Extreme heat threatens the health of Australians

Heatwaves have serious health impacts and we need a better approach to prevention and management

Last year was the world’s hottest on record, with anthropogenic global warming raising average temperatures about 1°C above pre-industrial levels.1 Even small increases in the average temperature influence extremes of hot weather. Heatwaves are becoming hotter, longer and more frequent, and are increasing the risk of bushfires.2 The number of record hot days in Australia has doubled in the past 50 years,2 and marine heatwaves are causing severe coral bleaching in the Great Barrier Reef.3

The recent summer of 2016–17 saw the highest monthly mean temperatures on record for Sydney and Brisbane. On 11 February, the average maximum temperature across New South Wales was 44°C, making it one of the hottest places on earth at the time.4

Heatwaves affect continuity of electricity supply and transport infrastructure,3,5 but often less visible is the accompanying surge in morbidity and mortality which in turn places enormous stresses on the health care system.

To understand extreme heat and its impacts, we need the geographical context. A heatwave is defined as three or more days of high maximum and minimum temperatures that are unusual for a particular location.6 Heatwaves of longer duration and higher intensity, and which occur earlier in the season, have higher impacts on mortality. Health impacts can be exacerbated by high relative humidity and poor air quality from urban pollution or bushfire smoke.

Human susceptibility to heat depends on the acclimatisation, age and existing disease burden of the population, and on local environmental factors. Extreme heat can affect health directly through heat-related illnesses when the body’s thermoregulatory mechanisms are unable to maintain a normal core temperature, or indirectly through exacerbation of pre-existing conditions in people with increased vulnerability.7

Direct heat illness ranges from syncope, muscle cramps and heat exhaustion, to life-threatening heat stroke and exertional heat stroke, with loss of internal temperature control, rhabdomyolysis, multi-organ failure, altered mental state and risk of death. It may be accompanied by dehydration and disorders of fluid, electrolyte or acid–base balance.8 High ambient temperatures may affect also perinatal outcomes such as pre-term birth.9

There are recognised categories of people who are more vulnerable to illness and death when exposed to very high ambient temperatures: older people; very young people; those with chronic comorbidities such as cardiovascular, respiratory and metabolic disorders; those with cognitive disorders and mental illnesses; and those taking a range of medications. The homeless and socially isolated, people of lower socio-economic status, and those living in poorly adapted urban environments are also at increased risk.7,10-12 Extreme heat can pose serious health risks for outdoor workers.10

Individual physiological factors such as age or chronic disease may increase risk in multiple ways, such as diminished cardiovascular reserve, and decreased mobility and decreased capacity to seek cooler environments or adequate hydration. Use of certain medications such as anticholinergics, neuroleptic agents, β-blockers, vasodilators and vasoconstrictors, diuretics and lithium add to the risk via mechanisms such as reducing sweating, altering central thermoregulation, reducing cardiac output, inducing postural hypotension, reducing peripheral blood flow, and increasing diuresis.7,13

Factors in the general and built environments are also extremely important. Parts of densely populated cities with dark absorptive surfaces and little green space tend to be several degrees hotter than surrounding areas, constituting an “urban heat island”.11 Heat absorbed during the day is radiated at night, interfering with cooling — a factor known to contribute to heat-related mortality. Access to air-conditioned cool spaces is protective, but may be unreliable during electricity outages in a heatwave.

Direct and indirect impacts on health have significant implications for our health care system. Our larger cities have experienced repeated heatwave events where health services have been severely stretched due to surges in illness and deaths. A summary of some recent reported impacts are shown in the Box. While direct comparisons are difficult due to lack of standardised reporting, increases are evident in ambulance and emergency department (ED) services, and mortality rates, with older people most severely affected. Direct heat-related conditions tend to be more easily identified and recorded compared with exacerbations of pre-existing conditions precipitated by heat. Excess deaths above expected numbers and increases in all-cause mortality have been reported.14

An extreme heatwave in January and February 2009 affected south-eastern Australia, with Melbourne experiencing three consecutive days above 43°C.14 Impacts were reported for 26 January to 1 February compared with the same period in previous years. There was an almost threefold increase in patients dead on arrival at EDs and, overall, there were 374 excess deaths.14 The Black Saturday bushfires the following week in Victoria contributed a further 173 deaths.15 Adelaide also experienced a heatwave in 2009, with maximum temperatures reaching 45.7°C,16 and NSW experienced an 8-day heatwave in 2011.17

Following the 2009 Melbourne heatwave, Victoria established a heat health surveillance system, operating annually over the summer. Analysis of data from its first two years revealed that heat-related health service activity in Melbourne increased within the first 24 hours of an extreme heat event and that people aged 65 years or over were four times more likely to present to an ED with a heat-related condition than people in other age groups.18

In 2014, Melbourne experienced temperatures over 41°C from 14 to 17 January, with 167 excess deaths and a new record set for the highest number of calls for ambulance services ever received in a day (Box).19

It is not only the increased rate of presentations that is of concern during a heatwave, but also the severity. For example, in Melbourne in 2009 and 2014, there were 64% and 60% increases in patients requiring immediate resuscitation, 26% and 27% increases in emergency presentations, and 25% and 16% increases in urgent presentations, respectively.14,19

There is no doubt that with increasingly intense and frequent heat events in the context of increasing urbanisation and ageing of the population, impacts on health services will become more the norm. So how can these be prevented or managed?

Effective policy and action on climate change with reduction of greenhouse gas emissions is the most fundamentally appropriate response to this growing health risk. However, with even the current level of warming, we need improved and effective adaptive responses.

There is evidence that the adverse health effects of heatwaves may be minimised through effective heat health action plans, which aim to prevent, respond to and manage heat-related risks to health.10,20-22 These include not only short term emergency measures but also longer term adaptive responses. Key elements include a heatwave warning system, providing heat health messaging to vulnerable people, primary health care providers reviewing and advising those at risk, activating social networks including outreach and monitoring older and socially isolated people, and timely surveillance of morbidity and mortality. A heat health warning system is a crucial component of a heatwave response plan and this service is now provided by the Bureau of Meteorology.6

Sectors traditionally outside health have a role to play in redesigning our cities and buildings through increased green infrastructure (such as tree canopy cover and parks), innovative water recycling and improved public transport and urban planning, making them more sustainable and energy efficient, with better temperature control and lower air pollution.

Effective preventive action by primary health care practitioners may help to stem the flows to EDs. General practitioners are being urged to identify patients at risk and educate them and their carers about heat illness and prevention, even including a pre-summer medical assessment for those with chronic disease. However, the extent to which this is actually implemented is unclear.23 Individual health protection messages against extreme heat include avoiding outdoor exposure and accessing cool environments, keeping well hydrated and knowing the symptoms of heat illness and how to access help.7,21,23

There is limited research measuring the effectiveness of public health interventions for extreme heat and there are many challenges in doing so. Attribution of success to specific measures is difficult when no two heat episodes are the same. Although lower than expected mortality for an event may be one measure, heat-related deaths may be misclassified and under-reported. Encouragingly, lower mortality has been reported in some locations where such interventions have occurred.20,22,24

Secondary prevention involves better preparing hospital and emergency services to respond. ED presentations during heatwaves tend to be in higher acuity groups, whose management may require aggressive resuscitation, intubation and intensive care unit support. However, even those who present less acutely are more likely to require admission.14,19 Many will require electrolyte and renal function analysis, rehydration and observation, necessitating time in the ED or acute medical or geriatric units where multiday beds are scarce. Heatwave plans need to prepare and assist hospitals to meet the added disease burden by ensuring availability of staff and beds and by treating patients in non-traditional locations.10

While Australian jurisdictions now have some form of heatwave response plan, a coordinating national framework, although recommended,5 is lacking. Heat health response and surveillance varies across the country and little evaluation has occurred. A national heatwave plan could provide some consistency across jurisdictional subplans,5 covering social and public health responses, communication, surveillance, additional resourcing, longer term capacity building and evaluation.

Medical leaders7,25-27 have been urging governments to recognise the health threats posed by climate change, develop a comprehensive national assessment of risks to human health, such as those of the United Kingdom and the United States, and boost national health spending on research, which has thus far been miniscule.28

Our responsibility is to meet the health threats from heatwaves and other climate-related risks to health with real action on climate change, improved short term disaster responses and better longer term adaptation strategies. Extreme heat will otherwise take an increasing toll on the health of Australians, particularly the most vulnerable.