IRR17 (21) - Designation of Classified Persons

An employer is required to designate a person as being classified if that person is likely to receive an effective (whole body) dose in excess of 6mSv/y, or more than three-tenths of the dose limit to the extremities (150 mSv/y). Furthermore, classified worker status will be required where 75% of the dose limit to the lens of the eye is likely to be exceeded (15mSv/y). Designation will be based on external exposure, internal exposure or both depending on the nature of the ionising radiation source.

Designation should be justified solely on the basis of an assessment of potential dose and not on the basis of previous dose history. Furthermore, medical exposure history should not be taken into account.

In deciding what is likely, the employer needs to consider routine occupational radiation exposures, as well as exposures from reasonably foreseeable incidents. This requires a careful assessment of the radiation work, likely exposures should be recorded in the radiation risk assessment, see IRR17 (8) Radiations Risk Assessment.

High hazard radiation sources

Anyone working with high hazard sources of ionising radiation, e.g. capable of delivering an effective dose greater than 20 mSv, or an equivalent dose in excess of a dose limit (e.g. > 500 mSv to the hands) within a few minutes, should be designated as classified. This should be on the basis that this occurrence is a reasonably foreseeable radiation incident.

Site industrial radiographers would for example be designated as Classified Persons as a matter of course since such persons have the potential to receive a significant exposure very rapidly should an incident occur which is incorrectly dealt with.

Example (1)

An industrial radiographer is undertaking site radiography using a 740 GBq (20 Ci) Ir-192 source. The source becomes detached from its control wire and the individual uses poor practice by grabbing the source briefly and placing it into an emergency lead pot. The dose rate at 1cm from the source will be of the order of 233 mSv/second, so is a good approximation of hand (extremity) dose rate. Therefore, just over 2 seconds of exposure will lead to an extremity dose exceeding 500 mSv. Whilst this is an extreme example, and no trained radiographer should be expected to do this, it is reasonably foreseeable, and that individual should be a classified person.

Example (2)

A hospital uses a linear accelerator (linac) in a concrete bunker for the purposes of cancer treatment (radiotherapy). The headline dose rate from the linac is 6Gy/min at 1m. For the purposes of this example, this is equivalent to 6 Sv/min at 1m.

Linear accelerator (linac) in a radiotherapy shielded bunker
Linear accelerator (linac) in a radiotherapy shielded bunker

The hospital and their RPA need to determine if the radiotherapy staff setting up patients and operating the linac from the control room need to be classified. Exposures outside the bunker are so close to background as to be ignored. To leave the bunker the hospital staff need to press a ‘last person out’ button and then confirm exit when leaving the bunker (via a maze or shielding door). This procedure, active signage (e.g. “Beam On) inside the bunker, and emergency-off buttons are designed to ensure no person, other than a patient undergoing treatment, could be exposed. However, suppose despite all these features, a staff member is left inside the bunker and the linac starts producing radiation? It is not reasonably foreseeable that a trained operator would align themselves with the collimated linac beam. It is, at a push, reasonably foreseeable that they might be elsewhere in the bunker and receive an exposure for a short duration (say 5 seconds) before pushing an emergency stop.

Dose rates within the bunker, outside the confines of the primary beam, will be in the order of 100’s micro Sv/h to 100 mSv/h. Taking the 100 mSv/h as an upper case, that is around 2mSv/ minute. Even at this high dose rate, it is not foreseeable that a person would remain in this area for more than a few seconds before stopping the linac and / or leaving the bunker (which would break interlocks and stop the linac). Therefore, it is common practice in this work sector that radiotherapy staff are not classified radiation workers since:

(a) Routine exposures will be very near background exposures.

(b) Reasonably foreseeable incident exposures will not breach the classified worker limits (see above), and in any case, would not deliver a radiation dose approaching an annual dose limit over the short period of an exposure.

Others reasons for classified persons

Some radiation employers tend to classify certain employees based on their position in the company (e.g. RPS), or for reassurance purposes. There is nothing particularly wrong in this, but it should be remembered that like designated areas, the administrative burden is much higher (formal dose records, health reviews etc).

Classified persons requirements

For a person to be designated as a classified person they must be at least 18 years old, and a relevant doctor (Appointed Doctor) needs to make an entry into the person's health record stating they are ‘fit’ to be classified.

Employees must be notified immediately on designation and cessation of designation (in writing). Cessation is only permitted at the end of a calendar year unless:

  • an Appointed Doctor requires this
  • the employee is employed for the remainder of the year, but in a capacity not likely to result in significant exposure (i.e. > 1mSv year)

Other matters which need to be considered include:

  • Employees or other persons who are not classified may only enter a controlled area under written arrangements (these will normally be included in the local rules).
  • Where female employees are involved, the employer should be aware of the special restrictions or potential changes to working practice once pregnancy is declared, and if breastfeeding (where unsealed radioactive materials are involved). This should be specified in local rules.
  • The employer should be aware that their classified employees will require pass books to enter the Controlled Areas of another employer (where they will be designated classified outside workers). This is particularly relevant for cyclotron service engineers who will be classified person (although if they take the controlled area over during service they are not outside workers).

In radiation protection I spend most of my time explaining to clients what they do not need to worry about.

– Mark Ramsay -