Texas Wastewater Class B Practice Exam

Which gas is known to be very toxic and can deaden the sense of smell?

1. Carbon Dioxide

2. Methane

3. Hydrogen Sulfide

4. Nitrous Oxide

The correct answer is: Hydrogen Sulfide

The gas known to be very toxic and that can deaden the sense of smell is hydrogen sulfide. This colorless gas is produced during the breakdown of organic matter and is commonly associated with wastewater treatment processes. At low concentrations, it's detectable by its characteristic rotten egg smell. However, at higher levels, hydrogen sulfide can cause olfactory fatigue, whereby the sense of smell becomes diminished, making it difficult for individuals to perceive its presence. This property contributes to the danger of exposure, as individuals may not realize the concentration is increasing to dangerous levels. In contrast, carbon dioxide is a naturally occurring gas that is not highly toxic at normal atmospheric levels, and while it can contribute to asphyxiation in high concentrations, it does not deaden the sense of smell like hydrogen sulfide does. Methane is primarily a combustible gas that poses explosion risks rather than immediate toxicity and does not impair the sense of smell. Nitrous oxide, often used as a sedative or analgesic, is not particularly toxic and does not exhibit the same olfactory fatigue characteristics as hydrogen sulfide.