The control of Noise at Work Regulations (2005) lays out three levels of responsibility. When working in noisy environments you must be aware of:

First Action Level - 80dB(A).
The regulations state that all employers must provide suitable hearing protection where noise levels exceed 80dB(A). It is the worker's choice whether to wear hearing protection between 80dB(A) and 85dB(A), however, continued exposure to noise above 80dB(A) can cause permanent hearing damage.

Second Action Level - 85dB(A).
At levels above 85dB(A) hearing protection is mandatory.

Maximum Exposure - 87dB(A).
The maximum exposure limit; 87dB(A) must never be exceeded even when hearing protection is worn.

The European Standards you need to be aware of are:
EN352-1 Standard Ear Defenders
EN352-2 Ear Plugs
EN352-3 Helmet Mounted Ear Defenders.

Additional Information
Choosing the right hearing protection