Employees who are likely to receive an effective dose of more than 6mSv per year, or an equivalent dose which exceeds three-tenths of any relevant dose limit should be designated 'classified persons' under The Ionising Radiation Regulations 1999. 

The dose limits are much less for persons under the age of 16 and less for persons under the age of 18 and for women of reproductive capacity.  Special rules apply to pregnant women and women who are breastfeeding.

Classified workers must have health surveillance by an appointed doctor.  The detailed requirements can be found in the Health and Safety Executive (HSE) approved code of practice and guidance L121 'Work With Ionising Radiation'.