The heat wave of 2003 caused an excess mortality of 150% across all age groups, with the largest increase (160-200%) in older people.1 Excess mortality was high across the whole of western Europe (>50 000 deaths) but highest in big cities such as Paris (1854 deaths).1 The death rate was particularly high in cities that were unaccustomed to high temperatures, and cities with warmer climates, such as those in southern France, were spared. This death toll was not followed by a subsequent decline in death rate, which can occur when serious environmental stress causes premature mortality in the frailest people.

No universally accepted definition of a heat wave exists. An alarm signal should be given for frail older people when the outdoor temperature exceeds 90°F (32°C) for three days or more, which is the most universally accepted definition of a heat wave. Meteorologists predict that global warming will dramatically increase the number of such periods.

The main causes …