Occupational dose limits for radiation workers follow ICRP Publication 103 and are given effect in IAEA GSR Part 3 and in national regulations. The effective dose limit is 20 mSv per year averaged over five consecutive years, that is 100 mSv in any five-year period, with the further condition that no single year may exceed 50 mSv. The averaging exists so that an unusual outage year does not by itself breach the limit, but the five-year total is a hard cap and the single-year ceiling stops the whole allowance being spent at once.

Limits are not targets. The optimisation principle requires dose to be kept as low as reasonably achievable below them, and employers implement that through a dose constraint – a self-imposed ceiling, commonly 6 mSv per year for industrial radiographers, that triggers investigation long before any legal limit is approached. In practice a well-run gamma crew records 1 to 3 mSv a year, so a constraint being approached is a signal that something in the working method has changed.

Projecting the year-end dose from the year to date is the routine control. Scaling the accumulated dose by the fraction of the year elapsed gives an early warning while there is still time to redistribute work, review techniques or increase barrier distances. The same arithmetic on the five-year running total shows whether an individual is on course to hit the 100 mSv cap.

Separate limits apply to the lens of the eye (20 mSv per year) and to skin and extremities (500 mSv per year), and different, much lower limits apply to members of the public and to declared pregnant workers. Those are not covered here – refer them to the radiation protection adviser.

Worked example

Dose this year to date	4.2 mSv
Months elapsed this year	5
Cumulative dose in the five-year period	31 mSv
Years elapsed in the five-year period	2.5 y
Employer dose constraint	6 mSv
Annual average limit	20 mSv
Five-year total limit	100 mSv
Projected dose this year	10.08 mSv
Remaining to the annual average limit	15.8 mSv
Remaining in the five-year period	69 mSv
Percent of annual limit used	21 %
Percent of employer constraint used	70 %
Five-year average to date	12.4 mSv
Remaining monthly budget to stay under 20 mSv	2.26 mSv

4.2 mSv in 5 months projects to 4.2 x 12/5 = 10.08 mSv for the year - above the 6 mSv constraint but below the 20 mSv limit. 15.80 mSv of the annual allowance and 69.00 mSv of the five-year allowance remain. The five-year average so far is 31.0/2.5 = 12.40 mSv per year. Spreading the remaining 15.80 mSv over the remaining 7 months gives 2.26 mSv per month.

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