Which gas would cause more immediate health hazards at elevated levels?

Carbon monoxide poses more immediate health hazards at elevated levels due to its ability to bind with hemoglobin in the blood more effectively than oxygen. When inhaled, carbon monoxide is absorbed quickly into the bloodstream, where it reduces the blood's oxygen-carrying capacity. This can lead to symptoms such as headaches, dizziness, confusion, and even loss of consciousness within minutes, depending on the concentration of carbon monoxide in the environment.

In mine rescue situations, the presence of carbon monoxide is particularly dangerous because it can accumulate in confined spaces and the initial symptoms may be mistaken for fatigue or other illnesses, delaying the recognition of danger. This rapid onset of harmful effects makes it critical for mine rescue teams to monitor and respond to carbon monoxide levels urgently.

In contrast, while carbon dioxide can also be dangerous, it typically requires higher concentrations over a longer duration to cause immediate health effects compared to carbon monoxide. Hydrogen, while flammable and potentially explosive, does not pose direct toxic health risks at lower concentrations like carbon monoxide does. Methane is primarily an asphyxiant and also poses explosion risks, but again, it is not as acutely toxic as carbon monoxide. Therefore, the significant immediate health hazards associated with carbon monoxide make it the correct choice in this context.