In a fire situation, which chemical would be least concerning for human health?

Oxygen is essential for human survival and is not considered toxic at normal atmospheric levels. Though an increase in concentration can lead to physiological effects, in a fire situation, the presence of oxygen is crucial for supporting combustion. Therefore, while excessive oxygen can create an enhanced fire risk, it does not pose an immediate threat to human health in the same way that the other substances do.

Carbon monoxide and nitrogen dioxide are both toxic gases that can lead to serious health effects; carbon monoxide can cause asphyxiation by binding to hemoglobin, while nitrogen dioxide can irritate the respiratory system. Sulfur dioxide is also harmful, potentially causing respiratory problems and irritation. Thus, while all substances require caution, oxygen, when encountered in the standard air mixture, poses significantly less risk than the others listed, especially during a fire scenario where toxic fumes are a primary concern.