Warm conditions affect human health, but the largest impacts are created by strong and prolonged events. These events, which are called ‘heatwaves’, are generally described as a period of abnormally high and quite often humid weather, usually lasting for a minimum of one day.
However, heatwaves that cause high or catastrophic impacts generally last considerably longer, sometimes even weeks at a time. The most hazardous conditions to human health are multi-day heatwaves where extreme daytime temperature is combined with high nocturnal temperatures, high-relative humidity and light wind conditions for a period of several consecutive days.
Heat is particularly a problem within large urban areas containing dense populations, because of the amplifying effects of the urban heat island in combination with atmospheric pollutants. The urban heat island effect is the thermal contrast between urban space and its surroundings, primarily occurring due to non-evaporating surface materials such as asphalt and concrete (Figure A). These materials retain the heat during periods when it would normally cool down, for example during the night.
The urban heat island effect is the clearest example of an anthropogenic impact of climate at the local level, and can exacerbate already high temperatures in cities. During the 2003 heatwave in Europe, anomalous heat produced nocturnal temperatures in London that reached 6-8 degrees higher than those found in rural environments.
The impacts of heatwaves represent an emerging environmental health concern. Recent heat events, in particular the 2003 event which accounted for up to 80,000 deaths (1), give an example of this health burden. For the period 2000-2016, at least 136,835 fatalities have been recorded in Europe due to heat-related health complications, which represents more than 87.1 % of all disaster-related deaths in that area (2). Figures that illustrate heat-related mortality are deeply alarming, and are likely to be underestimated because of lack of surveys and misreporting. Even when a heatwave is not technically in progress, warm temperature conditions are linked to mortality and morbidity (3).
Every year, a significant number of people die and/or require hospitalisation because of the physiological stress imposed by elevated levels of ambient heat. A ‘j-shaped’ (see Figure B) graph often represents the connection between mortality and both cold and warm temperatures. The optimum or ‘healthy’ temperature is dependent on average temperatures experienced in a geographical region (linked to latitude) as well as the implementation and effectiveness of adaptive measures designed to acclimatise populations to warmer or colder temperatures conditions.
While there is a predominance of research focused on heatassociated mortality in Europe, a significantly smaller number have been preoccupied with heat-related morbidity