The search function will throw up several discussions on clean shaven policy e.g.

http://forum.iosh.co.uk/posts/t124848-Clean-shaven

http://forum.iosh.co.uk/posts/t119945--Clean-Shaven--requirements-for-RPE

Ok- the law COSHH requires that employers identify and assess risks and devise controls to limit exposure to hazardous substances.  It is quite possible that having done this the employer may decide that to the most practicable way to prevent exposure to a particular substance is for employees to be required to wear a negative pressure face mask.

The employee has a duty under the law(various references notably Section 7 Health and Safety at Work Act) ) to cooperate with their employer, so if the boss says you must wear a face mask you must wear a face mask.

COSHH also requires that if a control is adopted then the employer must make sure that it is working correctly. The HSE suggest (very strongly) that the best way to do this is by applying a prescribed face fit test regime.

It is well established that face hair reducing the effectiveness overall of RPE and that it makes any assessment of effectiveness unreliable since the amount of hair can very day to day.

This leaves the employer with 3 choices:

1. Apply the rules strictly- if you have a beard you cannot have a face fit test, no test no mask, no job.
2. Look at using RPE that  does not require a face fit test –positive pressure equipment
3. Cross your fingers and hope for the best but the employer should note that of course if there is evidence that an employee has suffered harmful effects due to exposure to airborne substance and there was no adequate face fit testing regime they will be liable both criminally and civilly. Employees should also note that if they decide to work without allowing proper face fit testing they might lower out on any compensation due to contributory negligence.