Page 16 - Confined Space Training - Student Manual 2021

In storage tanks and pipelines, oxygen can be displaced when inert gases (nitrogen, carbon
dioxide, helium, or steam) are used to purge them of residual chemicals, gases, or vapors.

Simple repair or clean-up jobs may also reduce the oxygen level, such as exposure to vapors
from open containers of solvents within a confined space.

Oxygen enriched atmospheres, those with oxygen concentrations greater than 20.9% (up to
23.5%), represent a serious fire hazard.

Flammable material, such as clothing and hair, burn very rapidly in an oxygen-enriched
atmosphere. Unattended or leaking oxygen lines or cylinders can cause the oxygen concentration
to reach unsafe levels and should be recognized as hazards.

What about insects, rodents, and other animals? If these animals are present, does that
indicate a safe atmosphere?

It is possible for insects and other animals to survive at lower oxygen concentrations. That these
animals can exist in confined spaces does not mean the atmosphere is healthy for humans.

The following chart shows the effects of various oxygen concentrations:

23.5% or Greater Oxygen-enriched atmosphere (fire hazard)

20.9% Normal breathing air

19.5% Minimum safe entry level

Disturbed respiration, emotional upsets, abnormal
12% - 16%
fatigue or exertion and flames are extinguished.

Increased respiration and heart coordination may be
10% - 11% disturbed, some euphoria and possible headache

Nausea and vomiting; inability to move freely;
6% - 10% possible unconsciousness; will collapse while
remaining conscious, but helpless

Gasping respiration; respiration stops, followed by
Less than 6% cardiac arrest and death within minutes.

Flammable and Explosive Atmospheres

Within a confined space, the existence of a flammable gas such as methane or acetylene, fuel
vapors or grain dusts represents great danger. At appropriate concentrations, flammable gases
or vapors or dusts in combination with oxygen can be a deadly mixture. All that is necessary for
an explosion is a source of ignition.
Confined Space Training for Entrants, Attendants & Supervisors 16