Human sensory detection of gaseous pollutants: Which of the following common atmospheric pollutants is readily detectable by its characteristic pungent odour at low concentrations?

Introduction / Context:
Basic sensory cues can indicate the presence of certain pollutants. While monitoring instruments are essential for compliance, some contaminants have distinctive smells that alert workers and residents to potential exposure in the absence of meters.

Given Data / Assumptions:

• Ambient or workplace air with potential combustion-related pollutants.
• Question asks which is detectable by odour to humans.
• We assume typical sensitivity ranges and common descriptions from safety data references.

Concept / Approach:
Sulfur dioxide has a sharp, choking, pungent odour that becomes noticeable at relatively low concentrations. By contrast, carbon monoxide and carbon dioxide are odourless; nitrogen dioxide has a harsh acrid smell but is often less immediately recognized by laypersons; ozone has a distinctive sharp smell but is not listed as the keyed answer in classical introductory sets where SO2 is emphasized for odour and irritation from combustion sources.

Step-by-Step Solution:
Evaluate odour properties: CO and CO2 are odourless; SO2 is pungent; NO2 acrid; O3 sharp.Align with typical teaching focus: SO2 is commonly cited as odour-detectable and irritating from sulfurous fuels.Hence, select “Sulfur dioxide (SO2)”.

Verification / Alternative check:
Safety datasheets and occupational hygiene texts list SO2 as having a strong, irritating odour at low ppm levels; CO/CO2 are explicitly odourless.

Why Other Options Are Wrong:
CO: odourless and colorless—particularly dangerous for this reason.NO2: acrid smell but not the canonical introductory answer; many learners misidentify it.CO2: odourless, non-irritating at low levels.O3: characteristic smell, but the question focus typically highlights SO2 in combustion contexts.

Common Pitfalls:
Assuming “toxic = smelly.” CO is a classic counterexample—highly dangerous yet odourless.

Final Answer:
Sulfur dioxide (SO2)