Question 17 QMED04 - Boiler Technician-Watertender

The Correct Answer is A. **Explanation for A (paralyze your breathing system):** High concentrations of hydrogen sulfide ($\text{H}_2\text{S}$) gas are rapidly absorbed into the bloodstream. $\text{H}_2\text{S}$ is a potent cellular toxin, primarily acting by inhibiting cytochrome c oxidase, a crucial enzyme in the mitochondria necessary for cellular respiration and energy production (ATP synthesis). This inhibition leads to a rapid depletion of energy, particularly affecting organs with high oxygen demands, such as the brain and the heart. At lethal concentrations (above $\text{500-1000 ppm}$), $\text{H}_2\text{S}$ acts so quickly on the central nervous system (specifically the brainstem's respiratory centers) that it causes immediate respiratory paralysis, leading to "knockdown" (rapid unconsciousness) and death due to asphyxiation. This systemic toxic effect on the respiratory control center is the primary mechanism of danger. **Explanation of Incorrect Options:** **B) cause involuntary skeletal muscle contractions:** While $\text{H}_2\text{S}$ can affect the nervous system and muscle function, the primary and most immediate danger at lethal concentrations is the cessation of breathing (paralysis of the respiratory control system), not widespread involuntary muscle contractions (like seizures or tetany), which is a feature more commonly associated with certain other toxins (e.g., nerve agents or strychnine). **C) cause dizziness:** Dizziness (vertigo, lightheadedness) is a symptom of exposure, usually at lower, sub-lethal concentrations (e.g., $\text{50-100 ppm}$), reflecting mild CNS disruption. However, it is not the reason the gas is "most dangerous" at high concentrations. At extremely high levels, the effect is immediate paralysis and unconsciousness, bypassing the stage of mild symptoms like dizziness. **D) cause eye inflammation:** Hydrogen sulfide is a severe irritant, and exposure causes conjunctivitis (inflammation of the eyes, sometimes called "gas eyes") and respiratory tract irritation, even at relatively low concentrations. While this is a common and painful effect, it is a localized irritant effect, not the cause of death. The systemic toxicity leading to respiratory paralysis is the reason high concentrations are "most dangerous" (i.e., life-threatening).