Rank: Forum user 
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what do we mean by APF (assigned protection factor) and NPF, for RPEs?
Regards,
P
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Rank: Super forum user
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Hi, prads
In the UK, we used to use nominal protection factors (NPFs) to help people decide what type of RPE to use in a particular situation. However, they were ridiculously optimistic and were replaced by assigned protection factors (APFs) some time ago. The APFs represent a theoretical estimate of the level of protection offered by a piece of RPE but these days we have to confirm the suitability of the RPE for a given wearer by fit testing.
LB
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Rank: Forum user
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Hi LB,
Thanks for your reply. But what does the factor indicate, when we say the RPE has say an APF of 50 ppm?? This figure is calculated based on what?
Regards,
P
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Rank: Super forum user 
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The factor isn't 50ppm, but just 50 - it means that, if the maximum level someone should be exposed to (in UK the Workplace Exposure Limit [WEL]) is say 5ppm, that item of respiratory protective equipment will be OK for atmospheric levels up to 5 x 50 = 250ppm.
But that's a 'theorectical' factor so, as leadbelly says above, current practice in UK is to do a face-fit test on everyone and to also train them how to fit the item properly and do a basic self-check - in that way the NPF/APF is likely to be a real protection factor!
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Rank: Forum user
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Thank you, imwaldra!
Is there a minimum APF requirement for RPEs for different hazardous substance?
Also, what does NPF indicate?
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Rank: Super forum user
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The protection factor depends on the filter and mask type i.e.:
Half mask
p1 = 4
p2 = 10
p3 = 20
Powered respirators are described differently as:
TH2 = 20
TH3 = 40
Full face
P3 filter = 40
Self contained positive pressure breathing apparatus = 2000 (I think)
There are no specific masks types for different hazardous substances (though there are some exceptions) what you do is choose the correct mask for the concentration your employees are exposed to. This mask choice after engineering controls etc is the additional control required after all other controls to bring the exposure to below the maximum permissable exposure limits.
Worked example:
Workers on an oil rig where monitoring has shown that ambient air levels of benzene = 15ppm - exposure limit = 1ppm, there a protection factor of 15+ is needed to reduce their exposure to an acceptable level.
Exposure limits can be found in the HSE's EH40 although there are not always exposure limits for every chemical!
Des
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Rank: Super forum user
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Oh qualification of my original statement - although there are no mask types for specific substances you DO need to ensure that the filter attached to the mask P = particulate, however you can also get A, AX, B, E, K, Hg, CO, NO and maybe more filters.
Useful guide here:http://www.draeger.com/media/10/01/09/10010967/guide_for_selection_and_use_of_filtering_devices_9045782_en.pdf
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Rank: Super forum user 
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HSE guidance HSG53 sets out a process for selecting RPE and clearly explains all the terms.
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