I would start by doing a quantitative face fit test which doesn't rely on the operatives sense of taste/smell.

I would then make sure the operative is well trained on wearing the RPE correctly and user checks.

Thinking of it logically how do we know if a mask is malfunctioning for hazards such as asbestos where a sense of taste or smell cannot indicate any breakthrough? Is breakthrough achieved from a mask malfunction well before the indicator of taste or smell kicks in anyway?

Whether the person has a sense of smell is not relevant. As post 2 pointed out the majority of hazards that RPE protects against cannot be detected by smell or taste anyway (or if you can detect them it is too late). This is also why face fit testing for RPE is important, in that it gives you assurance that the mask is working. Of course as he cannot taste or smell you cannot use the more common qualitative and you will need to use the slightly more elaborate quantitative method based on counting particles in the air vs those in the mask but that is a reasonable approach.
If you are looking to dismiss the guy because he has no sense of smell, I think that your HR people will find that difficult, as I would suggest that providing him with regular face fit tests is what you be doing anyway.

Thanks for your comments. We know and have every intention of going down the quantitative fit test. However my concern remains - what happens should the mask malfunction?

The products we predominantly use can in 95% of cases be detected by smell or taste so therefore the lack of senses are not irrelevant. If the mask should fail - which can happen - and he doesn't detect anything and then suffers, becomes ill etc because of it where would we stand should there be an HSE investigation - no doubt hung out to dry because we were aware of his amosnia.

n3iloo you quite rightly say that a quantitative will ensure a good seal but if the mask failed his senses would indeed be relevant. It happened to us very recently onsite where one of our guys was working in a very confined area and it was very hot. He rubbed his face and hadn't realised he had dislodged his mask but he immediately knew he had a problem as he could smell/taste what was being given off by the product. If that were to happen to our anosmia guy he wouldn't have known he had a problem.

Apart from a quantitative test what other options are there or am I the only person perceiving a problem?

Reading this it seems to me that you may have more than a problem of the operatives lack of senses, i.e. confined space working with a hazardous substance and RPE not being suitable for the task i.e. it gets 'dislodged' with it failing to danger

RPE/PPE shouldn't be used as the first line of defence. Engineering controls, LEV etc should all be used first if you cannot find alternative methods to achieve the same.

It sound like you may be using RPE as the only method to ensure safety.

Nederman (& Others) now do portable LEV units if a permanent one is not suitable. RPE and face fit would still be relevant but would then be the back up system.