Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
6.3 Understanding Exposure Situations
(314) Workplace information should be gathered in order to understand the
exposure situations, e.g. radionuclides that may have been incorporated (including
equilibrium assumptions for the natural series), chemical form, presumed particle
size, likely time, pattern and pathway of any intake.
6.3.1 Time(s) and Pattern of Intake
(315) A principal source of uncertainty in the interpretation of bioassay data is the
assignment of the time(s) and pattern of intake. Since the bioassay function that gives
the predicted measurement depends on the time since the intake it follows that the
estimate of intake will vary, depending on when it is assumed the intake took place.
Consideration should be given to different possible patterns of intake, such as a single
contamination event, several individual events during the monitoring period, intakes
lasting a short period of time or chronic intakes.
(316) Where chronic intakes are expected, an assessment should be made as to
whether the working schedule should be taken into account when selecting the time of
measurement (or sampling), and interpreting the results from bioassay monitoring.
For elements where a fraction of the intake is rapidly excreted, the times and duration
of periods when no exposure could take place such as the weekend, days off or
vacations could strongly influence the assessed intake. With the exception of short
half-life radionuclides, the selection of a measurement or sampling time immediately
following such a period will reduce the uncertainty in assessed intake associated with
rapid excretion.
(317) For routine monitoring, when chronic exposures are not expected, it is
necessary to estimate an intake from a measurement made at the end of a monitoring
interval, often without knowing the time of intake.
(318) When a positive measurement appears from a routine bioassay programme, a
the review of workplace monitoring data, such as airborne or surface contamination
levels, can indicate a likely time for the intake to have occurred. Similarly, if other
workers in the same workplace have exhibited positive routine bioassay samples, a
review of the data and monitoring schedules for those individual workers will help
determine the time of intake for all. Workers interviews should elucidate whether if
an incident, an unusual procedure or equipment failure could have led to the intake.
Follow-up bioassay should be scheduled to confirm the positive measurement. When
several bioassay results are available, perhaps including different types of
measurement, a comparison of these results with the intake retention fractions tables
may help in narrowing the choice of the time the intake occurred.
(319) Another approach has been described by Miller et al (2002) in which
Bayesian-based dosimetry calculations are performed using a Markov chain Monte
Carlo algorithm. This method, which analyzes all available bioassay data
simultaneously, determines probabilistically the number, magnitude and times for N
possible intakes using a previously agreed set of biokinetic models. The Weighted
Likelihood Monte Carlo Sampling (WeLMoS) method is another Bayesian technique
(Puncher and Birchall, 2008). In this approach, biokinetic model parameters and times
of intake are sampled from probability distributions that express the state of
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