The 6th edition of the ACoP for COSHH covers this.  Parafraph 237 indicates where skin health surveillance is required:

Examples where health surveillance is appropriate under the criteria in regulation 11(2)(b) are:

where there have been previous cases of work-related ill health in the workforce/place;

where there is reliance on PPE, eg gloves or respirators, as an exposure control measure; eg printers wearing gloves to protect against solvents used during press cleaning, or paint sprayers using two-pack paints wearing respirators to prevent asthma. Even with the closest supervision there is no guarantee that PPE will be effective at all times;

where there is evidence of ill health in jobs within the industry; eg frequent or prolonged contact with water (termed ‘wet-working’) causing dermatitis in hairdressers and healthcare workers, or breathing in mists from chrome plating baths causing chrome ulcers in platers.

Paragraph 238 amplifies this:

This is not a definitive or exhaustive list and there will be many other instances where health surveillance is required. Employers will need to seek information or advice on the specific health risks identified in the risk assessment, or through any topic-specific HSE guidance, trade associations or other professional sources.

Note the last paragraph and think carefully about its implications!

Hope this helps

The bit in the ACoP that concerns me is the sub-paragraph that starts: 'where there is evidence of ill health in jobs within the industry...'  Taken at face value this appears to suggest that even if you are confident that your LEV systems are providing 'adequate control' as required by COSHH if others are having problems you should be doing health surveillance. If this is correct this really does shift the goal posts!