Which gas, commonly found in sewers, is not considered explosive?

The correct answer is carbon dioxide. This gas, while present in sewers, does not pose an explosion risk under typical conditions encountered in such environments. Carbon dioxide is a heavier-than-air gas that can accumulate in low-lying areas, but it does not ignite and does not have flammable properties, making it non-explosive.

In contrast, methane is highly flammable and can form explosive mixtures with air, especially when present in concentrations between 5% and 15%. Hydrogen is another flammable gas that poses a significant risk due to its wide range of explosive concentrations in air, as well as its rapid diffusion. Carbon monoxide is a toxic gas that, although not normally considered explosive, can still be present in environments with insufficient ventilation. However, its primary concern is its toxicity rather than flammability or explosiveness.

Thus, when considering the safety and risks associated with gases found in sewers, carbon dioxide stands out as a gas that does not contribute to explosion hazards.