Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
fundamental assumption made in calibrating a lung measurement system is that the
deposition of radioactivity in the lung is homogeneous, but depositions rarely follow
this pattern. The distribution of the particles in the lung is a function of particle size,
breathing rate, and health of the subject (Kramer and Hauck, 1999; Kramer et al,
2000).
(364) Measurement errors associated with counting statistics (Type A uncertainties)
decrease with increasing activity or with increasing counting time, whereas the Type
B components of measurement uncertainty may be largely independent of the activity
or the counting time. Therefore, when activity levels are low and close to the limit of
detection, the total uncertainty is often dominated by the Type A component (i.e. by
counting statistics). For radionuclides that are easily detected and present in sufficient
quantity, the total uncertainty is often dominated by the Type B components (i.e. by
uncertainties other than counting statistics).
6.5.2 Uncertainty in the Exposure Scenario
Time of Intake
(365) The uncertainty in the time pattern of intake can be the dominant source of
uncertainty in the estimated dose, or it can make or little or no contribution to it. For
example, if an intake is not recognised for some time after an incident and total body
retention and urinary and faecal excretion rates diminish quickly, the assumed time
pattern of intake could be the dominant uncertainty in the dose estimate. On the other
hand, if a worker is exposed in the vicinity of an immediately recognised accidental
release, or total body retention and excretion rates are fairly constant, the time pattern
of intake may be a negligible source of uncertainty in the dose estimate.
(366) In the case of routine monitoring, the intake can be assigned as being at the
mid-point of the monitoring interval, or the intakes corresponding to each possible
intake time can be calculated and then averaged. Either method may result in a large
uncertainty in the dose estimate. Puncher et al (2006) and Birchall et al (2007) argued
that intakes estimated from either of these methods have a tendency to overestimate
the true intake and showed that an intake obtained assuming a constant intake rate
throughout the monitoring interval (i.e., constant-chronic method) is an unbiased
estimate of the true intake when the measurement and the excretion/retention function
are accurately known or when they are uncertain but unbiased (i.e., the mean of the
distribution describing the uncertainty is the true value). If the uncertainties in the
measurement or in the excretion/retention function are affected by a bias, the
constant-chronic method produces a biased result, but the bias in the result can be
eliminated by the use of appropriate adjustment factors (Birchall et al, 2007).
Route of Intake
(367) In practice one may encounter situations when the mode of intake is unknown
and cannot be easily discerned on the basis of health physics records or available
bioassay data. For example, it may not be known if the intake took place by inhalation
only, by ingestion only, or by a combination of inhalation and ingestion. Even if it is
known that a combination of inhalation and ingestion occurred it may be impossible
to determine what fraction of activity was inhaled and what fraction was ingested. In
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