RIS-Skin Dose limit

["Michael Stabin" <michael.g.stabin@vanderbilt.edu>]

The following from Alan Roecklein <AKR@nrc.gov> Please respond to him with

any questions or comments.





> The attached Regulatory Issue Summary was signed out by the NRC 7/9/02.





UNITED STATES NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS



July 9, 2002





NRC REGULATORY ISSUE SUMMARY 2002-10

REVISION OF THE SKIN DOSE LIMIT IN 10 CFR PART 20





ADDRESSEES



All U.S. Nuclear Regulatory Commission (NRC) materials licensees.



INTENT



The NRC is issuing this Regulatory Issue Summary (RIS) to inform its

licensees of changes in the regulatory dose limit for the skin in 10 CFR

Part 20, effective as of June 4, 2002.  No specific action nor written

response is required.



BACKGROUND



The dose to the skin, referred to in 10 CFR Part 20 as the shallow dose

equivalent Hs, was formerly defined as the dose equivalent at a tissue depth

of 0.007 centimeters (7 mg/cm2) averaged over an area of 1 cm2 (10 CFR

20.1003).  Such a dose may result from exposure to an external radiation

field or from contamination on the skin or on protective clothing.  The

limit specified in Part 20 for Hs is 50 rem (0.5 Sv) [§20.1201(a)(2)(ii)].

In addition to the 50 rem (0.5 Sv) limit, Part 20 also required that the

assigned dose equivalent must be for the part of the body receiving the

highest exposure [10 CFR 20.1201(c)].



In enforcing this requirement, a distinction was made between exposure of

the skin resulting from hot particles and exposures resulting from other

sources of radiation.  Hot particles are very small, barely visible,

particles of radioactive material that are often electrically charged, and

tend to cling to the skin or to protective clothing.  Some of these

particles have a high specific activity, and are capable of delivering

shallow dose at a rate that could cause the shallow dose equivalent limit to

be exceeded within a few hours or sometimes in less time.  Hot particles

have been found at both nuclear reactor facilities and at nuclear materials

facilities.



NRC funded research that showed that the doses resulting from these hot

particles posed little risk to the exposed person's health, and that

exceeding the skin dose limit of 50 rems (0.5 Sv) did not pose a health

hazard, nor was it of a safety significance that is normally associated with

exceeding the corresponding limits on whole body or organ doses.  To

alleviate the regulatory burden resulting from this lack of correspondence

between the dose limit and the associated risk, NRC implemented a temporary

enforcement discretion policy in 1990.  According to this policy, exceeding

the shallow dose equivalent limit of 50 rem (0.5 Sv) was still to be

considered a violation of a  regulatory limit.  However, the enforcement

actions that would normally be taken when a regulatory limit is exceeded

were not implemented until the exposure from the hot particle exceeded 75

?Ci-hr, which corresponds to a shallow dose equivalent of about 300 - 500

rem (3 - 5 Sv), the exact value being dependent on the energy of the beta

radiation emitted by the hot particle.  Exposures from sources other than

hot particles were not subject to this enforcement discretion policy.

Licensees were still required, under this discretion policy, to report any

dose above 50 rem (0.5 Sv) shallow dose equivalent, to the NRC, as an

exposure above the regulatory limit.



Because workers who exceeded any NRC regulatory limit are required to stop

working in any activity that might result in additional occupation radiation

exposure, there have been cases in which radiation workers had to stop their

radiation work after a hot particle exposure that resulted in exceeding the

50 rem (0.5 Sv) limit.  This meant that these workers had to be re-assigned

to non-radiological duties for the remainder of the calendar year, if such

re-assignment were possible within the worker's organization.  Similar

situations occurred in medical settings, where a drop of very

high-specific-activity radiopharmaceutical could have the same effect as a

high-activity hot particle.  Again, interruption of a radiation worker's

duties was viewed as being an action that was not commensurate with the risk

resulting from the exposure, but the rules were written in such a manner

that there was no alternative course of action other than exemption of the

licensed activity from the rule.



SUMMARY OF ISSUES



The revision to the shallow dose equivalent limit was accomplished by making

two changes to Part 20: (1) The averaging area of 1 cm2 was removed from the

definition of shallow dose equivalent in 10 CFR 20.1003.  The shallow dose

equivalent is now defined only as the dose equivalent at a depth of 0.007 cm

(7 mg/cm2); (2) the averaging area was increased from 1 cm2  to 10 cm2, and

was inserted together with the requirement to assign the shallow dose

equivalent to the part of the body receiving the highest exposure in 10 CFR

20.1201(c).  This section now requires that the  assigned shallow dose

equivalent be the dose averaged over the contiguous 10-cm2 area of skin

receiving the highest exposure.



The effect of these changes is to gradually increase the shallow dose

equivalent limit for the exposed skin area from 50 rem (0.5 Sv) for exposed

skin areas larger than 10 cm2, to higher values as the area of exposed skin

decreases below 10 cm2.  For a hot-particle exposure, the revised limit, in

effect, permits a dose that under the old rule would be equivalent to 500

rem  (0.5 Sv) averaged over an area of 1 cm2.  Under the new rule, this dose

would be averaged over the contiguous 10 cm2 of skin containing the hot

particle, yielding a dose of 50 rem (0.5 Sv) averaged over 10 cm2.  This

latter quantity is considered to be commensurate with the health risk

resulting from such a shallow dose equivalent to small areas of skin.



It should be noted that, because the doses from small hot particles are very

localized, and could be quite high in the immediate vicinity of the

particle, exposures to a hot particle close to the new limit could result in

a small skin lesion.  The probability of such an occurrence is small, and

the lesion, if it does occur, would be barely perceptible, and would quickly

heal, leaving no lasting health effects.  Several benefits are gained by

radiation workers as a result of this rule change.  One is that less time

should be spent in attempts to identify hot particles on workers by frequent

exits from work areas for contamination surveys.  Such a change should

reduce whole body exposures and also increase industrial safety by reducing

stay times in radiation areas, and by reducing the number of times workers

have to traverse various areas of the plant on their way to and from the

survey locations.  Also, workers will not be required, as often, to stop

their radiation work when the 50 rem(0.5 Sv) limit is exceeded because of

contamination by hot particles or by high- specific-activity

radiopharmaceuticals.  These changes represent a net gain in safety and job

security for the radiation worker, and the rule is now more risk-informed

than had been the case previously.



The Commission believes that the less restrictive limit on dose to small

areas of skin might permit more observable, transient, deterministic

effects, but nonetheless represents a substantial increase in worker

protection because reduced use of protective clothing will result in a less

hazardous workplace and less frequent monitoring for hot particle

contamination will result in reduced whole-body occupational dose.  This

represents a shift in emphasis toward a risk-informed approach that would

possibly permit more frequent deterministic effects in order to avoid the

physical stress and whole-body doses associated with monitoring workers and

the use of protective measures.



The revised rule does not change the basic methods used to assess shallow

dose equivalent resulting from exposure to external radiation fields,

general skin contamination, or hot-particle skin exposures.  The same

methods of measurement or calculation that have been used in the past are

still applicable, with the only change being the use of a skin averaging

area of 10 cm2 instead of 1 cm2 when assessing shallow dose equivalent.  It

should be noted, however, that dose averaging must include the dose to all

exposed skin within the 10-cm2 averaging area. For example, assessing dose

based on only the highest exposed 1-cm2 area of skin and then dividing by 10

may seriously underestimate the dose if the skin surrounding this 1-cm2 area

also received a dose.  It should also be noted that licensees must still

comply with the limit on the total effective dose equivalent (TEDE).  In the

case of exposure of the skin of the whole body, the relevant component of

the TEDE is the deep dose equivalent, which is subject to a limit of 5 rem

per year (0.05 Sv/yr).  Because of the higher skin dose limit, skin

exposures caused by radiation fields that contain a significant component of

penetrating radiation may, in some situations, cause the DDE limit to be

exceeded before the skin dose limit is reached.



This RIS requires no specific action nor written response.  If you have any

questions about the information in this summary, please contact the

technical contact listed below or the appropriate regional office.





/RA/

Donald A Cool, Director

Division of Industrial and

   Medical Nuclear Safety

Office of Nuclear Material Safety

   and Safeguards



Technical contact: Sami Sherbini, NMSS

301-415-7853

E-mail:sxs2@nrc.gov









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