What atmospheric hazard is commonly encountered in manholes?

The presence of a lack of oxygen is a critical concern in manholes because these confined spaces can contain hazardous gases or materials, leading to an atmosphere that is insufficient for human respiration. In many cases, manholes can accumulate gases such as methane, hydrogen sulfide, or carbon dioxide, which can displace oxygen, creating an unsafe environment for personnel entering those spaces. This makes it essential for workers to have the appropriate safety measures in place, such as monitoring air quality and having rescue procedures planned.

In contrast, the other options do not accurately reflect typical hazards found in manholes. Manholes are not usually characterized by non-explosive or non-flammable atmospheres; rather, they can contain explosive gases depending on the environment and activities related to sewage or wastewater management. Additionally, a non-toxic atmosphere is not guaranteed, especially when dealing with hazardous materials that may produce toxic gases or chemicals within such confined spaces.