The main objective of this study was to assess the extent of health effects during the 2008 and 2009 extreme heatwaves in Adelaide and to compare to those for averaged heatwaves over the previous years. IRRs for total ambulance call-outs and for total mortality during both extreme events were higher than those for the previous averaged heatwaves, but the 2009 heatwave eclipsed the longer, but less intense 2008 event.
Although total hospital admissions and emergency department presentations were only modestly increased, specific health outcomes such as renal, mental and direct heat-related diseases were affected. Renal disease-related increases were of particular concern during the 2009 heatwave. As highlighted in a previous Adelaide study, and substantiated by international studies, heat-related dehydration appears to promote acute renal failure. Additional factors such as the physiological effects of ageing, personal behaviour, cognitive ability, pre-existing chronic disease and related medication use have also shown to contribute to acute renal disease in the elderly and the very young [8–11, 16–18].
A 14-fold increase in total direct heat-related hospital admissions was recorded during the 2009 event, compared to a three-fold increase in the previous averaged heatwaves and the 2008 heatwaves. Similar findings have been reported during the concurrent heatwave in Victoria (Australia) and in other cities [17, 19, 20]. However, these findings could possibly be explained by greater accuracy and expertise in reporting of ICD codes for direct heat-related diseases during the 2009 heatwave than in the past because of recurring extreme heat conditions. Although, heat-related disease has the potential for poor short-term and long-term prognosis, it is largely preventable [21, 22].
Surprisingly, with the exception of neurological-related ambulance call-outs in the 65-74 year age group, mental health conditions were not significantly affected during the two extreme events, contrary to what was expected based on our previous research and highlighted in the results from previous averaged heatwaves . However, the lack of effects on mental health-related hospital admissions and emergency department presentations may be explained by the relatively small number of mental health-related cases during single heatwave episodes or the effect of preventive actions recently put in place to support susceptible people in Adelaide. These initiatives were introduced during the 2009 heatwave as a result of previous local investigations, and included daily telephone follow up of patients at the severe end of the mental health disease spectrum.
The 2009 event was associated with a substantial increase in hospital admissions and emergency department presentations for ischaemic heart disease, particularly in the 15-64 year age group. Cardiac-related ambulance call-outs were also significantly increased during both extreme events. This is consistent with findings from the US, where cardiovascular-related hospital and emergency department admissions were increased during heatwaves [17, 23]. A recent study in Victoria found that men younger than 65 years have an increased risk of myocardial infarction when temperatures are elevated . This supports our findings of an increased ischaemic heart disease risk in this younger age group. Our previous averaged heatwave data did not suggest effects on cardiac or ischaemic heart disease outcomes and this may be explained by averaging the data over relatively mild heatwaves. This absence of an effect in cardiovascular morbidity agrees with results from a European study where also no effect on heart disease admissions was found . However, The sudden increase in cardiac-related mortality outside of health-care settings during the 2003 heatwave in Paris poignantly demonstrates the immediacy of cardiovascular mortality during extreme heat events suggesting that patients may die prior to presentation at hospital . This is plausible considering the biological impacts of heat on blood viscosity and heart rate, especially in people with pre-existing heart conditions [26, 27].
Results from previous averaged heatwaves implied that, on average total mortality during heatwaves is not higher than during non-heatwave periods. However, increased overall mortality was observed during both extreme events, with substantially greater excess deaths during the 2009 event. During the 2009 heatwave in Victoria, mortality increased by 62%, with the greatest increase in the 65+ years and over age groups which is consistent with findings in other parts of the world [19, 28]. Whereas in Adelaide there was an increase in total mortality of 10%, and surprisingly, the majority of excess deaths occurred in the 15-64 year age group. When health outcome-specific mortality data becomes available, it will offer further insight into the areas of concern for this age group. Meanwhile, a possible link between the rise of ischaemic heart disease morbidity and the increase in mortality in the 15-64 year age group is only speculation. It could be argued that the absence of excess elderly deaths in Adelaide may be due to the generally high standards of care provided to the elderly. Regular telephone calls offering assistance to people at risk during the excessive heat days during the 2009 heatwave may also have contributed to the prevention of heat-related deaths in the older age groups. The higher mortality in Victoria may have been influenced by location-specific parameters. For instance, the prevalence of air-conditioning in homes is 67% in Melbourne compared to more than 80% in Adelaide [29, 30]. This suggests greater preparedness in Adelaide where average summer temperatures are higher than in Melbourne (28.5°C versus 25.3°C for December, January and February). Acclimatisation to regular hot weather in Adelaide may also play a role. Similar large differences in mortality between cities have been observed overseas and the importance of location-specific parameters and investigations is strongly supported in the heatwave literature [1, 31].
In accordance with overseas studies, mortality was instantly elevated on the hottest day of the 2009 heatwave (Figure 1), with only two-day latency until daily mortality peaked leaving a very small window of opportunity for preventive action . This finding ties in with the observed dose response relationship between mortality and heatwave intensity but not with heatwave duration. However, the increasing duration of heatwaves was associated with increases in ambulance, hospital and emergency outcomes which may indicate that vulnerable people eventually visit health services due to accumulating heat effects.
This study design does not allow the recognition of the potential progression of heat-related disease to mortality after heatwaves have ceased, but this will be explored in a future study.