Which gas becomes hazardous when its concentration exceeds 5%?

The correct answer is methane. Methane is considered hazardous when its concentration exceeds 5% in the air because it can lead to an explosive atmosphere. At concentrations above this threshold, methane can displace oxygen in the environment, posing risks of asphyxiation and creating a flammable mixture that is dangerous in confined spaces.

In contrast, while oxygen is essential for life, elevated concentrations can also be hazardous in certain situations, but it becomes dangerous primarily at levels well above 21% due to fire risks. Nitrogen constitutes about 78% of the Earth's atmosphere and is generally not hazardous even at higher concentrations; it is inert and does not support combustion. Carbon dioxide, although it can be harmful at higher levels (typically around 10% or more), starts to pose health risks primarily at concentrations greater than 5%, but the explosive risk associated with methane at this level makes it particularly noteworthy in industrial contexts.