Radiation dose record and five-year headroom

The occupational effective dose limit is not a single annual number. It is 20 mSv per year averaged over five consecutive years, that is 100 mSv in any five-year block, with the additional constraint that no single year may exceed 50 mSv. The averaging exists so that a legitimately heavy year — a shutdown, a long pipeline campaign — does not by itself end a radiographer’s career, while the 50 mSv cap stops the whole five-year allowance being taken in one go. Both conditions apply at once, and the tighter of the two is the one that binds.

The consequence for a working radiographer is that the number to watch is not this year’s dose but the five-year running total. A technician at 8 mSv per year is comfortable on any single-year test yet is using 40 % of the five-year allowance every year, and a single 25 mSv shutdown on top of that puts the block over 100 mSv even though no individual year came close to 50. Employers therefore set investigation and dose-constraint levels well below the limits — commonly a few mSv per year — and ALARA obliges the reduction of dose below the limit wherever it is reasonably practicable, not merely up to it.

Effective dose is a whole-body quantity. Separate equivalent-dose limits apply to the lens of the eye and to skin and extremities, and they are not covered by the whole-body total: a radiographer handling a source guide tube can reach an extremity limit long before the whole-body figure looks interesting. Where source handling or hands-in-beam work is routine, extremity dosemeters are the only way to know.

This calculation is a planning and cross-check tool. The dose of record is the figure issued by the approved dosimetry service from the technician’s personal dosemeter, and it is that figure that goes on the statutory record. Use this to see where the five-year block stands before accepting the next job, not to replace the dose report.