Confined spaces are dangerous places. Several gases such as carbon monoxide, carbon dioxide, hydrogen sulfide and methane can accumulate inside a confined space. Carbon monoxide is a colorless, odorless gas that displaces oxygen in the blood stream - even when there is plenty of oxygen available in the air. It can lead to cardiac arrest.
Carbon dioxide displaces oxygen in the air and causes suffocation.
Hydrogen sulfide is a poisonous gas with a high concentration that causes almost immediate death. Confined spaces include manholes, tunnels, wells, cellars, tanks, sewers and trenches. Trenches Generally speaking, a trench is a cut in the ground that is deeper than it is wide. Trenches provide access to underground utilities for water, telephone, electrical, natural gas and sewer lines. Trenches are very dangerous and must be stabilized before anyone enters.