Which product of combustion is present at almost every fire?

• A. Hydrogen cyanide
• B. Carbon monoxide
• C. Nitrogen dioxide
• D. Formaldehyde

Answer: (B)

Incomplete combustion is common in real fires, especially in enclosed spaces where oxygen is limited and heat isn’t evenly distributed. When fuels don’t burn completely, carbon-containing material is oxidized only partially, producing carbon monoxide as a typical byproduct. CO is a colorless, odorless gas that readily binds to hemoglobin, depriving tissues of oxygen, so it becomes a constant hazard in most fires.

The other products don’t appear in every fire because they depend on specific materials or conditions. Hydrogen cyanide tends to come from burning nitrogen-rich polymers (like certain foams and plastics) and isn’t produced in every fire. Nitrogen dioxide forms primarily at very high flame temperatures and with ample oxygen reacting under those conditions, which isn’t guaranteed in all fires. Formaldehyde can form from burning a wide range of materials, but its appearance isn’t as universal as carbon monoxide.