What are the dangers?

Guidance from the Health and Safety Executive points to a number of situations that make confined spaces dangerous for workers.

The first, and perhaps most obvious risk is lack of oxygen. This can occur inside steel tanks and vessels due to the formation of rust, or in transport containers if a reaction occurs between the cargo being carried and the air inside the confined space. Similarly, oxygen levels can diminish as a result of a reaction between some soils and oxygen - groundwater on chalk and limestone can produce carbon dioxide and displace normal air.

Another risk is a build up of poisonous gases or vapour inside a confined space. Typically these occur in sewers or as a result of the inappropriate use of hazardous substances within a confined space. Workers have also been poisoned due to leakage into trenches on contaminated land - such as old refuse tips or industrial works - or as a result of poisonous fumes entering tanks or vessels from connecting pipes.

Less obvious are liquids and solids that can suddenly fill a space, or release gases into it, when disturbed. Free flowing solids such as grain can also partially solidify or "bridge" in silos causing blockages to collapse unexpectedly.

Other dangers include fire and explosions due to flammable vapours or excess oxygen, residues left in tanks or other vessels, high concentrations of dust or particles - such as in flour silos, or due to hot conditions leading to a severe increase in body temperature.

The HSE points out that although some of these conditions may already be present inside the confined space, some may arise through the work being carried out. When working in confined spaces, special consideration is needed with machinery - such as provision of dust extraction for a portable grinder and special precautions taken against electric shock.

Also, gas, fume or vapour can arise from welding processes or through the use of volatile and/or flammable solvents or adhesives.