Oxygen-enriched and Oxygen Deficient Atmospheres
Normal outside air contains about 21% oxygen. If the concentration of
oxygen exceeds 23% it is considered “enriched”. The primary
concern with oxygen-enriched atmospheres is the increased
flammability of materials. Things that would only smolder in normal air will burn vigorously in oxygen-enriched atmospheres.
Oxygen-enriched atmospheres are fairly rare in construction. They are usually associated with pure oxygen escaping from leaking or ruptured oxyacetylene hoses or, on projects in industrial plants, from an oxygen line in an industrial or manufacturing process.

Oxygen-deficient atmospheres, on the other hand, are fairly common. They may result from work being done (such as welding),
bacterial action (which consumes oxygen), or from chemical reactions (such as rusting).
Oxygen may also be displaced by another gas or vapour (e.g., carbon dioxide or nitrogen used to purge tanks, pipes, and vessels).

Table 2 lists (see attached to the paperclip at the top) the effects of oxygen deficiency.