You mean something like this?
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There has been a study where 3 out of 4 successfully ctb with these kind of a mask within 15 min. Unfortunately for the last subject it took over 30 min because of air leaking in and prolonging the ordeal. If the leak is big enough it could prolong it even further, possibly causing the gas to run out and leaving you grave brain damage.
These mask are fundamental not suited for our needs since they're design to let a small amount of air to enter for safety measures. If the oxygen were to run out in a hospital setting the patient would suffocate, so they designed it to not be completely air tight. The mask are also one size fits all kind, so fitting the mask securely can be an issue.
The whole idea of the inert method is to keep the oxygen level below 5% preferably even lower, using these kinds of mask is already setting you up for failure. Only a tiny airleak is sufficient to raise the oxygen to survivable levels.
Most users and publications would advise against these kind of mask, and so would I.
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