There are too many variables to effectively quantify CO
2 retention of a FFM. Military and commercial divers are trained to compensate for the added dead air space in an oral nasal mask compared to a mouthpiece by breathing deeper and ventilating the mask or hat.
As others have alluded to, greater dead air space inherent in an oral-nasal mask coupled to a demand regulator requires more gas exchange to dilute CO
2. Valid reasons to consider a FFM include:
- The need for audio communications or recording. Wireless communications is far less reliable than hardwire used in surface supplied diving. It can also be a PITA to fit ear phones to FFMs used in recreational diving. Commercial FFMs have earphone pockets built into the attached hood:
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- Cold and contaminated water, which is the main reason PSDs (Public Safety Divers) favor them.
- Deeper and/or long duration pure O2 in-water decompression where the risk of a convulsion is high. A rare example would be an IWR (In Water Recompression) treatment system that a trained and experienced team might consider in very remote locations. A whole lot more than just a FFM is involved though.
- Facial deformation or neurological disorders that make using a mouthpiece impractical — useful for divers with certain disabilities.
- Although it is rare, personal preference. There are pros and cons but the cons add up to a lot of complexity, especially when things go wrong.