Page 19 - Confined Space Training - Student Manual 2021

Toxic Atmospheres

Whenever operations are taking place in a permit-required confined space, exposure to toxic
chemicals cannot exceed the OSHA Permissible Exposure Level (PEL) for that toxic chemical.

Hydrogen Sulfide (H2S)

Hydrogen sulfide is a toxic, colorless, combustible gas that is heavier than air. It is formed by the
decomposition of organic plant and animal life by bacteria.

At low concentrations, hydrogen sulfide is irritating to the eyes, nose, and mucous membranes.
Hydrogen sulfide rapidly paralyzes one’s sense of smell. If atmospheric monitoring indicates
hydrogen sulfide concentrations greater than 10ppm, workers must not enter.

After a sufficient dose, hydrogen sulfide gas paralyzes the nerve centers in the brain that control
breathing. As a result, the lungs cease to function, and the individual is asphyxiated. Hydrogen
sulfide can be found in oil and gas refining and production, sewers, pulp mills and a variety of
industrial processes.

Hydrogen sulfide is easily detected by a strong “rotten egg” odor in low concentrations. However,
relying on this odor to warn of the presence of hydrogen sulfide can be very dangerous.

NOTE: Your nose is not an indicator of toxicity. Never rely on your sense of smell to assess
the level of toxic materials present or their potential toxic effect.

Symptoms of Exposure Levels (ppm) Exposure Time

Odor Threshold
0.1 --
Moderate odor
5.0 --
OSHA’s PEL
20 8 hours

NIOSH and ACGIH TLV
10 8 hours
Tolerable, but strong and unpleasant odor
25 --

Eye irritation, coughing, loss of consciousness
100 2 to 5 min.
Respiratory tract irritation loss of consciousness
200-300 1 hour
Rapid unconsciousness, possibly death
500-700 30-60 min.

Respiratory distress, unconscious almost immediately, 700-1000 0 min.
death follows
Respiratory stops, death within a few minutes.
Death may occur even if the victim is removed to fresh air. 1000-2000 0 min.
Sources: OSHA, NIOSH, ACGIH and ANSI

Confined Space Training for Entrants, Attendants & Supervisors 19