I.’ U... U,.... U.... U U U..
U U.
AAPM Tutorial Acceptable PaulH.
Risk
Murphy,
as a Basis
for
Regulatio&
PhD
In recent
years,
based
on
estimates
ments
regarding
guidelines for radiation of the risks associated acceptable
levels
protection have with radiation
of risk.
This
leads
increasingly
been
exposures
to a more
and
judg-
objective
ba-
are accurate. However, as enthusiasm for this approach expands, the large uncertainties in the risk estimates are often overlooked, and unreasonable, restrictive applications are possible. Several recent examples of radiation protection guidelines ifiustrate the riskbased emphasis underlying today’s radiation safety philosophy. sis for
regulations
if the
risk
estimates
INTRODUCTION The three fundamental tenets of a radiation protection program are that (a) the application of the radiation must be justfficd, meaning that there is an anticipated benefit; (b) dose limits should be set for workers and the public; and (C) there must be program to ensure that the radiation exposures are as low as reasonably achievable.
The purpose of this article dose limits are determined.
is to review
the philosophy
and process
of how these
includes a discussion of the history of radiation risk estimates, the current values, and the application of these values to arrive at dose limits in light of other occupational risks. Examples of regulations or guidelines from several governmental entities arc presented. The current approach to setting dose limits is based on the concept of acceptable risk. To discuss acceptable risk as a basis ofregulation, two aspects must be addressed. The first is, What is considered to be an acceptable level of risk? The second is, What
is the
magnitude
This
a
report
of the
risk
per
unit
radiation
dose
in the
dose
range
of in-
terest? Because radiation is only one of several occupational risks, others-such as fatal and nonfatal accidents, occupational illness, and travel on the job and to and from the workplace-should be considered when the radiation component is assessed. The fraction ofthe total occupational risk attributed to radiation should be considered in addition to its magnitude.
Abbreviations: national Commission
BEIR
Biological Radiological
on
tional
Council
on Radiation
Index
terms:
Radiations,
RadioGraphics 1
From
the
1991; Nudear
Department
30, 1991; 0PSNA,
revision
Protection
exposure
and
Measurements,
to patients
and
NRC
EPA = Environmental Aeronautics and = Nudear
Regulatory
#{149} Radiations,
personnel
Space
injurious
Protection Agency, Administration,
ICRP NCRP
= Inter. =
Na-
Commission effects
11:889-897
Medicine
of Radiology,
Effects of Ionizing Radiation, Protection, NASA National
Service, Baylor
requestedJune
3.261,
College 11 and
St Luke’s
Episcopal
ofMedicine,
Houston.
receivedjune
26;
Hospital, From
acceptedJuly
6720 the
1990
BertnerAve, RSNA
1. Address
scientific reprint
Houston, assembly. requests
TX 77030
and
Received to the
the
April
author.
1991
889
HISTORY OF RISK ESTIMATES
The
primary
assumption
risk
estimates
is that
the
underlying
forms
of this function,
Three
general
posed: models doses
the linear, linear-quadratic, of dose response that to predict the response
general
shapes
of the
of risk in the
the
magnitude
process
ofthe
risk
of developing is a known
regulations function
or the dose-response
based
ofradiation
curve,
have
on dose.
been
pro-
and threshold functions. These functions are use data from human populations exposed to high at lower doses. As ifiustrated in Figure 1, the three
dose-response
curve
result
in different
estimates
of the
magni-
region when data are extrapolated from the high-dose range. In human observations ofradiation-nelated carcinogenesis are statistically verified only at doses of hundreds of rem, but regulators are concerned with doses less than a few tens of rem for occupational and general public exposure limits. This is the low-dose range for which risk estimates are needed but are not tude
low-dose populations,
directly observable; thus, estimates must be made based on models dose data. The uncertainties in the resulting low-dose risk estimates The concept of acceptable risk as a basis for standard setting was National Council 1954 (1). Since ported
on Radiation then, several
their
conclusions
on
Radiological
mission
on the Effects Ionizing
and
ofAtomic
Radiation
Protection groups have
(BEIR)
and Measurements analyzed human
recommendations.
The
Protection
(ICRP),
Radiation,
and
sponsored
fitted to highare large. discussed by the (NCRP) as early as
the
United
the Committees
by the
National
exposure
NCRP,
Nations
the
data
Scientific
reCom-
Committee
on the Biological Research
and
International
Effects
Council
have
of
all ad-
dressed 1 and
this topic. In 1971, the NCRP estimated the risk ofleukemia to be between 2 x 106 per person per year per roentgen and that the risk for all neoplasia was two to 10 times that value, with an average ofabout 5 x 106 (2). In 1972, the BEIR I Committee proposed that the data available in the high-dose range could be extrapolated linearly to the low-dose range (3). In 1977, the risk estimates from hu-
man data mechanism 1980,
were updated, for specifying
and the process ofusing the effective doses of equivalent risk was proposed
dose equivalent by the ICRP
as a (4).
In
the
BEIR III Committee proposed a linear-quadratic dose-response curve (5). Finally, the BEIR V Committee, which reanalyzed the dosimetry for survivors of atomic bombings and returned to the linear dose-response relationship, proposed risk estimates that are three to four times higher than those previously reported (6). Several large populations of humans who have received fairly high doses of radiation arc the basis for the risk estimates (7). The major populations are the survivors of the World War II atomic bombings in Japan and several groups that underwent radiation therapy for a variety of conditions. With the evolution of risk estimates, the standards for radiation workers have de-
creased
from 26 rem (0.26 Sv) per year in 1934 to the present level of Sv) per year. At this time, with continuing discussion of the conclusions BEIR V Committee (6), one can only speculate on the impact of its risk the standards
for radiation
workers
1. Three general shapes of the dose-response curves permit prediction of different mcidences of radiation effects in the low-dose range when the curves are fitted to data in the high-dose range.
and
the general
5
rem
(0.05
from the estimates on
public.
Figure
uJ C-)
z
w C-)
z
DOSE
890
U
Ra4ioGrapbics
U
Murphy
Volume
11
Number
5
The
process
of associating
radiation
protection
levels
with
risk was probably
most
RISK-BASED
completely described in ICRP Report no. 26 in 1977 (4). In that publication, for all humans, a nominal risk of all cancers of 102 per sievert (1O per rem) was assumed, that is, a one in 10,000 chance ofdcveloping fatal cancer per rem dose equivalent. The authors supported the existing limit of 0.05 Sv (5 rem) per year for radiation workers, for which the average exposure is only one-tenth to one-twenti-
STANDARDS
average exposure results in a risk of approximately 0.5 to 1 x is comparable to the risk of a fatal accident in industries considered safe. Also, with a protection level of 5 mSv (500 mrem) per year for members of the general public, who are unlikely to receive an average exposure of more than 0.5 mSv per year, the resulting risk is less than 1 x 10’, which is comparable to other risks that all members of the population encounter frequently. The mechanism of specifying dose in terms of its risk equivalent was developed in ICRP Report no. 26 (4). This approach permits comparisons ofnonuniform radiation exposure to uniform whole-body exposure in terms of the risk associated with the exposures. Risk estimates for different tissues from this ICRP report sum to about 1.6 x 102 per sievert (1.6 x 1O per rem) (Table 1). Weighting factors were assigned to the different tissues to permit calculation of effective dose equivalent based on the relative risk per unit dose equivalent for the different tissues. Several sources of uncertainties in the risk estimates are particularly troublesome in the how-dose range. The primary source of information on radiation risk in humans is from the survivors of the atomic bombings. In such a catastrophic setting, dosimetry is complex, and the doses that are being assessed are in the high range and are delivered instantaneously. Also, the potential differences in cancer risk eth
io-4
of that limit. The per year, which
among
greatly
disparate
Japan and the ing those from
populations
with
United States, should the atomic bombing
very
different
be considered. survivors, age
together, although substantial differences in age and to exist for some cancers. In addition, the uncertainties from high-dose data make the low-dose risk estimates Most assessing since
risk estimates risk, looking the
Table
latency
are for death from at life shortening
period
lifestyles,
sex predilection introduced
as those
cancer
are believed imprecise.
cancers. is justified,
is typically
of
includaveraged
by extrapolating
uncomfortably
radiation-induced instead of death
for radiation-induced
such
For most risk estimates, and sex distributions are
However, particularly many
years.
in Also,
1
Weighting
Factors
and
Risk
Factors
for Tissues
at Risk
for Stochastic
Effects
Weighting Tissue
Risk
(Sv
‘)
Comments
Factor
Gonads
4.0 x i03
Genetic risk to first two generations
0.25
Breast
2.5 x i0
Averageforallagesand
0.15
both Red bone Lung
marrow
sexes
2.0
X
10
Leukemia
2.0
X
iO
Cancer
Thyroid
5.0
x i04
Bone surface Remainder
5.0 x 10 5.0 x i0-
Total
1.65
Source-Data
September
from
1991
reference
x 102
Fatal
0.12
0.12 cancer
0.03
Osteosarcoma Cancer, assuming that no single tissue contributes more than ‘/, of this total .
.
.
0.03 0.30
...
8.
Murphy
U
RadioGraphics
U
891
the ratio of the incidence of disease to the mortality rate is approximately 2: 1 on the average for all cancers but ranges from 10:1 for thyroid cancer to 1:1 for lung cancer. The aim of radiation protection is to make the radiation industry as safe as the industry, with an accidental (9) (ie, less than 1 X iO-
safest
years
among
different
industries
are
year
per
industries
and
trade,
manufacturing,
the
the radiation
industry
10,000
per
death year
rate ofone for individual
occupations, and
is compared.
The
The
annual
workers.
or fewer per 10,000 workerworkers). Fatality rates vary as shown in Table 2 (10). The safest
total
service
industries,
U.S. average risk
and
is about
to a radiation
one
it is to these
fatal
worker
that
accident
per
is estimated
to
be approximately 2 x iO-4 based on an average radiation exposure of 2.3 mSv (230 mrem) per year (1 1). The radiation risk is summed with the risk of a fatal accident and the rather high risk of travel to and from the workplace to arrive at this total occupational
risk. This risk level is comparable to that for other occupations The radiation component is about 25% ofthe total. most recent risk estimates were published by the BEIR V Committee
recog-
as safe.
nized
The reviewed
human
survivors. conclusions
The that
concluded
exposure
data,
particularly
committee updated the the neutron component
that
for the 76,000
estimates of the was less than
atomic
(6),
who
bombing
doses on the basis previously assumed.
of recent It also
function was the most appropriate and arThe risks for all cancers were estimated to be three to four times higher than previous estimates. The average lifetime excess risk ofdeath from cancer is 0.8% per 0.1 Sv (10 rem). With a 30% cancer frequency and 20% mortality, this accounts for about 4% of the baseline risk of death from cancen. The BEIR V Committee qualified these higher risk estimates by indicating that they should be used as that only, that is, as a risk estimate, and not for purposes of at new
rived
prediction
a linear
those
estimates
dose-response
ofradiation
or calculation
did not specify
risks.
ofthe
number
ofcancer
deaths.
rate reduction factor, which that the risks from doses in the low-dose and that the probability exists that there range of a few hundred millirem. In fact,
indicated
observed the dose
a dose
However,
the
committee
many believe is at least two (6). It range are too small to be directly may be no risks from exposures in some believe that exposure to low
levels ofradiation mayyield beneficial effects, a process known as hormesis (12). Hormesis is a stimulatory effect brought about by low-level exposure to a substance that is toxic at high levels. The potential mechanisms that have been proposed for radiation hormesis are increased free radical scavengers, enhanced DNA repair, and enhanced
factor
immune
cell
in any activity
Table
production.
of regulatory
Hormesis
agencies
is a controversial
Fatality
from
Rates
Accidents
in Different
No.
of
Annual
Fatal
Accident
Rate
24,000
0.5
Manufacturing
19,900
0.6
Service Government Transportation
28,900 15,900
0.7 0.9
5,500
2.7
5,700
3,400
3.9 4.6
1,000
6.0
and
util-
Construction Agriculture Mining, quarrying
All U.S. industries Source-Data
U
a
(per 10,000 workers)
Trade
ities
RadioGraphics
is not
Occupations
Workers (X 10)
Occupation
U
and
2
Annual
892
topic
to date.
Murphy
from
104,300
reference
1.1
10.
Volume
11
Number
5
A few recent examples of risk-related regulatory activities or risk-related standards being set for radiation exposure are (a) the NCRP recommendations to the National Aeronautics and Space Administration (NASA) on the astronauts’ exposure limits while in space (1 1); (b) the Environmental Protection Agency’s (EPA) proposal on limits
of radionuchide
10 of the Code Standards uhatory islature’s
proposal
on
(13);
against to low-level
goals
for
hustrates a regulatory agency’s risk estimates and a judgment At the request of NASA, the by astronauts
during
space
STANDARDS
to section 20 of title Commission’s (NRC) policy on “below reg(8); and (e) the U.S. leg(15). Each of these il-
(C)
Regulations,
for Protection concern,” relating
OF
the proposed revision the Nuclear Regulatory Radiation (14); (d) the NRC’s
emissions
of Federal
EXAMPLES RISK-BASED
radioactive
indoor
radon
materials concentrations
attempt to set limits on radiation exposure based on ofwhat constitutes an acceptable magnitude of risk. NCRP proposed standards for radiation dose received
activities
(1 1).
Radiation
limits
for
space
flight
were
re-
viewed from the standpoint of (a) stochastic effects (including primarily fatal cancer), with a nominal risk factor of 2 x 102 per sievert; (b) the genetic effects over the subsequent two generations, with an estimated risk of 1 x 102 per sievert; and (C) nonstochastic effects, including radiation-induced cataracts and adverse effects on fertility. In proposing limits for space activities, the NCRP looked at the extremes in risks for terrestrial radiation workers. For example, with a maximum permissible dose of 0.05 Sv (5 rem) pen year and a 50-year career, a radiation worker could conceivably receive a lifetime dose equivalent of 2.5 Sv (250 rem). This results in a lifetime risk ofdeveloping fatal cancer ofapproximately 5%. On the other hand, the average radiation worker receives an annual exposure ofabout 2 mSv (200 mrem), which results in a lifetime total dose equivalent of 0. 1 Sv (10 rem) and an excess cancer risk of less than 1%, which is comparable to risks of fatal accidents in safer industries. It did not seem reasonable to limit the exposure of astronauts to the average exposure of the terrestrial worker, given the other much more risky activities associated with space travel. It seemed more reasonable to limit the exposure to that of the more highly exposed terrestrial worker, that is, a dose of 0.05 Sv pen year for a potential 2.5 Sv pen lifetime. However, on the basis ofcurrent experience, it is unlikely that an astronaut’s total exposure will exceed about 0.7 Sv. Thus, the NCRP’s proposed limit for space workers is a career dose equivalent of 1.5 Sv (150 rem) or a risk ofless than 3% for fatal cancer. This 3% represents about one-sixth of the natural risk of fatal cancer and, in light of the other risks of space travel, was deemed to be acceptable. The age and sex differences for risk estimates were recognized by the NCRP, and the career whole-body for male and female
Table
dose-equivalent subjects as shown
limits were specified in Table 3 (1 1). These
3
NCRP Recommendations Astronauts’ Career
for Whole-Body
Dose-Equivalent
Limits
Age (y)
Female
Male
25 35 45 55
1.0 1.75 2.5 3.0
1.5 2.5 3.2 4.0
Limit
Source-Data
from
Note-Dose on a lifetime cancerof3
September
for several age groups values are the dose
reference
equivalent excess risk x
1991
(Sv)
1 1.
limits are based of death from
102.
Murphy
U
RadioGrapbics
U
893
equivalents Notice that
in sieverts the doses
that range
would from
risk of cancer mortality of 3%. rem) to a high of 4 Sv (400 rem).
produce a career a low of 1 Sv (100
The NCRP also recommended limits on dose equivalents for short-term exposures. For example, in Table 4 (1 1), the annual limit for irradiation of blood-forming organs
tion
is 0.5 Sv (50 workers.
Another
rem)
example
or 10 times
of risk-based
Standards for Hazardous 1979, when radionuclides
the
limit
presently
regulatory
Air Pollutants were listed
applied
activity
(13). under
to earth-bound
radia-
is the EPA’s National
Emission
The history ofthis activity goes the Clean Air Act as a hazardous
back to air pol-
lutant. In 1985, the EPA proposed final rules for several categories of radioactive material users, including licensees of the NRC. The EPA was forced by the federal court to propose standards for radionuclide exposure, following the precedent set for vinyl chloride, and the final proposal was published in 1989. The standards followed the two-part guidelines defined in the vinyl chloride case: (a) to limit risks for individuals most highly exposed to one in 10,000 and (b) to reduce the risk for as many people as possible to one in 1,000,000. On the basis ofcurrent risk estimates, this
results
mSv)
in a maximum
per
year,
ofwhich
to radioactive posed
iodine.
continuously
allowable
dose
no more
This to the
than
dose
to a member
3 mrem
estimate
maximum
of the
(0.03
assumed
mSv)
that
concentration
public
of
year
can
per
an individual
for 70 years.
10 mrem
would The
(0.1
be attributed
be ex-
EPA approach
to estimating the risks from radionuclide emissions is to either measure or estimate the emissions, predict concentrations as a function of distance from the source with use of a dispersion model, and calculate the exposures of the populations. The estimates of risk per unit dose equivalent are then used to set limits on emissions to
Table 4 NCRP
Career
Recommendations for Astronauts’ Short-term Limits for Protection against Nonstochastic
Dose-Equivalent Effects
Limit
Time Period
Blood-forming Organs
30d Annual
.
from
Source-Data
2. comparison Figure
reference
.
and
(Sv)
Lens of the Eye
Skin
1.0 2.0 4.0
1.5 3.0 6.0
0.25 0.5
Career
Limits
.
11.
The NRC’s of below-regu-
hatory-concern doses to doses ral background cal exposures.
(BRC)
from and
I
natumedi-
I 40
p [T#{244} I
6
Fi 0
All Medical Exams
50
Natural Radioactive Materials in the Body
BRC
Practice
Chest
X-ii
BRC
50
Affecling
Urnited
Numbe r of People
I
Practice Affecting Large Number of Pei 100
150
200
250
300
Radiation Dose (mrem)
894
U
RadioGraphics
U
Murphy
Volume
11
Number
5
keep the risks less than those the emissions are below these problem, the EPA has provided phiance. The program, named relative
leveh
With
use
of emissions of cost
previously limits can
mentioned. be extremely
a computer
program
Comply,
has
to assist
a tiered
options,
safety
from
the
EPA
for
for
is 500
times
radionuchide the
ganizations
greater
emissions.
of implementing
that
implementing
com-
depends
on the
standard
associated
for
with
than
the
Because these
number
of the
the
saved
with
perceived
medical
and
licensed
activities
various
acceptable
dollar spent to reduce the risk of of magnitude, if the money were nadionuclide emissions (16). For spent per year for automobile
analysis
restraints
limit
that this
of radionuchides.
estimates
one
by a licensee To address
in documenting
structure
concluded that for every death, the yield would be much higher, by orders spent on safety programs rather than on reducing example, the number of lives saved per $ 1 million risk
Verification complicated.
the
large
academic
EPA’s
proposed
cost-to-benefit
licensees,
ratio
professional
aggressively
oppose
or-
the
regula-
tion.
Another risk-based regulatory action is the proposed revision to section 20 of title 10 of the Code of Federal Regulations: the NRC Standards for Protection against Radiation (14). This regulatory action has been in development for several years and would essentially implement the effective dose-equivalent approach as proposed by the
ICRP
in
general risk
1977.
public
It would
that
are
establish
based
radiation
on risk
exposure
estimates
and
levels
for
a decision
workers
and
regarding
the
acceptable
levels.
A more recent NRC initiative based on risk estimates is the NRC’s Below Regulatory Concern Policy Statement for exempting low-level radioactive materials from regulations (8). The NRC believes that this statement will establish a consistent risk framework for regulatory decisions. It establishes dose criteria of 1 mrem (0.01 mSv)
and
and
10 mrem
from
sievert)
(0. 1 mSv)
all sources per
year
for
the
fact that variations vide some perspective the
on
the
hypothesis
is linearly
(0. 1-mSv)
which
individuals
regions
and
that and do
country
for
individual
respectively,
population.
exposures
and
It recognizes
1,000 and
from
single
person-rem
uses
sources
(10
as a basis
for
personjustification
from natural sources of radiation proof risk. The policy is explicitly based
the
fatal
risk
to the
medical
of the
year
in individual exposures on the acceptableness
proportional
mrem ground
per
ofradiation,
of developing dose
at 5 X iO
1-mrem
(O.01-mSv)
exposures
(Fig
not
associate
or living
levels
2) and an
also
unacceptable
in a brick
versus
cancer
per
rem.
from The
exposure NRC
by comparing
to doses risk,
a wood
to radiation
justified
the
them
to natural
from
other
activities
such
as living
home
(Fig
3.
Figure
comparison
other
for
in different
3).
The NRC’s of below-regu-
latory-concern doses to doses
lected
10back-
(BRC)
from
se-
radiation
sources.
30 40 Radiation Dose (mrem)
September
1991
Murphy
U
RadioGrapbics
U
895
The
response
to this policy
of acceptable vigorously
risk opposed
is not the
statement
has demonstrated
necessarily the same policy by associating
lem of disposal
oflow-level
radioactive
have
suggestions
that
ing
also
been
in contaminated
consumer
some time. A final example
waste
some
of risk-based
waste
power
is that
groups to the
companies.
will the
activity
perception
Several solution
materials
Undoubtedly,
regulatory
the public’s
regulators’. a careless
by nuclear
of the
products.
that
as the it with
have prob-
There
be recycled,
debate
will
relating
result-
continue
to indoor
for
radon
concentrations. The U.S. Public Health Service has declared that indoor radon gas is a national health problem and that radon causes thousands of deaths pen year. This has led the U.S. Congress to pass legislation that establishes as a national goal the reduction of indoor radon concentrations to the level of outside concentrations (15). Indoor radon is currently perceived to be the most important problem involving
radiation
exposure
the concentration would correspond This
of the
public.
recommendation
is quite
different
whose approach is somewhat the Food and Drug Act. This cinogenic,
the
cinogen CONCLUSIONS
(ie,
Most tude
people of the
basis
assumption
any
of these
for
to automated
limit
automatically
benefit purposes,
risk
estimates
that Also,
dose
estimate,
Most
people
conservatism
when
quotes
today
(17).
“For
practical
been
used
consistently
sents
a conservative
problems the
assessment
RadioGrapbic.s
U
as then,
is how
uncertainties
U
from
Murphy
although
these
the
because upper
uncertainties of the
dose
of a car-
of risk for
proceedings
linear,
estimates
regulatory
assessment
process but
rather
other
as pretypes
the
risk
estimates
doses,
with
their
regulations
when
of frequency
of cancer
of interest.
as relevant
to the
risks
dose
of
of the
the association
usually
addressed
non-threshold
such
with
setting
ranges are
the
the interpretation
predictions
in the dose
to
protection
reasons,
risks
at low
for
proceedings
of risk
other
estimates
as a
tends
the dose
for
Because
as a basis
risk doses
on deemphasize
established for
magniis the
acceptable at low
to ignore
monitoring.
an effective
assessment
regulatory
legislature,
is adjusted,
of radiation importance on
of its plausible
limit
in the risk
use
NCRP
purposes
to develop
in making
the
Once
estimates
tends
is inappropriate
1980
U.S.
Amendment to found to be car-
whole-body irradiation, difficult to interpret.
may be zero
the
range.
approach
appropriate
but
the
as a safe
a risk estimate
personnel
that
is still
issue
ation
agree
is justified
this
to uniform risk is very
in fact this frequency
following
896
would
uncertainty,
when
is proportional to the The major disagreement
As it becomes
comparison increased
from
are based on dose-equivalent between badge reading and
by
thing
of risk
approach
to apply
for example,
exposure range.
is, once
this
or
action
the Delaney food additive
such
low-dose
ratio.
is a tendency
of frequency of cancer Also, this approach places
proposed
is no
application
of the benefit-risk
there
there
in the
regulations; adjusts.
that
remedial
in the United States, which of approximately 4 x 10.
logic behind banned any
risk of radiation in the high-dose
diction risks.
significant
recommends
is unacceptable).
is established,
portion
from
that
cancer
risk
regulation
lead
NCRP
akin to the amendment being
excess
agree that the dose equivalent
application
The
exceeds 10 times the average level to an annual lung cancer incidence
from
The
radiation
carcinogens
response
model
has
scientific basis and because it preIndeed one of the unsolved .
.
.
program .
the
.
.
The
uncertainties
taking
central
into
issue in how
consider-
is not the to use
risk
decisions.”
Volume
11
Number
5
1.
2.
3.
4.
5.
Permissible
Research 6.
7.
8.
9.
September
dose
from
external
sources
of ionizing radiation. NCRP Report no. 17. Bethesda, Md: National Council on Radiation Protection and Measurements, 1954. Basic radiation protection criteria. NCRP Report no. 39. Bethesda, Md: National Council on Radiation Protection and Measurements, 1971. BEIR I Committee. The effects on populations of exposure to low levels of ionizing radiation. Washington, DC: National Academy of Sciences/National Research Council, 1972. Recommendations of the International Commission on Radiological Protection. ICRP Publication 26. New York: Pergamon, 1977. BEIR III Committee. The effects on populations ofexposure to low levels of ionizing radiation. Washington, DC: National Academy of Sciences/National
Council,
11
Accident facts, 1982 tional Safety Council, .
Guidance
data. Chicago: 1985.
on radiation
received
Na-
REFERENCES
in
space activities. NCRP Report no. 98. Bethesda, Md: National Council on Radiation Protection and Measurements,
1989. 12.
Macklis hormesis.J
RM, Beresford NucI Med
B. Radiation 1991; 32:350-
359. 13.
National ous
emission
standards
air pollutants:
for hazard-
radionuclides-final
rule
14.
and notice of reconsideration. FedRegister, December 15, 1989; 54(240):51654 (40 CFR § 61). Standards for protection against radia-
15.
tion: final rule. Federal Register, 21, 1991; 56(98):23360. U.S. Environmental Protection
eral
and Centers citizen’s
what
1980.
BEIR V Committee. Health effects of exposure to low levels of ionizing radiation. Washington, DC: National Academy of Sciences/National Research Council, 1990. Sinclair WK. The scientific basis for risk quantification. In: Quantitative risks in standard setting: proceedings of the 16th annual meeting, National Council on Radiation Protection and Measurements. Washington, DC : National Academy ofSciences, 1981; 3-33. Below regulatory concern policy statement. Federal Register,July 23, 1990; 55(141):29925. Hall EJ. Radiobiology for the radiologist. 3rd ed. Philadelphia: Lippincott, 1988; 516.
1991
10.
guide Health
ofHealth Collopy
and P.
Pennsylvania.
it.
Service,
Human Chapter Health
Agency
Control.
to radon:
to do about
Public
16.
for Disease
May
what
A it
Washington, U.S.
is and
DC:
Department
Services, 1986. news: western Physics
17.
1989; 17(12):8-10. Albert RE. Federal regulatory approaches to the assessment trol of risk from carcinogens.
Society
Newsletter
agency and conIn: Per-
ceptions of risk: proceedings of the 1 5th annual meeting of the National Council on Radiation Protection and
Measurements.
Washington,
DC: Na-
tional 6-14.
of Sciences,
1980;
Academy
Murphy
U
RadioGraphics
U
897
U U.
AAPM Tutorial Acceptable PaulH.
Risk
Murphy,
as a Basis
for
Regulatio&
PhD
In recent
years,
based
on
estimates
ments
regarding
guidelines for radiation of the risks associated acceptable
levels
protection have with radiation
of risk.
This
leads
increasingly
been
exposures
to a more
and
judg-
objective
ba-
are accurate. However, as enthusiasm for this approach expands, the large uncertainties in the risk estimates are often overlooked, and unreasonable, restrictive applications are possible. Several recent examples of radiation protection guidelines ifiustrate the riskbased emphasis underlying today’s radiation safety philosophy. sis for
regulations
if the
risk
estimates
INTRODUCTION The three fundamental tenets of a radiation protection program are that (a) the application of the radiation must be justfficd, meaning that there is an anticipated benefit; (b) dose limits should be set for workers and the public; and (C) there must be program to ensure that the radiation exposures are as low as reasonably achievable.
The purpose of this article dose limits are determined.
is to review
the philosophy
and process
of how these
includes a discussion of the history of radiation risk estimates, the current values, and the application of these values to arrive at dose limits in light of other occupational risks. Examples of regulations or guidelines from several governmental entities arc presented. The current approach to setting dose limits is based on the concept of acceptable risk. To discuss acceptable risk as a basis ofregulation, two aspects must be addressed. The first is, What is considered to be an acceptable level of risk? The second is, What
is the
magnitude
This
a
report
of the
risk
per
unit
radiation
dose
in the
dose
range
of in-
terest? Because radiation is only one of several occupational risks, others-such as fatal and nonfatal accidents, occupational illness, and travel on the job and to and from the workplace-should be considered when the radiation component is assessed. The fraction ofthe total occupational risk attributed to radiation should be considered in addition to its magnitude.
Abbreviations: national Commission
BEIR
Biological Radiological
on
tional
Council
on Radiation
Index
terms:
Radiations,
RadioGraphics 1
From
the
1991; Nudear
Department
30, 1991; 0PSNA,
revision
Protection
exposure
and
Measurements,
to patients
and
NRC
EPA = Environmental Aeronautics and = Nudear
Regulatory
#{149} Radiations,
personnel
Space
injurious
Protection Agency, Administration,
ICRP NCRP
= Inter. =
Na-
Commission effects
11:889-897
Medicine
of Radiology,
Effects of Ionizing Radiation, Protection, NASA National
Service, Baylor
requestedJune
3.261,
College 11 and
St Luke’s
Episcopal
ofMedicine,
Houston.
receivedjune
26;
Hospital, From
acceptedJuly
6720 the
1990
BertnerAve, RSNA
1. Address
scientific reprint
Houston, assembly. requests
TX 77030
and
Received to the
the
April
author.
1991
889
HISTORY OF RISK ESTIMATES
The
primary
assumption
risk
estimates
is that
the
underlying
forms
of this function,
Three
general
posed: models doses
the linear, linear-quadratic, of dose response that to predict the response
general
shapes
of the
of risk in the
the
magnitude
process
ofthe
risk
of developing is a known
regulations function
or the dose-response
based
ofradiation
curve,
have
on dose.
been
pro-
and threshold functions. These functions are use data from human populations exposed to high at lower doses. As ifiustrated in Figure 1, the three
dose-response
curve
result
in different
estimates
of the
magni-
region when data are extrapolated from the high-dose range. In human observations ofradiation-nelated carcinogenesis are statistically verified only at doses of hundreds of rem, but regulators are concerned with doses less than a few tens of rem for occupational and general public exposure limits. This is the low-dose range for which risk estimates are needed but are not tude
low-dose populations,
directly observable; thus, estimates must be made based on models dose data. The uncertainties in the resulting low-dose risk estimates The concept of acceptable risk as a basis for standard setting was National Council 1954 (1). Since ported
on Radiation then, several
their
conclusions
on
Radiological
mission
on the Effects Ionizing
and
ofAtomic
Radiation
Protection groups have
(BEIR)
and Measurements analyzed human
recommendations.
The
Protection
(ICRP),
Radiation,
and
sponsored
fitted to highare large. discussed by the (NCRP) as early as
the
United
the Committees
by the
National
exposure
NCRP,
Nations
the
data
Scientific
reCom-
Committee
on the Biological Research
and
International
Effects
Council
have
of
all ad-
dressed 1 and
this topic. In 1971, the NCRP estimated the risk ofleukemia to be between 2 x 106 per person per year per roentgen and that the risk for all neoplasia was two to 10 times that value, with an average ofabout 5 x 106 (2). In 1972, the BEIR I Committee proposed that the data available in the high-dose range could be extrapolated linearly to the low-dose range (3). In 1977, the risk estimates from hu-
man data mechanism 1980,
were updated, for specifying
and the process ofusing the effective doses of equivalent risk was proposed
dose equivalent by the ICRP
as a (4).
In
the
BEIR III Committee proposed a linear-quadratic dose-response curve (5). Finally, the BEIR V Committee, which reanalyzed the dosimetry for survivors of atomic bombings and returned to the linear dose-response relationship, proposed risk estimates that are three to four times higher than those previously reported (6). Several large populations of humans who have received fairly high doses of radiation arc the basis for the risk estimates (7). The major populations are the survivors of the World War II atomic bombings in Japan and several groups that underwent radiation therapy for a variety of conditions. With the evolution of risk estimates, the standards for radiation workers have de-
creased
from 26 rem (0.26 Sv) per year in 1934 to the present level of Sv) per year. At this time, with continuing discussion of the conclusions BEIR V Committee (6), one can only speculate on the impact of its risk the standards
for radiation
workers
1. Three general shapes of the dose-response curves permit prediction of different mcidences of radiation effects in the low-dose range when the curves are fitted to data in the high-dose range.
and
the general
5
rem
(0.05
from the estimates on
public.
Figure
uJ C-)
z
w C-)
z
DOSE
890
U
Ra4ioGrapbics
U
Murphy
Volume
11
Number
5
The
process
of associating
radiation
protection
levels
with
risk was probably
most
RISK-BASED
completely described in ICRP Report no. 26 in 1977 (4). In that publication, for all humans, a nominal risk of all cancers of 102 per sievert (1O per rem) was assumed, that is, a one in 10,000 chance ofdcveloping fatal cancer per rem dose equivalent. The authors supported the existing limit of 0.05 Sv (5 rem) per year for radiation workers, for which the average exposure is only one-tenth to one-twenti-
STANDARDS
average exposure results in a risk of approximately 0.5 to 1 x is comparable to the risk of a fatal accident in industries considered safe. Also, with a protection level of 5 mSv (500 mrem) per year for members of the general public, who are unlikely to receive an average exposure of more than 0.5 mSv per year, the resulting risk is less than 1 x 10’, which is comparable to other risks that all members of the population encounter frequently. The mechanism of specifying dose in terms of its risk equivalent was developed in ICRP Report no. 26 (4). This approach permits comparisons ofnonuniform radiation exposure to uniform whole-body exposure in terms of the risk associated with the exposures. Risk estimates for different tissues from this ICRP report sum to about 1.6 x 102 per sievert (1.6 x 1O per rem) (Table 1). Weighting factors were assigned to the different tissues to permit calculation of effective dose equivalent based on the relative risk per unit dose equivalent for the different tissues. Several sources of uncertainties in the risk estimates are particularly troublesome in the how-dose range. The primary source of information on radiation risk in humans is from the survivors of the atomic bombings. In such a catastrophic setting, dosimetry is complex, and the doses that are being assessed are in the high range and are delivered instantaneously. Also, the potential differences in cancer risk eth
io-4
of that limit. The per year, which
among
greatly
disparate
Japan and the ing those from
populations
with
United States, should the atomic bombing
very
different
be considered. survivors, age
together, although substantial differences in age and to exist for some cancers. In addition, the uncertainties from high-dose data make the low-dose risk estimates Most assessing since
risk estimates risk, looking the
Table
latency
are for death from at life shortening
period
lifestyles,
sex predilection introduced
as those
cancer
are believed imprecise.
cancers. is justified,
is typically
of
includaveraged
by extrapolating
uncomfortably
radiation-induced instead of death
for radiation-induced
such
For most risk estimates, and sex distributions are
However, particularly many
years.
in Also,
1
Weighting
Factors
and
Risk
Factors
for Tissues
at Risk
for Stochastic
Effects
Weighting Tissue
Risk
(Sv
‘)
Comments
Factor
Gonads
4.0 x i03
Genetic risk to first two generations
0.25
Breast
2.5 x i0
Averageforallagesand
0.15
both Red bone Lung
marrow
sexes
2.0
X
10
Leukemia
2.0
X
iO
Cancer
Thyroid
5.0
x i04
Bone surface Remainder
5.0 x 10 5.0 x i0-
Total
1.65
Source-Data
September
from
1991
reference
x 102
Fatal
0.12
0.12 cancer
0.03
Osteosarcoma Cancer, assuming that no single tissue contributes more than ‘/, of this total .
.
.
0.03 0.30
...
8.
Murphy
U
RadioGraphics
U
891
the ratio of the incidence of disease to the mortality rate is approximately 2: 1 on the average for all cancers but ranges from 10:1 for thyroid cancer to 1:1 for lung cancer. The aim of radiation protection is to make the radiation industry as safe as the industry, with an accidental (9) (ie, less than 1 X iO-
safest
years
among
different
industries
are
year
per
industries
and
trade,
manufacturing,
the
the radiation
industry
10,000
per
death year
rate ofone for individual
occupations, and
is compared.
The
The
annual
workers.
or fewer per 10,000 workerworkers). Fatality rates vary as shown in Table 2 (10). The safest
total
service
industries,
U.S. average risk
and
is about
to a radiation
one
it is to these
fatal
worker
that
accident
per
is estimated
to
be approximately 2 x iO-4 based on an average radiation exposure of 2.3 mSv (230 mrem) per year (1 1). The radiation risk is summed with the risk of a fatal accident and the rather high risk of travel to and from the workplace to arrive at this total occupational
risk. This risk level is comparable to that for other occupations The radiation component is about 25% ofthe total. most recent risk estimates were published by the BEIR V Committee
recog-
as safe.
nized
The reviewed
human
survivors. conclusions
The that
concluded
exposure
data,
particularly
committee updated the the neutron component
that
for the 76,000
estimates of the was less than
atomic
(6),
who
bombing
doses on the basis previously assumed.
of recent It also
function was the most appropriate and arThe risks for all cancers were estimated to be three to four times higher than previous estimates. The average lifetime excess risk ofdeath from cancer is 0.8% per 0.1 Sv (10 rem). With a 30% cancer frequency and 20% mortality, this accounts for about 4% of the baseline risk of death from cancen. The BEIR V Committee qualified these higher risk estimates by indicating that they should be used as that only, that is, as a risk estimate, and not for purposes of at new
rived
prediction
a linear
those
estimates
dose-response
ofradiation
or calculation
did not specify
risks.
ofthe
number
ofcancer
deaths.
rate reduction factor, which that the risks from doses in the low-dose and that the probability exists that there range of a few hundred millirem. In fact,
indicated
observed the dose
a dose
However,
the
committee
many believe is at least two (6). It range are too small to be directly may be no risks from exposures in some believe that exposure to low
levels ofradiation mayyield beneficial effects, a process known as hormesis (12). Hormesis is a stimulatory effect brought about by low-level exposure to a substance that is toxic at high levels. The potential mechanisms that have been proposed for radiation hormesis are increased free radical scavengers, enhanced DNA repair, and enhanced
factor
immune
cell
in any activity
Table
production.
of regulatory
Hormesis
agencies
is a controversial
Fatality
from
Rates
Accidents
in Different
No.
of
Annual
Fatal
Accident
Rate
24,000
0.5
Manufacturing
19,900
0.6
Service Government Transportation
28,900 15,900
0.7 0.9
5,500
2.7
5,700
3,400
3.9 4.6
1,000
6.0
and
util-
Construction Agriculture Mining, quarrying
All U.S. industries Source-Data
U
a
(per 10,000 workers)
Trade
ities
RadioGraphics
is not
Occupations
Workers (X 10)
Occupation
U
and
2
Annual
892
topic
to date.
Murphy
from
104,300
reference
1.1
10.
Volume
11
Number
5
A few recent examples of risk-related regulatory activities or risk-related standards being set for radiation exposure are (a) the NCRP recommendations to the National Aeronautics and Space Administration (NASA) on the astronauts’ exposure limits while in space (1 1); (b) the Environmental Protection Agency’s (EPA) proposal on limits
of radionuchide
10 of the Code Standards uhatory islature’s
proposal
on
(13);
against to low-level
goals
for
hustrates a regulatory agency’s risk estimates and a judgment At the request of NASA, the by astronauts
during
space
STANDARDS
to section 20 of title Commission’s (NRC) policy on “below reg(8); and (e) the U.S. leg(15). Each of these il-
(C)
Regulations,
for Protection concern,” relating
OF
the proposed revision the Nuclear Regulatory Radiation (14); (d) the NRC’s
emissions
of Federal
EXAMPLES RISK-BASED
radioactive
indoor
radon
materials concentrations
attempt to set limits on radiation exposure based on ofwhat constitutes an acceptable magnitude of risk. NCRP proposed standards for radiation dose received
activities
(1 1).
Radiation
limits
for
space
flight
were
re-
viewed from the standpoint of (a) stochastic effects (including primarily fatal cancer), with a nominal risk factor of 2 x 102 per sievert; (b) the genetic effects over the subsequent two generations, with an estimated risk of 1 x 102 per sievert; and (C) nonstochastic effects, including radiation-induced cataracts and adverse effects on fertility. In proposing limits for space activities, the NCRP looked at the extremes in risks for terrestrial radiation workers. For example, with a maximum permissible dose of 0.05 Sv (5 rem) pen year and a 50-year career, a radiation worker could conceivably receive a lifetime dose equivalent of 2.5 Sv (250 rem). This results in a lifetime risk ofdeveloping fatal cancer ofapproximately 5%. On the other hand, the average radiation worker receives an annual exposure ofabout 2 mSv (200 mrem), which results in a lifetime total dose equivalent of 0. 1 Sv (10 rem) and an excess cancer risk of less than 1%, which is comparable to risks of fatal accidents in safer industries. It did not seem reasonable to limit the exposure of astronauts to the average exposure of the terrestrial worker, given the other much more risky activities associated with space travel. It seemed more reasonable to limit the exposure to that of the more highly exposed terrestrial worker, that is, a dose of 0.05 Sv pen year for a potential 2.5 Sv pen lifetime. However, on the basis ofcurrent experience, it is unlikely that an astronaut’s total exposure will exceed about 0.7 Sv. Thus, the NCRP’s proposed limit for space workers is a career dose equivalent of 1.5 Sv (150 rem) or a risk ofless than 3% for fatal cancer. This 3% represents about one-sixth of the natural risk of fatal cancer and, in light of the other risks of space travel, was deemed to be acceptable. The age and sex differences for risk estimates were recognized by the NCRP, and the career whole-body for male and female
Table
dose-equivalent subjects as shown
limits were specified in Table 3 (1 1). These
3
NCRP Recommendations Astronauts’ Career
for Whole-Body
Dose-Equivalent
Limits
Age (y)
Female
Male
25 35 45 55
1.0 1.75 2.5 3.0
1.5 2.5 3.2 4.0
Limit
Source-Data
from
Note-Dose on a lifetime cancerof3
September
for several age groups values are the dose
reference
equivalent excess risk x
1991
(Sv)
1 1.
limits are based of death from
102.
Murphy
U
RadioGrapbics
U
893
equivalents Notice that
in sieverts the doses
that range
would from
risk of cancer mortality of 3%. rem) to a high of 4 Sv (400 rem).
produce a career a low of 1 Sv (100
The NCRP also recommended limits on dose equivalents for short-term exposures. For example, in Table 4 (1 1), the annual limit for irradiation of blood-forming organs
tion
is 0.5 Sv (50 workers.
Another
rem)
example
or 10 times
of risk-based
Standards for Hazardous 1979, when radionuclides
the
limit
presently
regulatory
Air Pollutants were listed
applied
activity
(13). under
to earth-bound
radia-
is the EPA’s National
Emission
The history ofthis activity goes the Clean Air Act as a hazardous
back to air pol-
lutant. In 1985, the EPA proposed final rules for several categories of radioactive material users, including licensees of the NRC. The EPA was forced by the federal court to propose standards for radionuclide exposure, following the precedent set for vinyl chloride, and the final proposal was published in 1989. The standards followed the two-part guidelines defined in the vinyl chloride case: (a) to limit risks for individuals most highly exposed to one in 10,000 and (b) to reduce the risk for as many people as possible to one in 1,000,000. On the basis ofcurrent risk estimates, this
results
mSv)
in a maximum
per
year,
ofwhich
to radioactive posed
iodine.
continuously
allowable
dose
no more
This to the
than
dose
to a member
3 mrem
estimate
maximum
of the
(0.03
assumed
mSv)
that
concentration
public
of
year
can
per
an individual
for 70 years.
10 mrem
would The
(0.1
be attributed
be ex-
EPA approach
to estimating the risks from radionuclide emissions is to either measure or estimate the emissions, predict concentrations as a function of distance from the source with use of a dispersion model, and calculate the exposures of the populations. The estimates of risk per unit dose equivalent are then used to set limits on emissions to
Table 4 NCRP
Career
Recommendations for Astronauts’ Short-term Limits for Protection against Nonstochastic
Dose-Equivalent Effects
Limit
Time Period
Blood-forming Organs
30d Annual
.
from
Source-Data
2. comparison Figure
reference
.
and
(Sv)
Lens of the Eye
Skin
1.0 2.0 4.0
1.5 3.0 6.0
0.25 0.5
Career
Limits
.
11.
The NRC’s of below-regu-
hatory-concern doses to doses ral background cal exposures.
(BRC)
from and
I
natumedi-
I 40
p [T#{244} I
6
Fi 0
All Medical Exams
50
Natural Radioactive Materials in the Body
BRC
Practice
Chest
X-ii
BRC
50
Affecling
Urnited
Numbe r of People
I
Practice Affecting Large Number of Pei 100
150
200
250
300
Radiation Dose (mrem)
894
U
RadioGraphics
U
Murphy
Volume
11
Number
5
keep the risks less than those the emissions are below these problem, the EPA has provided phiance. The program, named relative
leveh
With
use
of emissions of cost
previously limits can
mentioned. be extremely
a computer
program
Comply,
has
to assist
a tiered
options,
safety
from
the
EPA
for
for
is 500
times
radionuchide the
ganizations
greater
emissions.
of implementing
that
implementing
com-
depends
on the
standard
associated
for
with
than
the
Because these
number
of the
the
saved
with
perceived
medical
and
licensed
activities
various
acceptable
dollar spent to reduce the risk of of magnitude, if the money were nadionuclide emissions (16). For spent per year for automobile
analysis
restraints
limit
that this
of radionuchides.
estimates
one
by a licensee To address
in documenting
structure
concluded that for every death, the yield would be much higher, by orders spent on safety programs rather than on reducing example, the number of lives saved per $ 1 million risk
Verification complicated.
the
large
academic
EPA’s
proposed
cost-to-benefit
licensees,
ratio
professional
aggressively
oppose
or-
the
regula-
tion.
Another risk-based regulatory action is the proposed revision to section 20 of title 10 of the Code of Federal Regulations: the NRC Standards for Protection against Radiation (14). This regulatory action has been in development for several years and would essentially implement the effective dose-equivalent approach as proposed by the
ICRP
in
general risk
1977.
public
It would
that
are
establish
based
radiation
on risk
exposure
estimates
and
levels
for
a decision
workers
and
regarding
the
acceptable
levels.
A more recent NRC initiative based on risk estimates is the NRC’s Below Regulatory Concern Policy Statement for exempting low-level radioactive materials from regulations (8). The NRC believes that this statement will establish a consistent risk framework for regulatory decisions. It establishes dose criteria of 1 mrem (0.01 mSv)
and
and
10 mrem
from
sievert)
(0. 1 mSv)
all sources per
year
for
the
fact that variations vide some perspective the
on
the
hypothesis
is linearly
(0. 1-mSv)
which
individuals
regions
and
that and do
country
for
individual
respectively,
population.
exposures
and
It recognizes
1,000 and
from
single
person-rem
uses
sources
(10
as a basis
for
personjustification
from natural sources of radiation proof risk. The policy is explicitly based
the
fatal
risk
to the
medical
of the
year
in individual exposures on the acceptableness
proportional
mrem ground
per
ofradiation,
of developing dose
at 5 X iO
1-mrem
(O.01-mSv)
exposures
(Fig
not
associate
or living
levels
2) and an
also
unacceptable
in a brick
versus
cancer
per
rem.
from The
exposure NRC
by comparing
to doses risk,
a wood
to radiation
justified
the
them
to natural
from
other
activities
such
as living
home
(Fig
3.
Figure
comparison
other
for
in different
3).
The NRC’s of below-regu-
latory-concern doses to doses
lected
10back-
(BRC)
from
se-
radiation
sources.
30 40 Radiation Dose (mrem)
September
1991
Murphy
U
RadioGrapbics
U
895
The
response
to this policy
of acceptable vigorously
risk opposed
is not the
statement
has demonstrated
necessarily the same policy by associating
lem of disposal
oflow-level
radioactive
have
suggestions
that
ing
also
been
in contaminated
consumer
some time. A final example
waste
some
of risk-based
waste
power
is that
groups to the
companies.
will the
activity
perception
Several solution
materials
Undoubtedly,
regulatory
the public’s
regulators’. a careless
by nuclear
of the
products.
that
as the it with
have prob-
There
be recycled,
debate
will
relating
result-
continue
to indoor
for
radon
concentrations. The U.S. Public Health Service has declared that indoor radon gas is a national health problem and that radon causes thousands of deaths pen year. This has led the U.S. Congress to pass legislation that establishes as a national goal the reduction of indoor radon concentrations to the level of outside concentrations (15). Indoor radon is currently perceived to be the most important problem involving
radiation
exposure
the concentration would correspond This
of the
public.
recommendation
is quite
different
whose approach is somewhat the Food and Drug Act. This cinogenic,
the
cinogen CONCLUSIONS
(ie,
Most tude
people of the
basis
assumption
any
of these
for
to automated
limit
automatically
benefit purposes,
risk
estimates
that Also,
dose
estimate,
Most
people
conservatism
when
quotes
today
(17).
“For
practical
been
used
consistently
sents
a conservative
problems the
assessment
RadioGrapbic.s
U
as then,
is how
uncertainties
U
from
Murphy
although
these
the
because upper
uncertainties of the
dose
of a car-
of risk for
proceedings
linear,
estimates
regulatory
assessment
process but
rather
other
as pretypes
the
risk
estimates
doses,
with
their
regulations
when
of frequency
of cancer
of interest.
as relevant
to the
risks
dose
of
of the
the association
usually
addressed
non-threshold
such
with
setting
ranges are
the
the interpretation
predictions
in the dose
to
protection
reasons,
risks
at low
for
proceedings
of risk
other
estimates
as a
tends
the dose
for
Because
as a basis
risk doses
on deemphasize
established for
magniis the
acceptable at low
to ignore
monitoring.
an effective
assessment
regulatory
legislature,
is adjusted,
of radiation importance on
of its plausible
limit
in the risk
use
NCRP
purposes
to develop
in making
the
Once
estimates
tends
is inappropriate
1980
U.S.
Amendment to found to be car-
whole-body irradiation, difficult to interpret.
may be zero
the
range.
approach
appropriate
but
the
as a safe
a risk estimate
personnel
that
is still
issue
ation
agree
is justified
this
to uniform risk is very
in fact this frequency
following
896
would
uncertainty,
when
is proportional to the The major disagreement
As it becomes
comparison increased
from
are based on dose-equivalent between badge reading and
by
thing
of risk
approach
to apply
for example,
exposure range.
is, once
this
or
action
the Delaney food additive
such
low-dose
ratio.
is a tendency
of frequency of cancer Also, this approach places
proposed
is no
application
of the benefit-risk
there
there
in the
regulations; adjusts.
that
remedial
in the United States, which of approximately 4 x 10.
logic behind banned any
risk of radiation in the high-dose
diction risks.
significant
recommends
is unacceptable).
is established,
portion
from
that
cancer
risk
regulation
lead
NCRP
akin to the amendment being
excess
agree that the dose equivalent
application
The
exceeds 10 times the average level to an annual lung cancer incidence
from
The
radiation
carcinogens
response
model
has
scientific basis and because it preIndeed one of the unsolved .
.
.
program .
the
.
.
The
uncertainties
taking
central
into
issue in how
consider-
is not the to use
risk
decisions.”
Volume
11
Number
5
1.
2.
3.
4.
5.
Permissible
Research 6.
7.
8.
9.
September
dose
from
external
sources
of ionizing radiation. NCRP Report no. 17. Bethesda, Md: National Council on Radiation Protection and Measurements, 1954. Basic radiation protection criteria. NCRP Report no. 39. Bethesda, Md: National Council on Radiation Protection and Measurements, 1971. BEIR I Committee. The effects on populations of exposure to low levels of ionizing radiation. Washington, DC: National Academy of Sciences/National Research Council, 1972. Recommendations of the International Commission on Radiological Protection. ICRP Publication 26. New York: Pergamon, 1977. BEIR III Committee. The effects on populations ofexposure to low levels of ionizing radiation. Washington, DC: National Academy of Sciences/National
Council,
11
Accident facts, 1982 tional Safety Council, .
Guidance
data. Chicago: 1985.
on radiation
received
Na-
REFERENCES
in
space activities. NCRP Report no. 98. Bethesda, Md: National Council on Radiation Protection and Measurements,
1989. 12.
Macklis hormesis.J
RM, Beresford NucI Med
B. Radiation 1991; 32:350-
359. 13.
National ous
emission
standards
air pollutants:
for hazard-
radionuclides-final
rule
14.
and notice of reconsideration. FedRegister, December 15, 1989; 54(240):51654 (40 CFR § 61). Standards for protection against radia-
15.
tion: final rule. Federal Register, 21, 1991; 56(98):23360. U.S. Environmental Protection
eral
and Centers citizen’s
what
1980.
BEIR V Committee. Health effects of exposure to low levels of ionizing radiation. Washington, DC: National Academy of Sciences/National Research Council, 1990. Sinclair WK. The scientific basis for risk quantification. In: Quantitative risks in standard setting: proceedings of the 16th annual meeting, National Council on Radiation Protection and Measurements. Washington, DC : National Academy ofSciences, 1981; 3-33. Below regulatory concern policy statement. Federal Register,July 23, 1990; 55(141):29925. Hall EJ. Radiobiology for the radiologist. 3rd ed. Philadelphia: Lippincott, 1988; 516.
1991
10.
guide Health
ofHealth Collopy
and P.
Pennsylvania.
it.
Service,
Human Chapter Health
Agency
Control.
to radon:
to do about
Public
16.
for Disease
May
what
A it
Washington, U.S.
is and
DC:
Department
Services, 1986. news: western Physics
17.
1989; 17(12):8-10. Albert RE. Federal regulatory approaches to the assessment trol of risk from carcinogens.
Society
Newsletter
agency and conIn: Per-
ceptions of risk: proceedings of the 1 5th annual meeting of the National Council on Radiation Protection and
Measurements.
Washington,
DC: Na-
tional 6-14.
of Sciences,
1980;
Academy
Murphy
U
RadioGraphics
U
897
