Radon Exposure of the United States Population - Status of the Problem

31 Pages·1991·1.074 MB·31\31

NCRP COMMENTARY No.6
RADON EXPOSURE OF THE
UNITED STATES POPULATION -
STATUS OF THE PROBLEM
Issued March 15, 1991
National Council on Radiation Protection and Measurments
7910 WOODMONT AVENUE / BETHESDA, MARYLAND 20814
This report was prepared by the National Counci1,on Radiation Protection and
Measurements (NCRP). The Council strives to provide accurate, complete and
useful information in its reports. However, neither the NCRP, the members of
NCRP, other persons contributing to or assisting in the preparation of this
report, nor any person acting on the behalf of any of these parties: (a) makes
any warranty or representation, express or implied, with respect to the accuracy,
completeness or usefulness of the information contained in this report, or that
the use of any information, method or process disclosed in this report may not
infringe on privately owned rights; or (b) assumes any liability with respect to
the use of, or for damages resulting from the use of any information, method or
process disclosed in this report, under the Civil Rights Act of 1964, Section 701
et seq. as amended 42 U.S.C. Section 2000e et seq. (Title VIIJ or any other
statutory or common law theory governing liability.
Library of Congress Cataloging-in-Publication Data
National Council on Radiation Protection and Measurements.
Radon exposure of the U.S. population, status of the problem
-
P cm. -- (NCRP commentary; no. 6)
Includes bibliographical references.
ISBN 0-929600-17-7: $15.00 (est.)
1. Lungs--Cancer--UnitedS tates--Risk factors. 2. Radon--Health
aspects--United States. 3. Indoor air pollution--United States.
I. Title. 11. Series.
RC280.L8N37 1991
363.17'99--dc20
Copyright O National Council on Radiation
Protection and Measurements 1991
All rights reserved. This publication is protected by copyright. No part of
this publication may be reproduced in any form or by any means, including
photocopying, or utilized by any information storage and retrieval system without
written permission from the copyright owner, except for brief quotation in
critical articles or reviews.
1. Background
The National Council on Radiation Protection and Measurements
(NCRP) published two reports in 1984 that ,focused attention on the
need for better information concerning the exposure of the U.S.
population from radon and its decay products and the estimated lung
.
cancer risks from such exposure (NCRP, 1984a; 1984b) The purpose
of this commentary is to consolidate information fromthose reports
and from some more recent studies into a document that summarizes
information on radon and the Council's recommendations for a wide
readership.
Exposure of the general population to indoor radon was first
noted as a public health concern in the United States in Grand
Junction, Colorado, where uranium milling wastes were used as earth
fill under and around buildings. Later, concern was expressed for
residents in land areas of Florida reclaimed,f ollowing mining for
phosphate rock. A study of phosphate wastes in Montana led to the
discovery that the high levels of radon found in some homes came
from natural sources and not from mine wastes. As additional
surveys were made, it was evident that many geographical areas had
homes with elevated natural levels of radon. The Reading Prong
area in Pennsylvania, New Jersey, and New York has received the
greatest public attention, although it is not known how exceptional
this area is. It is clear, however, that there is a considerable
potential for widespread exposure to radiation from radon and its
decay products.
2. Radon Concentrations
2.1 Outdoor Radon Concentrations
Radon is the immediate decay product of radium which is
present at low concentrations in most materials. For example,
soils and rocks contain radium at a concentration of about 40 Bq
kg-'. Some of the radon produced by radium escapes into the air
spaces around soil particles and then diffuses into the atmosphere.
Average soil releases about 0.02 Bq m-2 s-' so that the earth's
surface contributes about 100 EBq annually to the atmosphere. This
leads to an average outdoor air concentration of approximately 8 Bq
m-3o ver the northern hemisphere continents. This calculated value
is in agreement with measured values. Releases from the oceans are
much smaller, with the result that air concentrations at sea are
about one .percent of those over land.
The radon released at the soil surface is dispersed upward by
convection. Radon is found in the troposphere but stratospheric
concentrations are too low to measure. Under inversion conditions,
upward dispersion is limited, and most locations show a diurnal
cycle of concentration, rising at night and falling in the morning
when the inversion breaks up. There is also a seasonal cycle which
is dependent on location. The concentration in air is also
affected by ground freezing, rainfall and other factors. Both
diurnal and seasonal factors cause radon concentration to vary by
a factor of two to three in temperate areas. The diurnal factor
may be larger in coastal areas where the change from offshore to
onshore winds can produce greater swings.
2.2 Indoor Radon Concentrations / 3
Radon decay products outdoors are at about 70 percent of
equilibrium e , the concentration of the short lived decay
products of radon are about 70 percent of the concentration of the
radon), with the unattached fraction1 at somewhat below 10 percent.
With this degree of equilibrium, the estimated average outdoor
concentration of 8 Bq m-3o f radon would be equivalent to a working
level (wL)~va lue of about 0.001 and an annual exposure of 0.08
working level month (WLM)~fo r an individual outdoors full time.
2.2 Indoor Radon Concentrations
When radon is released into an enclosed space, it cannot
disperse into the atmosphere and continued release results in a
buildup in concentration. This is the case both in mines and in
houses.
The major source of radon in houses is that formed in the
soil beneath and immediately around the house. Releases from
building materials and domestic water are almost always secondary
sources and natural gas is a negligible source of radon and radon
decay products in houses (NCRP, 1984a).
l~hef raction of short lived radon decay products not attached to the
ambient aerosol.
2 Any combination of short-lived radon decay products in one liter of air
that will result in the emission of 1.3 x lo5 MeV of potential alpha energy. The
SI unit of WL is J m-' or the radioactivity concentration of radon in Bq m-3 may
be used for WL when the equilibrium fraction of the short lived decay products
of radon are known, i . e. , 218~o2,1 4~abn d 214~i.
30ne working level month (WLM) is equivalent to an exposure to an air
concentration of 1 WL for a working month of 170 hours., The SI unit for exposure
to the short lived decay products of radon is Jh m-' where 1 WLM is equal to 3.5
x Jh mT3.
4 / 2. Radon Concentrations
Measured values of indoor radon in houses show a log-normal
distribution. NCRP Report No. 77 (NCRP, 1984a) presented data
demonstrating that with a log-normal distribution a large number of
houses will show concentrations of 10 or even 100 times the average
value, which is not the case for a normal distribution. Different
surveys have shown log-normal distributions but with a
significantly different distribution of readings. This can have a
large effect on the predicted number of houses with high
concentrations as demonstrated by the data in Table 2.1.
TABLE 2.1 Reported distribution of radon in U.S . living areas
Average Radon Concentration Percent Greater than
~q m3 150 Bq m3 Reference
3 7
5 5 7 Nero et al., (1986)
26 0 2 3 Alter and Oswald (1987)
12 0 19 Cohen (1988)
Living areas in close proximity to soil have the highest
radon concentrations. In most cases, the basement of a house has
the highest concentration with the radon concentration at
successive higher floors decreasing by a small factor.
Concentrations in public buildings, such as offices and schools,
are generally lower than those in Table 2.1, largely because of
greater ventilation along with a lower contribution from soil due
to more substantial foundations. High-rise apartments are also
lower since the living areas are well removed from soil.
2.2 Indoor Radon Concentrations / 5
Many indoor radon measurements have been made, but their
value in estimation of the average exposure is doubtful. The
United States Environmental Protection Agency (EPA) and the state
programs that they support use radon screening measurements
designed to determine the potential maximum radon concentration
rather than the average exposure of the occupants. Their protocol
requires measurement in the lowest livable level of the house under
closed conditions that assures the accumulation of data that
overestimates the exposure of the occupants. The data bases of
commercial radon measurements made by vendors are biased by the
fact that the customers often have a reason to believe the radon
concentration is high in their houses. One vendor, the Radon
Project in Pittsburgh, has attempted a random sampling by mail
(Cohen and Pondy, 1987), but the low response rate maym have
produced a bias toward higher values. Considering that many
vendors have difficulty measuring radon concentrations below the
EPA guideline of 4 pCi 1-I (150 Bq m-3),i t is likely that the bulk
of current data is biased toward high value^.^
There is a need for a program of random sampling,
statistically stratified, to estimate the average exposure and the
exposure distribution of occupants of'houses in the United States
as was recommended in NCRP Report No. 77 (NCRP, 1984a) . The EPA
has had such a national program in the planning stage for some time
and it has now been implemented. Results from this study are
planned for release in 1991.
4 ~ h est ated unit followed by the SI unit in parentheses is utilized in
those instances that refer to a specific guideline or recommendation.
6 / 2. Radon Concentrations
2.3 Radon Exposures
The annual exposure of workers working for 170 hours per
month is 12 times the average WL they are exposed to during the
year, i.e. twelve 170 hour periods in a year. For the general
population, total outdoor plus indoor exposure is continuous for
730 hours per month, rather than 170 hours and, therefore, their
annual exposure is about 50 times the average exposure rate they
are exposed to for the year. A United States average indoor
concentration of 8 x lo-' J m-3,e stimated from the measurement data
of George and Breslin (1980), gives an annual exposure of 7 x
Jh m-3 or 3.5 x Jh m-3 in 50 years ( e - g . , 8 x lo-' J m-) x 8.76 x
lo3 h y-I = 7 x Jh m-3y -l, -a nd 7 x Jh m-3 y-' x 50 y = 3.5 x
Jh m-3 in 50 years) . This estimate was based on a small sample
of houses and the average exposure rate is probably higher, from 8
x lo-' up to 2 x J m'3. This gives a 50 year exposure range of
3.5 x lo-' up to 9 x lo-' Jh m-3.
ological Studies
Introduction
Epidemiological studies aimed at elucidating the lung
cancer risk attributable to radon exposure have focused primarily
on miners. However, because lung cancer incidence is high both in
the miner groups that have been studied and in the general
population, an excess incidence in the miner groups due to radon is
difficult to ascertain accurately. Identification of appropriate
control study groups that allow for age, sex, cigarette smoking
history, age at exposure, magnitude of radon decay product
exposure, etc., is exceedingly complicated and introduces
considerable statistical variation.
Nevertheless, estimation of the risk of exposure to radon
decay products by epidemiological studies of the miner groups and
then the projection of these risks to lifetime risk is the only
method currently available to estimate the risk to the general
population, provided the exposure of the population is known.
3.2 Underground Miner Studies
There have been four major retrospective epidemiological
studies of underground miners for which there have been updates
since NCRP Report No. 78 (NCRP, 1984b) was published. Three of
these are of uranium miners, the Colorado Plateau series (Hornung
and Meinhardt, 1987), the Ontario series (Muller et dl., 1985) and
8 / 3. Epidemiological Studies
the Czechoslovakian series (Sevc et al., 1988); the fourth study is
of the Swedish iron miners (Radford and Renard, 1984). Table 3.1
indicates the size of the study groups, their average exposure, and
their observed, expected and, excess lung cancer mortality. Two
additional studies, the Eldorado (Canada) cohort (Howe et
dl.,
1986) and the Newfoundland fl uorspar cohort (Morrison et al., 1988)
may provide further information when complete.
TABLE 3.1 Mortality from lung cancer in major mining cohorts as of
the most recent follow-up
Number in Averaqe Lunq Cancer Deaths
Cohort Cohort Exposure Observed Expected Excess
(Jh KI-~)
Colorado 3360 2.8 256 5 9 197
Ontario 10661 0.13 8 0 5 6 2 4
( Canada )
Czechoslovakia 3043 0.79 4 8 4 9 8 3 8 6
Malmberget 1292 0.33 5 1 15 3 6
( Sweden )
Total excess deaths 643
There are a number of problems with these retrospective
studies that introduce uncertainties into the risk estimates. The
exposures of the miners are poorly documented and, in fact, many
early exposures were not measured. The contribution of cigarette
smoking to lung cancer is high in some studies and the smoking
histories are not well known. Epidemiologists have attempted to