Introduction

The United Nations, in their Demographic Year Books, publish, approximately every six years, details of monthly mortality statistics (deaths from all causes) gathered from many countries throughout the world. For many of these countries monthly mortality figures are strongly correlated with season. Seasonal fluctuations in mortality can be attributed to many factors depending on the region, for example to seasonal availability of food, to climatic phenomena such as monsoons, and to seasonality in the occurrence of disease epidemics. Climatic temperature itself is a primary component, with highest death rates coinciding with seasonal temperature extremes of heat in low latitudes and cold in high latitudes. Seasonal swings in morbidity and mortality are not confined simply to regions with extremes of temperature. Many temperate countries, including the British Isles, exhibit strong seasonal mortality trends (Fig. 11.1).

It is of considerable concern that there is an excess winter mortality in the British Isles that is of a magnitude much higher than in many other comparable countries, particularly Scandinavia and neighbouring parts of Western Europe. Many of these European countries experience much more severe cold winters than in Britain. Certain ‘at-risk’ groups such as infants, the sick and disabled, and the elderly, are the most vulnerable to the effects of seasonal temperature changes. In Great Britain the high excess winter mortality, especially in the elderly, is well recognised, but the aetiology of this phenomenon is not fully understood.

Early studies on seasonal mortality

A brief mention of the influence of season on mortality was made in the Second Annual Report of the Registrar General, representing England in 1840. A more extensive discussion of the effect of low temperature on mortality in London (1838–41) appeared in the subsequent report of 1841.