Controlled Area is defined in the Ionising Radiations Regulations 2017 (IRR17). A Controlled Area is an area where any person is likely to receive more than 6 mSv Effective dose (greater than 15mSv to the lens of the eye, or 3/10 of any other UK Dose Limit) and / or an area where specific and detailed procedures need to be followed in order to restrict exposure from Ionising Radiation and ensure that doses are ALARP. The area can also be designated on the basis of dose rate, such that a Controlled Area will generally be required where the dose rate exceeds 7.5 micro Sv/h when averaged over a working day. Conversely, an area may not need to be designated as Controlled where the dose rate is less than 7.5 micro Sv/h averaged over the working day, and where the instantaneous dose rate (IDR) does not exceed 100 micro Sv/h. However, where high IDR dose rates arise careful consideration of the ALARP concept is required.

The above concise definition avoids over complicating this glossary entry, a more detailed explanation can be found here: IRR17 (17) - Designation of controlled or supervised areas . There are a number of important caveats and conditions and the reader is encouraged to visit this additional resource.