Appreciate your question is slightly differently worded to the usual RPE / Facial Hair enquiry but plenty of threads that may supply your answer, including:

http://forum.iosh.co.uk/posts/t126600-facial-hair

http://forum.iosh.co.uk/posts/t125192-Facial-hair-and-FIT

http://forum.iosh.co.uk/posts/t122830-Face-Fit-Testing-and-facial-hair

Remember you get the RPE to fit the individual not make the individual fit the RPE - there are options which do not require a tight face seal

I am presuming that even with engineering controls in place there is a requirement for wearing RPE, so there are other types of RPE you can select which can be worn without having to be clean shaven e.g. hoods etc. You could go down this route. However, this normally increases costs as the RPE tends to be more expensive.  

It is a legal requirement to wear tight fitting half and full masks for certain activities and with that comes the legal requirement in order to achieve a good seal that the wearer is clean shaven (COSHH Regulations). In these cases you should have a robust policy, procedures including risk assessment of the activities which clearly explain the requirement to be clean shaven.

Remember employees also have a duty of care to ensure they are following training instruction and utilising RPE as intended etc. to assist employers with complying with their legal duty.

If you have the odd employee who is refusing then simply they cannot do the assigned tasks safely and you should look at assigning them somewhere else. If this an issue and your orgnisational size/tasks do not allow this then they should not be working for you in those environments. Ultimately and this sounds brutal, if it is a legal requirement and your company policy to comply with the law then they have a choice plain and simple, either comply with the requirements or look elsewhere for work.

You would not tolerate them not wearing a seatbelt when driving on work business, the principle is the same.

Edited by user 21 November 2018 13:32:03(UTC)  | Reason: Typo

You also need to consider whether someone who has a tight fit at the beginning of a shift will still have one a few hours later.

There was research ages ago to indicate that facial growth during a shift negated the efficacy of RPE that is dependent on that tight fit.

Which means considering an alternate type of RPE which is not reliant on passing face fit test, which then might deal with your current HR issue.

I can't find any legal requirement for face fit testing per se - am I missing something? ACOPS supporting the COSHH and CAW Regulations recommend that the initial selection of tight fitting facepieces should include a fit test, and I accept that RPE - if required which surely is key - must be 'adequate / suitable' etc.

One of my bugbears is our construction clients insisting on RPE - against our own thorough RA - when a small amount of drilling creates some dust which is collected and contained as part of the SSW (think drilling a few holes in plaster). If they insist on RPE, then FFT then also comes in - but we don't think RPE is needed in the first place. Now the job becomes unviable.

Which is why I was interested to see the discussion on HSE construction forum - is there really value in FFT of RPE for 'dust' where exposure is limited to say the least? I have no argument if exposure is to a high hazard substance, or for extended lengths of time or volumes. Controversial I know.

Face fit testing is required under regulation 9 (Maintenance, Examination and Testing of Control Measures) of COSHH which requires you to prove that any control that are adopting as based on YOUR risk assessment actually works. Note that the risk assessment is YOURS not what the bloke on the site office thinks.