A known relationship exists between temperature and mortality.1–5 In general increased death rates occur principally in the elderly6 and the relationship is considered to take the form of a ‘V’ or a ‘U’ shape, with the lowest death rates occurring on days of moderate temperature and highest rates at either end of the temperature range.7,8 In the UK a 2% increase in mortality has been estimated for every one degree fall in temperature from 18°C.9 Most of the excess mortality is due to respiratory, cardiac and cerebrovascular disease. Furthermore, the effect on these diseases of cold temperatures has been reported to occur over prolonged periods, in some cases up to a few weeks duration.10,11
A study of the relationships between registered deaths in the elderly and weekly data for the incidence of respiratory disease presenting to general practitioners (GPs) found a strong positive association between the two health outcomes.12 Despite this, one study could not establish any relationship between environmental factors and accounts of asthma or exacerbation of chronic bronchitis on the Swiss sentinel reporting system.13 However, more recent work from the UK has shown that a drop in temperature is associated with a rise in bronchitis consultation rates some 10–17 days later.14