Long-term effects of flooding on mortality in England and Wales, 1994-2005: controlled interrupted time-series analysis

Background: Limited evidence suggests that being flooded may increase mortality and morbidity among affected
householders not just at the time of the flood but for months afterwards. The objective of this study is to explore
the methods for quantifying such long-term health effects of flooding by analysis of routine mortality registrations
in England and Wales.
Methods: Mortality data, geo-referenced by postcode of residence, were linked to a national database of flood
events for 1994 to 2005. The ratio of mortality in the post-flood year to that in the pre-flood year within flooded
postcodes was compared with that in non-flooded boundary areas (within 5 km of a flood). Further analyses
compared the observed number of flood-area deaths in the year after flooding with the number expected from
analysis of mortality trends stratified by region, age-group, sex, deprivation group and urban-rural status.
Results: Among the 319 recorded floods, there were 771 deaths in the year before flooding and 693 deaths in the
year after (post-/pre-flood ratio of 0.90, 95% CI 0.82, 1.00). This ratio did not vary substantially by age, sex,
population density or deprivation. A similar post-flood ‘deficit’ of deaths was suggested by the analyses based on
observed/expected deaths.
Conclusions: The observed post-flood ‘deficit’ of deaths is counter-intuitive and difficult to interpret because of
the possible influence of population displacement caused by flooding. The bias that might arise from such
displacement remains unquantified but has important implications for future studies that use place of residence as
a marker of exposure.