Death in heat waves

Increasing evidence shows that atmospheric carbon dioxide levels are rising and are causing global warming.¹ Record air temperatures were recorded in Britain during the last month. Accurate estimates of the consequences of these must wait until daily mortality data are available, but press reports indicate that the hot weather caused around 1000 deaths during one week alone in Britain and perhaps 10 000 overall in France, where temperatures were higher.

Mortality in Britain is lowest when the mean daily temperature is 17-18°C. The number of heat related deaths per year, obtained as the number of excess deaths on days hotter than this, has averaged around 800 in recent years.² Most of those deaths are of people over 70 years of age, and most occur in the first day or two of a period of high temperature.³

Few of these deaths are recognisable clinically as being due to heat.⁴,⁵ Heat stress causes loss of salt and water in sweat, causing haemoconcentration, which in turn causes increases in coronary and cerebral thrombosis.⁶ Other deaths in heat waves are probably due to overload of already failing hearts, unable to meet the need for increased cutaneous blood flow in the heat. Very few heat related deaths in British conditions are caused by hyperthermia, overheating sufficient to cause denaturation of the body tissues, but patients unable to sweat because they have diabetic peripheral neuropathy or are taking anticholinergic drugs are known to be at risk in American heat waves. So are patients taking drugs such as barbiturates or phenothiazines, which depress reflex regulation of body temperature. Alcohol can also be dangerous in the heat, both through depression of the central nervous system and by causing diuresis and consequent dehydration.

According to some predictions heat related mortality will increase drastically as global warming develops,⁷ but recent evidence is relatively reassuring. Heat related mortality is similar in hot and cold parts of western Europe⁸ and in hot and cold parts of the United States.⁹ This implies that the populations of hot regions have adjusted by physiological or other means to their hotter summers. In Britain annual heat related deaths are in any case far fewer than cold related deaths, so that the initial effect of increased temperatures all year round, before such adjustment, would be to reduce net annual mortality.²

Analysis of actual changes in heat related mortality during global warming since 1971 is even more reassuring. Despite rises in mean summer temperatures of at least 1°C in southeast England and North Carolina heat related mortality has not risen in southeast England and has virtually disappeared in North Carolina.¹⁰ The latter represents something more than adaptation as it could prevent the mortality rising with higher temperatures but could not make it fall. The likely explanation is the increase in air conditioning (from 57% to 72% between 1978 and 1997) that has occurred in households in that region of the United States and is in turn attributable to increasing prosperity.

However reassuring these facts may be, they do not mean that nothing needs to be done. As the recent hot weather has reminded us climatic warming is not continuous but is interrupted by unpredictable fluctuations. As in the recent heat wave these can suddenly present populations with temperatures that they have never encountered before and are not prepared for. People with heat exhaustion need to be given fluids by mouth and sometimes intravenous saline and dextrose, but by far the most important measures are preventive ones.

Air conditioning can allow people to continue to work effectively in hot weather and may become necessary to prevent mortality, but it uses a large amount of energy, which can itself accelerate global warming. Simpler measures can be very effective in protecting elderly and other vulnerable people from the levels of heat likely to occur in Britain over the next decade. Continuing to eat regular meals and drink enough water will normally prevent dehydration during heat stress. An open window, fan, light and loose fitting clothing, avoidance of unnecessary exertion, and if necessary sprinkling water on the clothing, can prevent the heat stress.

Such action is effective only if it is taken in time, and it needs to be taken by the general public as well as staff working in homes caring for elderly people. The time to check that windows can be opened, and that a fan is available, is when hot weather is forecast, not when it occurs; delay until temperature reaches a peak may mean that nobody can free a jammed window, or find a fan, for many hours. Elderly people unable to do this themselves, or to get help from relatives or neighbours, need to be given a number to call for help. Brief messages, giving simple advice on these lines in news broadcasts and daily press when hot weather is forecast, could be the most effective way to reduce illness and death in heat waves.