The following article from WorkSafe WA highlights the dangers of H2S in confined spaces.  Unlike most confined space incidents, this worker survived but not by chance... by prepartion and planning! See alert

Hydrogen Sulphide (H2S) is a toxic gas that has the following properties:

• Occurs naturally. Generated by bacteria.
• Heavier than air
• Easy to smell at low levels (0.05 ppm). Smells of rotten egg.
• Deadens sense of smell at high levels (50 - 100 ppm)
• Deadly health effects may be immeditate (respiratory paralysis) or delayed (pulmonary oedema)
• Disolves in water.  May be released when water is disturbed.