Exam Question

### (a) Gases Usually Found Inside Enclosed Spaces
1. **Oxygen (O₂)**: The concentration can be either deficient ( 23.5%). Deficiency can be caused by oxidation (rusting), microbial action, combustion, or displacement by other gases. Enrichment, often from leaking oxygen cylinders or lines, creates a severe fire hazard.
2. **Hydrocarbon Vapours (Flammable Gases)**: These are present in spaces that have contained petroleum products, solvents, or paints (e.g., cargo tanks, fuel tanks, cofferdams, paint lockers). They are measured as a percentage of the Lower Explosive Limit (% LEL).
3. **Hydrogen Sulphide (H₂S)**: A highly toxic and flammable gas produced by the anaerobic decomposition of organic matter containing sulphur. It is commonly found in sewage tanks, ballast tanks with stagnant water, and tanks that have carried sour crude oil. It has a characteristic 'rotten egg' smell at low concentrations but deadens the sense of smell at higher, lethal concentrations.