Occupation and cancer - European Trade Union Institute (ETUI)

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768 E. Pukkala et al.
1980 and 1990 censuses. For Norway the censuses
of 1960, 1970, and 1980 have been included,
as the 1990 census included only a sample of the
Norwegian population.
In Norway control surveys based on personal
interviews with random samples of the census
population were undertaken both in 1970 and in
1980. Compared with the census, the interview data
gave similar occupational codes on the two digit level
of the NYK-code. Compared with the census, overall
2.1% more men and 13.8% more women in the
survey reported to have more than 500 hours of
income producing work and should have been
classified as economically active. The main deficits
in the census data came from agriculture and public
and private services [151].The validity was considered
to be equally good for the 1960 census [41].
In Sweden, a control study of the 1970 census
data showed an underreporting of economic activity
in the census data for persons with 20 or less hours
of work per week coming especially from women in
agriculture [152]. The distributions by the main
occupational categories for those recorded economically
active were fairly similar in the census and
control surveys. The accuracy of the occupational
codes used in Finland has also been proven to
be high [153].
In Denmark, no coding errors were detected in a
sample of 1970 census questionnaires retrieved for a
nested case-control study [154]. In general, the
validity studies indicate that the classification by
occupation in the Nordic censuses has been reasonably
accurate, but that economic activity has been
somewhat underestimated, especially among the
women.
We grouped the economically active population
into relatively specific categories that are large
enough to get stable numbers of observed cases
and that can be defined similarly in different coding
systems in the countries. From the point of view of
work place exposures, some of the occupational
categories are heterogeneous and may therefore
hide occupational risks. For example, nickel smelting
workers, who are known to have about a 30-fold
excess risk of nasal cancer [80], are here grouped
together with other occupations in the category of
smelters and metal foundry workers, and the SIR
for nasal cancer in this broad group was only 1.20
(95% CI 0.93 1.53; Table 30). Still, a major part of
the known associations between occupations and
specific cancer diseases was seen in the present
study.
Occupational mobility
Occupational classification in this study is based on
the occupation recorded in the first census the
person participated in the age range of 30 64 years.
The extent to which the first census occupation
reflects the lifetime experience varies considerably
across occupational categories. We estimated the
occupational stability proportion of individuals who
had the same occupational category (of the 53
categories defined for the present study) in the first
and second census available to us, i.e., 1960 and
1970 censuses in Norway and Sweden, and 1970
and 1980 censuses in Finland (Table 82). In general,
stability was higher among men than women and
highest in occupational categories where a long
education is required such as physicians, dentists
and teachers. Nurses were more likely to keep their
occupation than assistant nurses, except in Sweden,
where female assistant nurses also had a high rate
of staying in the occupation. Hairdressers in all
countries and of both genders tended to remain
in the occupation. Among men, the category of
domestic assistants was the least stable. Among
women, the least stable categories were occupations
with male dominancy, and occupations requiring
little specific education.
There are several reasons for low occupational
stability rates:
(1) The number of employed persons in an
occupational category may have decreased
in the society as a whole. This applies, for
instance, to agricultural work (farmers) or
industrial plants which were closed down,
such as asbestos mines in Finland [155];
(2) Individual career development may have
moved workers to leading positions. To avoid
the occupational misclassification related to
the beginning of the work career, the followup
in our study started with the occupation
held at the age of 30 or older;
(3) The changes in coding practises between
subsequent censuses could have influenced
classifications. This is believed to have only a
minor impact, except for some new occupations.
In terms of traditional occupational
exposures, it is assumed that the movement
has been from the more to the less exposed
categories.
An earlier study from Finland analysed occupational
mobility between the censuses from 1975, 1980, and
1985 [153]. In both genders, 85% of persons worked