Reasons for needing a SCBA rather than a Gas Mask

Hopefully a more clear explanation of reasons you may need a SCBA/Self Contained Breathing Apparatus or an Air-Supplied-Respirator as opposed to the standard gas mask/Air-Purifying-Respirator.
The benefits of an air supplied mask is it will work almost anywhere including areas with no oxygen or unknown chemicals that may be able to get through a standard filter.

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