Confined or Enclosed Spaces and Other Dangerous Atmospheres >> Oxygen-Deficient or Oxygen-Enriched Atmospheres

Oxygen-deficient atmospheres are the leading cause of confined space fatalities in the shipyard. While normal atmosphere contains between 20.8 and 21 percent oxygen, OSHA defines as oxygen deficient any atmosphere that contains less than 19.5 percent oxygen, and as oxygen enriched, any atmosphere that contains more than 22 percent.

Oxygen-deficient atmospheres may be created when oxygen is displaced by inerting gases, such as carbon dioxide, nitrogen, argon, or the ship's inert gas system or firefighting system. Oxygen can also be consumed by rusting metal, ripening fruits, drying paint, or coatings, combustion, or bacterial activities.

Oxygen-enriched atmospheres may be produced by certain chemical reactions, but in a shipyard they are typically caused by leaking oxygen hoses and torches. Oxygen enriched atmospheres present a significant fire and explosion risk.

  • The following spaces must be tested for oxygen content before entry: [29 CFE 1915.12(a)]
    • Spaces that have been sealed, such as spaces that have been coated and closed up, and non-ventilated spaces that have been freshly painted;
    • Spaces and adjacent spaces that contain or have contained combustible or flammable liquids or gases;
    • Spaces and adjacent spaces that contain or have contained liquids, gases, or solids that are toxic, corrosive, or irritant;
    • Spaces containing materials or residues of materials that create an oxygen-deficient atmosphere.
  • Spaces that are oxygen deficient must be labeled "Not Safe for Workers", and spaces that are oxygen enriched must be labeled "Not Safe for WorkersNot Safe for Hot Work." [29 CFR 1915.12(a)]