I.3.1. Risk of mortality and morbidity during periods of extreme heat

The exposure of an individual to a high temperature environment can lead to insufficient response of thermoregulatory mechanisms in human body which in its turn can lead first to discomfort, then to a heat stroke, which increases mortality risk. People with already sensitive health condition (elderly, people with chronic illnesses, infants, etc) are particularly vulnerable. During a heat wave, they can experience dehydration, worsening of their chronic illness or heat stroke. A healthy adult can tolerate a variation of about 3 ° C in body temperature without the physical and mental performance being significantly affected. (Ministère de la santé et de la protection sociale, 2004). However, even people in good health condition are also vulnerable to heat-related illness if they don’t take elementary protective measures (as decreasing exposure to heat, reducing physical activity, drinking extra liquids, and increasing time spent in air-conditioned places) (Kilbourne et al., 1982) (See table 4 for heat-related illnesses)

Table 4. Heat related illness Adapted from Gauthier et al 2005

Heat-related condition Symptoms
Heat stress

(body temperature t°C >37°C but <40°C)

Anxiety , headache, discomfort

Dehydration, hypovolemia

Dizziness, fainting, generalized weakness

Hypotension, tachycardia

Intense thirst ( may be absent in the elderly )

Heat stroke

(body temperature t°C > 40°C )

Skin is warm and dry

Confusion, delirium

Convulsions, coma

hyperventilation

Hypotension (25 % of patients)

Heat waves are associated to short-term mortality rise (Fig.10) (Kovats et al. 2006, Knowlton et al. 2009; Naughton et al. 2002; Basu and Ostro 2008; Whitman et al. 1997; Poumadere et al. 2005; Reid et al. 2009) Yet it’s difficult to attribute mortality directly to heat, as epidemiological studies show that excess mortality during heat waves is caused by cardiovascular, respiratory and neurological illness (Kovats and Ebi 2006). There’s also high confidence that during a heat wave outdoor workers, children, homeless people, elderly, women are especially vulnerable (IPCC 2014)

During the 2003 heat wave in France three groups of mortality causes were identified: the causes directly related to heat, as heat stroke, hyperthermia and dehydration; other causes for which mortality was also significant: mental disorders, respiratory disease, infectious disease, etc and other medical causes. (Laaidi et al 2012)

10

Fig.10 Death rate (absolute) per 1000 in France 1960- 2012: evident pic of mortality in 2003. Source: adapted from World Bank, University of Sherbruck, 2014

 There are two ways of assessing mortality during a heat wave: determine either the number of heat –related deaths or excess mortality rate.

A heat related death is usually defined as a death with a core body temperature ≥ 40.6ºC/105ºF with no other reasonable explanation of death (Donoghue et al., 1997). Yet, people with chronic disease like cardiovascular disease experience the aggravation of their health condition during a heat wave. So, in case they die, they die not directly because of heat, but because of illness, aggravated by heat. (Kovats and Ebi, 2006) Sometimes the definition of heat related deaths is widened to include the deaths indirectly related to heat, so the estimates may vary (Donoghue et al, 1997)

Another way to assess the number of heat wave related deaths is to calculate the excess mortality rate. Excess mortality rate is the difference in the mortality rates during and before the heat wave. For example, in a very early study directly after the 2003 heat wave in France Inserm (National Institute of Health and Medical Research) estimated the excess mortality by comparing the average daily number of deaths during July to September period of 2000, 2001 and 2002 and the actual daily number of deaths during the same months of 2003 (Hémon and Jougla, 2004)

Unlike deaths of heat stroke, heat-related deaths can occur with a short-term forward displacement of deaths with respect to the heat wave period, meaning that heat waves can cause mortality even after the heat wave is over. (Huynen 2001)

Morbidity and mortality issues differ across regions due to differing behavioral adaptation, psychological readiness and housing conditions and, thus, population in Northern France, for example, is more vulnerable than that of Mediterranean region (Kovats and Ebi 2006). Yet, according to the same source, no population is completely acclimatized or adapted to very hot weather, so even for adapted population risk will increase at extreme temperatures (Kovats and Ebi 2006)

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