MDL and Dose Effects

[kmq@cosmail1.ctd.ornl.gov (Kim McMahan)]

This article appeared in today's Knoxville News-Sentinel. It is available on
the www at:

        http://www.knoxnews.com/features/munlab/fm110496.html

______________________________________________________________________________

Research finds unrecorded radiation data was significant

By Frank Munger News-Sentinel staff writer


A researcher at the Oak Ridge Institute for Science and Education has thrown
another kink into the controversy about low-dose radiation. 

Bill Tankersley's study shows that some nuclear workers at Oak Ridge -- and
elsewhere -- received radiation doses significantly higher than they thought
because records did not reflect doses at low levels. 

This stems from the historic practice of recording a zero if the worker's
dosimeter measured radiation at or below the ''minimum detectable level.'' 

The MDL is the measured level at which authorities believe it is difficult
to ascertain whether an exposure took place. It also represents a belief
that even if a worker did receive an exposure at or below the minimum
detectable level, it was probably insignificant from a health perspective. 

Tankersley's research, however, found that the magnitude of unrecorded doses
was significant in the 1940s and '50s.

During that period, the Atomic Energy Commission used an MDL of 30 to 50
millirems. At the same time, film badges used to measure doses were changed
weekly -- instead of quarterly or even annually as some radiation readings
are handled today. 

Therefore, an employee whose job regularly exposed him to low-level
radiation in the range of 30 millirems could have received an unrecorded
dose of several rems or more over his career. 

This information might not have any direct relevance in determining whether
an individual worker suffered health effects as a result of radiation doses,
but Tankersley said the data could provide more insights for epidemiologists
studying cause-and-effect issues in a population of nuclear workers. 

''Today . . . even very low exposures are suspected of contributing to
excess disease,'' Tankersley wrote in a paper published earlier this year in
the Journal of Applied Occupational and Environmental Hygiene. ''Therefore,
there is increased interest in dose previously considered unimportant.'' 

While reviewing records at Oak Ridge National Laboratory, Tankersley and
colleagues found a ''substantial and otherwise inexplicable'' increase in
radiation dose levels among employees after 1956. 

The increase coincided with ORNL's changeover from weekly to quarterly
monitoring of employee doses. 

''Although the same practice of censoring readings below (30 millirems) was
continued, the magnitude of the possibly missed or unrecorded dose during
the period when monitoring was done weekly was many times greater than that
likely missed for the same reason during quarterly monitoring,'' Tankersley
wrote. 

He and other researchers also looked at sets of raw dosimetry data from
workers at the Oak Ridge facilities to evaluate the reliability of low-dose
results, which traditionally were reported as zero in employee records
instead of the actual measurement. 

They found that low doses -- ''even those near the zero level'' -- can be
''rather accurately measured with a relatively large set of data.'' 

In an interview, Tankersley said there is a level of impreciseness with any
monitoring system, but he said it was particularly important to researchers
to have as much actual data on radiation exposures as possible when studying
nuclear workers. 

The low-dose information may be helpful in explaining some reported health
effects among workers that previously were not understood, he said. 

Donna Cragle, who heads ORISE's Center for Epidemiologic Research, said
epidemiologists are now trying to incorporate these findings into current
studies and, in some instances, retrofitting old studies with new
information if available. 

In the study's conclusion, Tankersley wrote: 

''It is not possible to determine accurately the fraction of total dose
unrecorded due to procedures where all readings less than the applicable MDL
are set to zero. However, our investigations indicate that, in some cases,
the amount is considerable.'' 

The administrative practice of recording zeros for radiation doses at or
below the MDL continues today at the government's Oak Ridge plants and other
nuclear facilities around the country. However, the minimum detectable level
for dosimeters is much lower than it used to be, and monitoring period is
longer, too, which reduces the chance that a significant dose of radiation
will be missed. 

Nonetheless, Tankersley urged authorities to change the policy and report
all radiation doses as measured. 


Kim McMahan, CHP
Office of Radiation Protection				    Sola fide ...
Oak Ridge National Laboratory				  Sola gratia ...
P.O. Box 2008   Oak Ridge, TN  37831-6290	       Sola scriptura ...
Ph:  (423) 576-1566				      Soli Deo gloria .
e-mail: mcmahankl@ornl.gov