The following table illustrates the kinds of harmful atmospheres that may be present in a confined space, and how they may be created.

Source Examples
Substance stored in the confined space or its by-product(s)
  • build-up of hydrogen sulphide in sewers and pits
  • release of toxic substances e.g. hydrogen sulphide in tanks of decomposing organic material, especially when the material is disturbed
Work performed in the confined space
  • use of paints, adhesives, solvents or cleaning solutions
  • welding or brazing with metals capable of producing toxic fumes
  • exhaust fumes from engines used in the confined space
  • painting or moulding glass-reinforced plastics
Entry of natural contaminants e.g. groundwater and gases into the confined space from the surrounding land, soil or strata
  • acid groundwater acting on limestone with the potential to produce dangerous accumulations of carbon dioxide
  • methane released from groundwater and from decay of organic matter
Release of airborne contaminants
  • when sludge, slurry or other deposits are disturbed or when scale is removed
  • residues left in tanks, vessels etc., or remaining on internal surfaces can evaporate into a gas or vapour
Manufacturing process

Gas detection trianing