A couple of things....
1) the level of attenuation achieved with a custom ear mould can vary between ~ 20dB and 40dB. This is a worthwhile reduction, and allows the sound of the IEM mix to be heard over the stage sound, but it may not reduce the background noise to a "safe" level. Of course, if all musicians are on IEMs, then the stage level may be pretty quiet, and the external noise level isn't then a problem. IEMs should allow the musician to monitor at relatively lower levels, so a combination of
some isolation and (hopefully) lower listening levels should achieve the improvement WAL mentions.
However, if generic buds are being used, the degree of isolation realised can be much less. Also, some bands use a mix of IEM and wedges - leading to potentially high background levels that then require high IEM levels. Furthermore, if the musician wishes to monitor at high levels (whatever else might be going on), there is little to stop him or her doing this. It is considerably easier to achieve high sound pressure levels at the ear drum using a tight fitting efficient earpiece than it is to achieve the same SPL from a stage monitor.
Therefore, there is a risk arising from prolonged use of IEMs at high listening levels.
2) In answer to the OP, if the user is "at work" in this country, then
yes, use of earpieces, headphones, IEMs etc. are subject to the Control of Noise at Work Regulations 2005. These have been discussed at some length on these pages.
Edit... Of course all noise exposure at work (including playing / working on a stage with loud wedges or mixing FOH) is also subject to these regulations!!The problem is that although it is fairly easy to measure exposure on stage, it is harder to do this for IEMs. The only acceptable measurement would be to use an earpiece fitted to a
measurement mannequin, determine the typical output, and then work out the user's exposure over a working day.
Given that an IEM user could listen at an average of 85dB(A) for 8 hours a day, 5 days a week and be within the CONAWR limit, means that these devices are very useful.
If the user wants to listen at higher levels, then the time / intensity trade off means halving the exposure time for every 3dB increase LEP,d.
The CONAW regulations
do not state that IEMs must be limited in output. However, they do require that you assess the exposure level and the risk to hearing, and take appropriate action to minimise such exposure and risk. The BBC's approach to this was to fit inline limiters to headphones made available for staff.
Overall, IEMs have the potential to improve monitoring clarity and reduce noise exposure, but like most things in life, they have to be used responsibly.
Simon