protection. See Safety and Waste.
What’s Next
- Learn next: Waste Treatment — absorbing and neutralising what is left
- Combine with: Electrolysis Setup — the case where this has killed people