Question 65

The Correct Answer is C **Explanation for Option C (Correct Answer):** Asphyxia occurs when there is an insufficient supply of oxygen to the body, typically defined by oxygen concentration in the atmosphere falling significantly below the normal level. Normal breathable air contains approximately **20.9% to 21% oxygen**. If the oxygen concentration remains at 21%, there is no deficiency of breathable air, and therefore, asphyxia will not be caused by lack of oxygen. This condition represents a safe and breathable atmosphere. **Explanation of Incorrect Options:** **A) Where rusting of internal tank surfaces has taken place:** Rusting (oxidation) is a chemical reaction that consumes oxygen ($\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$). In an enclosed space like a tank, this process can rapidly deplete the available oxygen, causing the concentration to fall below safe limits, thus leading to asphyxia. **B) Where large quantities of inert gas is present:** Inert gases (such as nitrogen, carbon dioxide, or argon, often used for purging) displace oxygen. If a large quantity of inert gas is present in an enclosed space, it will lower the partial pressure and concentration of oxygen below the necessary level (typically $19.5\%$ minimum for safe entry), causing asphyxia. **D) When large quantities of cargo vapor is present:** While some cargo vapors may be toxic, their presence primarily displaces breathable air, similar to inert gases. If the vapor concentration is high enough, it significantly reduces the concentration of oxygen (the remaining portion of the atmosphere), leading to an oxygen-deficient environment and causing asphyxia. (Note: While toxicity is a separate hazard, the displacement of oxygen is the mechanism causing asphyxia in this context.)