Why might toxic gases accumulate in a trench?

Toxic gases can accumulate in a trench primarily due to the lack of air movement. When a trench is cut into the ground, especially if it's deep or has limited openings for ventilation, the exchange of fresh air with contaminated air is restricted. This stagnation can cause gases released from decomposing materials, sewage, or other sources to build up within the confined space.

In such environments, without sufficient airflow, heavier-than-air gases can also settle at the bottom of the trench, but the critical factor leading to dangerous concentrations is the limited movement of air that prevents dilution and dispersal of these gases. Therefore, the presence of stagnant air significantly increases the risk of toxic gas accumulation, creating hazardous working conditions for drainlayers and other workers in the area.

Other factors such as the specific properties of gases, like being lighter than air or evaporating quickly, do not directly contribute to accumulation in a trench as much as the absence of air movement does.