Basic HTML Version
Methods
Included studies
This collaboration included all 13 European studies that
registered over 150 people with lung cancer and 150 controls,
incorporated detailed smoking histories, and sought radon
measurements
7
in homes that these individuals had lived in dur-
ing the past 15 years or more. Information on demographic and
lifestyle variables was compiled for each person by using a com-
mon format, and radon measurements were expressed in
becquerels (Bq) (radon disintegrations per second) per cubic
metre of household air.
On the basis of information from the studies on miners,
2 3
we
assumed that the period of radon exposure most relevant to the
risk of lung cancer was the 30 years ending five years before the
diagnosis of (or death from) lung cancer or, for those in the con-
trol group, before a corresponding index date. We excluded indi-
viduals for whom no radon measurements for this 30 year
period were available or with unknown smoking status. The
available radon measurements covered a mean of 23 years. For
relevant homes where radon measurements could not be
obtained (for example, because the house had been demolished),
we estimated the concentration indirectly as the mean of all the
radon measurements in the residences of control group
members in the relevant study area. Finally, to obtain the “meas-
ured radon concentration” for each individual, we calculated a
time weighted average of the radon concentrations in all the
homes occupied over the past 5-34 years with weights
proportional to the length of time the individual had lived in
each.
Statistical methods
We assessed the association between radon and lung cancer in
two ways. Firstly, a model was fitted in which the risk of lung can-
cer was proportional to (1
+
x
) where
x
is measured radon
concentration and the proportionate increase in risk per unit
increase in measured radon. Secondly, we subdivided cases and
controls by categories of measured radon concentration and
plotted relative risks across different categories against estimated
mean exposure levels in those categories. In both types of analy-
sis, confounding was controlled through stratification.
Radon measurements made in the same home but in differ-
ent years show considerable random variability, indicating some
uncertainty in the measured radon concentration for each indi-
vidual. Further random uncertainty arises as radon concentra-
tions in some homes could not be measured and were estimated
indirectly. Both types of uncertainty lead to “regression dilution,”
whereby the relation of risk to measured radon concentration is
substantially weaker than the relation of risk to “usual” (that is,
true long term average) concentration.
5 6 8
We calculated
dose-response relations both with and without correction for this
and estimated a time weighted average usual radon concentra-
tion for each individual (see www.ctsu.ox.ac.uk/radonmethods
for further details).
Results
Our analysis included 7148 people with lung cancer and 14 208
controls. For cases of lung cancer the mean measured radon
concentration was 104 Bq/m
3
while for controls the weighted
average of the study specific means, with weights proportional to
numbers of cases of lung cancer, was 97 Bq/m
3
(table 1). Among
controls, the percentage who were lifelong non-smokers
increased as radon concentration increased (percentages were
39%, 40%, 41%, 46%, and 48% for measured radon < 100, 100-
199, 200-399, 400-799, and
≥
800 Bq/m
3
after stratification for
study, age, sex, and region of residence; P = 0.001 for trend).
Risk of lung cancer versus measured radon concentration
After we stratified for study, age, sex, region of residence, and
smoking the risk of lung cancer increased by 8.4% (95%
confidence interval 3.0% to 15.8%; P = 0.0007) per 100 Bq/m
3
increase in measured radon concentration. We stratified for
smoking by first subdividing the individuals into seven categories
(lifelong non-smokers, current smokers of < 15, 15-24, or
≥
25
cigarettes a day, ex-smokers for < 10 years or
≥
10 years, and
others) and then further subdividing each group of current
smokers by the age at which they started smoking ( < 15, 15-17,
18-20, or
≥
21 years or unknown) and each group of ex-smokers
by amount previously smoked ( < 15, 15-24, or
≥
25 a day or
unknown). If smoking had been omitted from the stratification,
the risk of lung cancer would have increased by only 2.3% per
100 Bq/m
3
increase in measured radon, and if it had been
included with only seven categories, the estimated increase
would have been 5.2%. In all subsequent analyses we used the full
smoking stratification.
The proportionate increase in risk was not strongly
influenced by any one study. When we re-estimated the risk
omitting each study in turn, it changed at most by a fifth. Nor did
it vary substantially according to the period used to calculate
radon exposures. The above analyses relate to measured radon
concentrations 5-34 years earlier. Measured radon in periods
5-14, 15-24, and 25-34 years earlier were highly correlated, so the
relation of risk to radon in each of these three periods was simi-
lar to that for the entire period (7.5%, 7.6%, and 6.6%,
respectively). When we considered radon concentrations
throughout the period 5-34 years earlier but with contributions
from periods 5-14, 15-24, and 25-34 years earlier weighted in
proportions 1.0:0.75:0.50, as suggested by the miners’ studies,
2
the risk was unaltered, at 8.4% per 100 Bq/m
3
of measured
radon.
When we subdivided study participants according to seven
categories of measured radon (table 2), the results were consist-
ent with a linear dose-response relation (fig 1). There was no sig-
nificant curvature of the best fitting regression line, and no point
differed significantly from this line. The linear relation remained
significant even when we limited analysis to measured
concentrations < 200 Bq/m
3
(P = 0.04). When we compared
Table 1
European case-control studies of residential radon and lung cancer
Study
Mean year of
diagnosis
Mean measured radon concentration
(Bq/m
3
)*
Lung cancers
Controls
Austria
9
1983
267
130
Czech Republic
10
1981
528
493
Finland (nationwide)
11
1989
104
103
Finland (south)
12
1982
221
212
France
13
1995
138
131
Germany (eastern)
14
1994
78
74
Germany (western)
14
1993
49
51
Italy
15
1995
113
102
Spain
16
1993
123
137
Sweden (nationwide)
17
1982
99
94
Sweden (never smokers)
18
1990
79
72
Sweden (Stockholm)
19
1985
131
136
United Kingdom
6
1991
57
54
All studies
1990
104
97†
*Estimate for each individual is time weighted average of measurements in different
residences 5-34 years earlier.
†Weighted average, with weights proportional to study specific numbers of lung cancer cases.
Papers
page 2 of 6
BMJ
Online First bmj.com