Confined Space Training Certificate Confined Space Definition & Hazards 2 — Questions and Answers
Question 1: What level of hydrogen sulphide (H2S) in ppm is considered immediately dangerous to life and health (IDLH)?
- 100 ppm (Correct answer)
- 10 ppm
- 50 ppm
- 500 ppm
Correct answer: 100 ppm
The IDLH level for hydrogen sulphide is 100 ppm. At this concentration, rapid loss of consciousness and death can occur within minutes. H2S is particularly dangerous because it deadens the sense of smell at moderate concentrations.
Question 2: Why is methane particularly dangerous in confined spaces beyond its toxicity?
- It is highly flammable and explosive in air concentrations of 5–15% (Correct answer)
- It is heavier than air and displaces oxygen from the floor level
- It reacts with water to form corrosive acid
- It is colourless and has a distinctive sweet odour
Correct answer: It is highly flammable and explosive in air concentrations of 5–15%
Methane (CH4) is flammable in concentrations of 5–15% in air (its Lower and Upper Explosive Limits). A single spark can ignite the mixture causing an explosion. It is also an asphyxiant in high concentrations.
Question 3: Which confined space hazard involves the gradual displacement of oxygen by an inert gas?
- Simple asphyxiation (Correct answer)
- Chemical asphyxiation
- Toxic gassing
- Flammable atmosphere
Correct answer: Simple asphyxiation
Simple asphyxiation occurs when an inert gas such as nitrogen, argon, or carbon dioxide displaces oxygen, reducing its concentration below safe levels. There is no toxic effect from the gas itself — the danger is purely from oxygen depletion.
Question 4: What is the Upper Explosive Limit (UEL) and why is it relevant to confined space safety?
- The highest gas concentration at which ignition can occur; above this the mixture is too rich to ignite (Correct answer)
- The maximum gas level permitted before evacuation
- The highest safe oxygen reading on a gas detector
- The point at which a gas becomes toxic
Correct answer: The highest gas concentration at which ignition can occur; above this the mixture is too rich to ignite
The UEL is the highest concentration of a flammable gas in air at which ignition can occur. Above the UEL the mixture is too fuel-rich to burn. The danger zone is between the Lower Explosive Limit (LEL) and UEL.
Question 5: Which physiological effect makes carbon monoxide especially dangerous in confined spaces?
- It binds to haemoglobin 200 times more readily than oxygen, preventing oxygen transport (Correct answer)
- It destroys lung tissue on contact
- It paralyses the respiratory muscles
- It causes instant unconsciousness at any concentration
Correct answer: It binds to haemoglobin 200 times more readily than oxygen, preventing oxygen transport
Carbon monoxide (CO) binds to haemoglobin to form carboxyhaemoglobin with an affinity approximately 200–240 times greater than oxygen. This prevents blood from carrying oxygen to tissues, causing cellular hypoxia even when oxygen levels in the air are normal.
Question 6: A grain silo is a confined space. What unique hazard does grain dust present?
- Explosive dust cloud when suspended in air (Correct answer)
- Oxygen enrichment from biological decomposition
- High carbon dioxide production causing toxic atmosphere
- Physical entrapment only with no atmospheric risk
Correct answer: Explosive dust cloud when suspended in air
Grain dust suspended in air forms an explosive mixture. Static electricity, friction, or any ignition source can trigger a dust explosion. Grain silos also present engulfment and oxygen deficiency risks from decomposing grain.
Confined Space Training Certificate Confined Space Definition & Hazards 2
What level of hydrogen sulphide (H2S) in ppm is considered immediately dangerous to life and health (IDLH)?