Answer to Question #13595 Submitted to "Ask the Experts"

What period should be considered after which a personnel monitoring device like TLD can be declared as lost? In case of a TLD lost and found after some time lapse, should that TLD be processed and read? How should exposure be assigned to the worker?

While I am not aware of any mandatory criteria regarding specific times are concerned, the reality is that as soon as a dosimeter that is used for personal dose estimation as part of the permanent record of occupational exposure is found to be missing, and the affected worker will be continuing with his duties involving potential exposure, he/she should be issued a temporary badge. My own preference, when I was radiation safety officer at a university, was to issue the spare badge as soon as the worker realized he/she could not find his/her badge and to declare the original badge "lost" if the usual time for routine processing arrived, and the badge had not been found. The temporary badge would be worn at least until the original badge is found or the usual time for processing of badges has arrived. Some dosimetry vendors have their own operational definition of when a badge is considered as "lost"; an elapsed time of 90 days beyond the last date that the badge was to be worn before being processed is common. From a vendor's viewpoint this may be reasonable, considering the vendor's ability to provide an acceptable evaluation from the dosimeter reading, but it is too long, in my opinion, for the users, considering their needs for a reliable ongoing record of doses to workers.

If the original badge is found, but its history while missing (locations and possible effects of influencing factors, such as ionizing radiation exposure, heat, moisture, mechanical damage, etc.) cannot be confidently determined, the worker should continue to wear the temporary badge until the usual processing time arrives. The vendor should be made aware of the situation when both badges are sent for processing so that possible adjustments may be made if necessary. If a badge has been missing for more than a month, I do not believe it is worth processing unless you can confidently determine the badge's whereabouts during that time and that its exposure to ionizing radiation or to influencing environmental and other factors were insignificant. It is sometimes acceptable to use the response from the spare badge that was issued to prorate the dose received during the entire wear period. At times it can also be justified to use coworker doses for the same period to generate a suitable dose for the affected worker. Clearly, this works best for workers doing very similar jobs and whose dose histories are similar,

If the lost badge is found and processed, the magnitude of the dose(s) to be assigned to the affected worker will depend on what readings are obtained from the lost badge and the determined history of the badge while missing. If the badge was missing for a relatively short time, perhaps a week or two, the dose results of processing appear typical for this worker based on the period worn, and the worker can confirm that his work pattern and likely exposure were typical of his usual behavior, then you may be able to use the readings from the badge for the period it was worn and reasonably assign a dose to the worker for the period when the badge was missing and he/she was not wearing a spare badge, if such was the case. As long as the worker's recent badging history demonstrated a reasonably consistent pattern with no unusual elevations in doses, this dose estimation could be based on the average dose received for a usual wear period, based on the last several months of readings. If the worker's past history showed a number of incident of elevated exposures, then it may be preferable to assign a dose for the entire normal wear period of the badge based on the highest dose received in a wear period over the previous several wear periods. These decisions must be made based on all the available information, including the worker's past history and his/her recollections of his/her potential exposures.

When the processed badge results reflect significantly higher doses, or in some cases lower doses, than are typical for the affected worker, then the recollections of the worker become more important. Typically, the longer a badge has been missing, the more difficult it will be to reconstruct the worker's behavior and potential exposure during the period when the badge was missing. This constitutes a strong argument for letting workers know that (1) they must be fastidious about wearing their badges whenever they are on the job and letting the person in charge know as soon as they find their badges are missing and (2) they must always leave their badges in an assigned and safe location when their work day is over.

There have been numerous cases of a worker leaving his/her badge in a location where nearby sources could potentially expose the badge; unfortunately, there have also been some instances of intentional exposure of badges. It may then become difficult to resolve what has happened by talking with the affected worker, and decisions may have to made based on as much other information as can be obtained. In instances when a dosimeter has been exposed to a single fixed source it may be possible for the dosimeter vendor to obtain information about the nature of the exposure. For example, there are some current dosimeters, such as some OSL devices, that allow the processor to develop the optical image associated with the readout process in a fashion to obtain additional information. A single source, fixed exposure may yield a sharper image than an exposure associated with either multiple sources or with a worker moving around during exposures. Such information can be very helpful in resolving some cases of intentional exposures of badges. Of course, this information is not always available and responsible parties must do whatever detective work they can to attempt to ascertain the history of a dosimeter while it was missing or when it may not have been missing but registered an abnormal dose.

I hope this has been helpful to you.

George Chabot, CHP, PhD