Rank: Forum user 
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I understand that either a disposable p3 mask or a half mask with a p3 filter has a protection factor of 20 and a full face mask has a protection factor of 40. Although i have worked and trained with these types of mask for many years i still dont undestand why there is a differance in protection, given that both types of mask should be facefit tested and only protect the nose and mouth. Am i missing something obvious?
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Rank: Super forum user 
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As far as I recall it is to do with the usage and construction of the masks. The full face is more likely to be worn properly thus giving better quantified protection. Its the 'Applied Protection Factor', not the theoretical maximum that the device will give by design. See HSG53, available free at: http://www.hse.gov.uk/pubns/books/hsg53.htm
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Rank: Super forum user
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Pretty much what SNS says, the protection factors are a combination of both the efficiency of the filter as well as the risk of "inwards leakage". The risk of inwards leakage on a full face mask is lower - the ability to get a better seal is greater, thus a higher level of protection is possible than with a half mask The same is for powered respirators which also use a P3 filter, these are also able to provide a proteciton factor up to 40 as there is a lower risk of inwards leakage
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