In excavation work, the risk of gases and fumes contaminating the air must be controlled. Heavier-than-air gases and fumes can gather in tunnels and excavations, including sulphur dioxide, engine exhaust, carbon monoxide and carbon dioxide, and leaks from LPG tanks, gas pipes, gas bottles, drains, sewers and fuel tanks. Plant with a combustion engine, such as compressors and generators, should never run inside a trench or other confined excavation while anyone is working inside, because exhaust gases, especially carbon monoxide, can build up and kill.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.