Dangers can arise in confined space ,What are the dangers from confined spaces?

Dangers in confined Spaces such as 

Presence of toxic gases 

Limited Oxygen Level

Entrapment

Engulfment

The dangers of confiend spaces are:

Reduced oxygen level

Fire

Flooding

Drowning,

Loss of consciousness from gas, fumes, vapour or lack of oxygen.

I assume the most important hazard would be reduced oxygen levels and possibly risk of fire. Asphyxiation is a risk as well and can be triggered even by an innocent factor such as dust.