Childhood leukaemia and non-Hodgkin's lymphoma near large rural construction sites, with a comparison with Sellafield nuclear site

RURAL INDUSTRIAL SITES

The following criteria were used to select for study rural non-nuclear projects outside the North Sea oil industry: (a) situated in a (pre-1974) local authority rural district; (b) over 20 km (as in a previous study6) from a large town (population over 70000; an arbitrary definition that embraced most of the former county boroughs); (c) built over at least three successive calendar years; (d) involving a peak workforce of more than 1000. In the absence of any single source of relevant information details of rural projects over the past 50 years were compiled with the help of the annual official handbook on Britain10 (section on large construction projects), the managers of the major fossil fuel power stations in rural Britain, senior members of the construction industry, and a limited search of civil and mechanical engineering publications. Though we tried to be comprehensive, one or two projects may have been overlooked or inappropriately excluded because of incorrect information on, say, the peak workforce. The projects (table I) were mainly power stations, hydroelectric schemes, and oil refineries. Most required work camps, reflecting the need for outside labour. Some were very large, such as Longannet and Drax, successively the largest fossil fuel power stations in Western Europe, and Dinorwig, the world's largest (hydroelectric) pumped storage scheme.

The extensive postwar hydroelectric work in Scotland concerned some of the remotest parts of Europe. By 1965, apart from the laying of thousands of miles of electric cable in the highlands, this included 56 power stations and 54 major dams.11 Individually most of these projects were carried out with workforces of less than 1000, though the Loch Sloy scheme in Dunbartonshire was an exception.12 Collectively, however, several thousand men were involved in the construction at any one time over a wide period. These projects fulfilled conditions (a) and (b) individually, but conditions (c) and (d) only collectively. They were therefore treated collectively in this study, on a county basis.

Several otherwise suitable construction projects such as the Channel tunnel were not eligible for inclusion because they were within 20 km of a large town. However, the Fawley oil refinery in New Forest Rural District, built in 1949-51 by a workforce of over 5000, seemed to warrant inclusion though within 20 km of Southampton; an unbridged inlet of the sea (Southampton Water) prevents direct access and the journey by road exceeds 20 km.

STUDY AREAS AND STUDY PERIODS

In England and Wales the study areas comprised those parishes largely within 10 km of each site, whereas in Scotland the (smaller) local authority areas were used before 1974 and afterwards postcode sectors. The 10 km distance was arbitrary but, given the minimum 20 km separation from a large town, seemed reasonable. In the case of Scottish hydroelectric schemes the local population was usually very small. In view of this and the fact that the hydroelectric board as policy employed a high proportion of men from the highlands,11 12 the whole of the relevant counties (apart from any large burgh) were included so as to cover (as in a previous study6)the home areas of those local workers. Inevitably this crude approach included many unaffected communities. (Argyll was not included because of the fewer men employed in this work in this very large county (of over 800000 hectares).)11

Three periods were chosen for study: (a) from the calendar year in which construction began to the year after its completion, with different construction phases on the same site separated by two or more years being examined separately; (b) parts of the above that overlapped with the operation of the plants; and (c) the five year periods that preceded and followed (a).

LEUKAEMIA AND NON-HODGKIN'S LYMPHOMA

The incidence of leukaemia and non-Hodgkin's lymphoma below age 15 in study areas was examined by using registration data (available from 1962) from the national registry maintained by the Childhood Cancer Research Group. Before 1962 mortality was our measure of incidence, the diseases being then almost invariably rapidly fatal, and we used leukaemia death certificates for 1945 and later years, assembled many years previously for certain Medical Research Council studies.13 14 Corresponding details for non-Hodgkin's lymphoma deaths were available for 1953-62 from the Childhood Cancer Research Group.

ANALYSES

In each study area observed numbers of cases were compared with expected numbers calculated by applying the relevant national rates at ages 0-4, 5-9, and 10-14 years to the corresponding local populations. Local population details were taken from the (partly unpublished) small area statistics of the relevant censuses, supplemented in intercensal years by estimates obtained by linear interpolation.

Observed and expected numbers of cases were examined in terms of the proportions of social classes I and II combined (approximated as in a previous study15) among economically active men. These were derived from parish data from a 10% sample of the decennial censuses (1961, 1971, 1981) made available by the Office of Population Censuses and Surveys. Social class details for Scotland were also available from the censuses. The constituent parts (mainly parishes) of all the study areas were first ranked on the basis of these proportions in 1971, the census year in which more areas were under study than any other. In this order their childhood populations (0-14 years) were accumulated and then divided into two similar sized groups of respectively “lower” and “higher” social classes. It was then convenient to apply the 1971 cut point (15.3% of social classes I and II) to similarly ranked data from the other censuses when there were much smaller numbers of areas to consider. This procedure was not intended to correspond to any independent measure of social class but simply to provide a basis for deriving two groups of contrasting social class in a way that was independent of the leukaemia findings. Finally, comparisons of the incidence of these malignancies were made between the study areas and a similar area around Sellafield in Cumbria, where an excess of leukaemia and non-Hodgkin's lymphoma has aroused much concern.

COMPARISON WITH SELLAFIELD AREA

The population mixing hypothesis was partly suggested by the excess of childhood leukaemia and non-Hodgkin's lymphoma near the nuclear site at Sellafield.1 The relevance of construction workers to this hypothesis was suggested by observations in a study of the North Sea oil industry,6 implying a similar role at Sellafield.6 18 Despite support for the present hypothesis1 2 3 4 5 6 7 8 9 and the finding that an alternative hypothesis19 about paternal preconceptional irradiation was incorrect,20 the magnitude of the excess in Seascale, near Sellafield, and possibly also the obvious source of radiation nearby have weighed in the minds of some against population mixing as the explanation for this particular increase. However, two considerations tend to be overlooked. The first is that the well known excess in Seascale tends to restrict interest to this parish taken in isolation from the entire area close to Sellafield. A geographical study prompted by concern about Sellafield would initially be directed to the area surrounding the plant rather than to a single part or segment of this (see below). When the area within 10 km of Sellafield was considered as a whole (table VI) the excess of leukaemia and non-Hodgkin's lymphoma (observed to expected ratio 1.67) was similar to that in the study areas in the overlap periods (1.72, table III).

However, the increase of these diseases near Sella-field was concentrated in Seascale, where eight cases in children below age 15 occurred in the period 1947-93 compared with about 0.9 expected (observed to expected ratio 8.9, table VII). Comparisons of the magnitude of this excess with those in the study parishes are influenced by the fact that, as emphasised recently,21 it was the Seascale cases themselves (specifically those before the report by the Black committee in 1984) that drew attention to this parish, thereby making conventional statistical tests inappropriate. Somewhat comparable would be for us to single out one parish near the Drax power station simply because of the five cases in a prolonged overlap period (7.9-fold increase). In Seascale only the cases after 1983 are amenable to hypothesis testing since appreciable opportunities exist for chance to inflate the excess among the earlier cases, a group that was in effect stumbled on.21

The second consideration is the highly unusual demography of Seascale, which greatly limits testing any hypothesis at the parish level, comparable places being so rare. Situated more than 60 km from a population centre, it is additionally secluded by being away from a main road and, with the sea beyond, it is effectively a cul de sac. It is also distinctive in being the Cumbrian parish with the highest proportion of residents working at Sellafield; it also has an 18-fold higher proportion of Sellafield workers born outside Cumbria than among such workers in the rest of West Cumbria.22 The nuclear authority was largely responsible for the building programme that resulted in it growing threefold between 1948 and 1961, and the houses were originally allocated to staff mainly on the basis of grade. As a result Seascale has an exceptionally high proportion of people in social class I: in 1961 this was 43% compared with 4% nationally, and in 1971 it was 46% compared with 7% nationally.23 These proportions were responsible for the high social class structure of the whole Sellafield area shown in table VI.

Among the 124 constituent units in this study, only one resembled Seascale in having 41% of men in social class I (the next highest having only 20% and 22%). Interestingly, a case of leukaemia occurred in this small parish (with less than a tenth of Seascale's population), which is a significant excess (expected 0.017; observed to expected ratio 58.8) for it represents a test (the only one possible in this study) of the importance of Seascale's high social class structure. The confidence interval around the 58-fold increase easily encompasses both the unbiased 10-fold excess in Seascale since 1984 (also based on one case under 15 years) and the potentially biased ninefold increase before 1984 (see table VII).

A remarkable aspect of the excess of leukaemia and non-Hodgkin's lymphoma in the Sellafield area is its duration, for it has been present from the 1950s. This might seem to argue against an infective explanation, as most infective epidemics are self limiting, ending when the number of susceptible people has declined below some critical level. However, two facts are relevant. Firstly, the level of movement among the administrative and scientific staff there both to other parts of the nuclear industry in Britain and also from Seascale to more traditional villages in Cumbria is high; the large numbers of new arrivals who replace them include susceptible children23 (and perhaps some adults). The presence of these new arrivals combined with the effects of hygiene (high social class) and geography (isolation) would ensure a continuing high level of susceptible people, a potent factor in promoting and maintaining an epidemic.24

Secondly, the other requirement for a prolonged epidemic—namely, a source of infection—is adequately provided by the large numbers of construction workers continuously over a long period at Sellafield, an aspect that distinguishes it from other rural industrial sites in Britain. Quite independently of the large nuclear workforce (up to more than 9000), a large contractors' workforce (reaching a peak of 7800 in 1991) has also been present on the site in most of its 45 years of operation. Only one site in our study (Drax) has had a prolonged period of construction work after its operational start, and it alone has shown an excess of childhood leukaemia and non-Hodgkin's lymphoma over an extended period (see tables II, III, and VI).

In a recent study25 of the distribution of cases of childhood leukaemia and non-Hodgkin's lymphoma children within 25 km of the 29 nuclear installations in England and Wales only Sellafield showed a noticeable distance effect, entirely due to the Seascale cases. However, in contrast with the present study all but one site was studied after completion of construction; few of the 29 had a later construction phase, and these were not examined separately. The findings therefore do not conflict with those of this study.

CONCLUSIONS

Overall these findings provide further support for the hypothesis that prompted the study—that rural population mixing is conducive to the transmission of the underlying infective agent(s) among susceptible people so as to increase the incidence of childhood leukaemia and non-Hodgkin's lymphoma. These excesses had not been noticed before because no one had looked for them. No doubt if any of the situations had included a source of radiation the excesses would have attracted attention sooner. The study also finds that the increased incidence near Sellafield (the source of the hypothesis) is not very different from that near large non-nuclear rural construction sites, particularly when (as at Sellafield) operating staff and construction workers are present on the same site. The findings also predict that in a parish like Seascale the excess would be particularly noticeable as a result of its high social class make up and relative isolation—and also prolonged because of the continual influx of new susceptible people.