Which pollutant is a colorless, odorless gas from incomplete combustion that can impair oxygen transport?

• A. Sulfur dioxide
• B. Nitrogen oxides
• C. Ground-level ozone
• D. Carbon monoxide

Answer: (D)

Incomplete combustion produces a colorless, odorless gas that binds strongly to hemoglobin, hindering the blood’s ability to carry oxygen. That gas is carbon monoxide, which forms carboxyhemoglobin and reduces oxygen delivery to tissues even when oxygen levels are normal. This makes CO especially dangerous in enclosed or poorly ventilated spaces, such as from vehicle exhaust, faulty furnaces, or gas appliances. The other pollutants have different characteristics and health effects: sulfur dioxide is a pungent gas that irritates the lungs and contributes to acid rain; nitrogen oxides are formed in high-temperature combustion and also irritate the airways and form smog; ground-level ozone is a reactive oxidant produced by sunlight-driven reactions involving NOx and VOCs, not by incomplete combustion itself.