The impacts of heat waves on mortality and morbidity are now well documented in France. Nearly 15,000 deaths were recorded during the 2003 heat wave, and nearly 2,000 during the 2006 heat wave. During heat waves, the relative impact is greater in large urban areas, and the most vulnerable people are the elderly, whose numbers are projected to double by 2050 (see box). One of the reasons cited for this greater vulnerability in urban areas is the existence of urban heat islands (UHIs). UHIs are characterized by the fact that ambient temperatures in urban areas are generally several degrees higher than those in surrounding rural areas. This is due to the replacement of bare soil and vegetation in urban areas with impervious surfaces that heat up more easily, to urban structures that hinder heat exchange between the ground and the air, and to heat emissions from human activities. Urban heat islands therefore expose the urban population to higher temperatures, with temperature differences that can be considerable from one neighborhood to another. This phenomenon tends to increase over time. For example, the measured difference between the city and the surrounding countryside was +1 °C in 1868 in Paris, whereas it reached +10 °C in 2003. Several studies have estimated that the UHI increases the risk of mortality during heat waves, as seen, for example, in Philadelphia, Berlin, or Shanghai. (R.A.)