Which indoor pollutant is a by-product of combustion and has no odor at low concentrations?

• A. CO
• B. CO2
• C. Mold
• D. Radon

Answer: (A)

Odorless, poisonous gases produced by incomplete combustion are a common indoor hazard, because you can’t rely on smell or taste to notice them. Carbon monoxide is formed when fuels such as gas, oil, wood, or propane burn without enough oxygen. It has no color, no odor, and no taste at low concentrations, so people can be exposed without realizing it.

The danger comes from how it behaves in the body: carbon monoxide binds to hemoglobin much more readily than oxygen, forming carboxyhemoglobin and reducing the blood’s ability to carry oxygen to tissues. This can cause headaches, dizziness, confusion, and nausea, and at higher levels can lead to loss of consciousness or death. Because these effects can occur quietly at low levels, carbon monoxide is the classic indoor pollutant to be aware of in relation to combustion.

Other options aren’t the same. Mold is a biological contaminant, not a combustion by-product. Radon is a radioactive gas from soil and rocks, not produced by combustion. Carbon dioxide is also a combustion by-product, but it is not the primary acute hazard associated with indoor combustion at low concentrations in the way carbon monoxide is. To stay safe, ensure proper ventilation, maintain appliances, and use working carbon monoxide detectors.