What is a heatwave?

A few years ago, while chatting with a former government employee about climate change and public health, he mentioned that the most common question he encountered was ‘what is a heatwave?’

So, what is heatwave, how hot is ‘heat’ and how long is a ‘wave’? Residents in Vancouver or Moscow may think 30 °C is very hot because they rarely experience it, but in Guangzhou or Brisbane, 30 °C is common in summer.

According to the World Meteorological Organization, a heatwave is a period of unusually hot weather that exceeds records and persists for at least two consecutive days during the hottest time of the year in a region. This means local weather conditions are very important when considering ‘how hot is too hot?’.

Over the past decade, researchers worldwide have increasingly recognized that heatwaves need to be assessed against health impacts. For instance, a Brisbane study found when maximum temperatures exceed 33 °C, the risk of ambulance service use, emergency department attendance, and hospitalization increases significantly.1  

How do heatwaves impact health?

When we are exposed to heat, our body pushes temperature-regulating mechanisms, such as heart rate and function, hard to release internal heat. However, once heat goes beyond a certain limit, our thermoregulation system cannot cope, increasing the risk of illness or death. This is why health events often peak during heatwaves.

Recent studies in Brisbane have found exposure to heat increases the risk of acute myocardial infarction, mainly in diabetes patients,2 and increases the risk of stroke in people with hyperlipidemia.3 These findings remind us that people with diabetes or hyperlipidemia should minimize their exposure to heatwaves to prevent cardiovascular disease.

Not everyone is equally vulnerable to heatwaves. A person’s heatwave vulnerability is generally determined by their exposure, sensitivity, and adaptive capacity. Outdoor workers, for example builders, are vulnerable because of high exposure to heatwaves. Infants and elderly people are more vulnerable than healthy adults because they have less developed and compromised thermoregulation systems, which makes them highly sensitive. Pregnant women are increasingly being recognized as vulnerable to heatwaves because physiological and anatomical changes during pregnancy compromise their ability to thermoregulate. A recent study published in the British Medical Journal found heat exposure may increase a pregnant woman’s risk of having a preterm birth by 16 per cent.4 People who are socioeconomically-disadvantaged are also more vulnerable to heatwaves because they have less capacity to access cooling measures, such as air conditioning.    

So how can we increase our community’s heat adaptation capacity?

Air conditioning may be the most widely adopted heat reduction strategy globally, but it is not affordable for the most vulnerable groups, and it is environmentally costly.

The University of Sydney’s Professor Ollie Jay recently published a summary of viable heat adaptation methods in the Lancet,5 suggesting heat-health action plans could be a sustainable way to efficiently adapt to heatwaves. Several Australian jurisdictions already have heat-health action plans, although details of these plans vary across jurisdictions. Heat-health action plans require collaboration across several departments to coordinate responses.

In Australia, heatwaves are currently our most lethal natural disaster and are becoming more dangerous.6 Record breaking heatwaves, previously rare, are now common. As climate change progresses, future heatwaves will become more frequent, longer lasting, and intense, posing huge threats to the health of our children and their children. Tackling the health impacts of heatwaves in response to climate change requires urgent action by governments, society, and individuals. If there is a best time to act, it is now.    

Dr Zhiwei Xu is a Postdoctoral Research Fellow with the UQ School of Public Health

1. Xu Z, Cheng J, Hu W, Tong S.. Heatwave and health events: A systematic evaluation of different temperature indicators, heatwave intensities and durations. Science of The Total Environment. 2018/07/15/ 2018;630:679-689. doi:https://doi.org/10.1016/j.scitotenv.2018.02.268

2. Xu Z, Tong S, Ho HC, Lin H, Pan H, Cheng J. Associations of heat and cold with hospitalizations and post-discharge deaths due to acute myocardial infarction: what is the role of pre-existing diabetes? International Journal of Epidemiology. 2021; doi:10.1093/ije/dyab155

3. Xu Z, Tong S, Pan H, Cheng J. Associations of extreme temperatures with hospitalizations and post-discharge deaths for stroke: What is the role of pre-existing hyperlipidemia? Environmental Research. 2021/02/01/ 2021;193:110391. doi:https://doi.org/10.1016/j.envres.2020.110391

4. Chersich MF, Pham MD, Areal A, et al. Associations between high temperatures in pregnancy and risk of preterm birth, low birth weight, and stillbirths: systematic review and meta-analysis. BMJ. 2020;371:m3811. doi:10.1136/bmj.m3811

5. Jay O, Capon A, Berry P, et al. Reducing the health effects of hot weather and heat extremes: from personal cooling strategies to green cities. The Lancet. 2021;398(10301):709-724. doi:10.1016/S0140-6736(21)01209-5

6. Jyoteeshkumar reddy P, Perkins-Kirkpatrick SE, Sharples JJ. Intensifying Australian Heatwave Trends and Their Sensitivity to Observational Data. Earth's Future. 2021;9(4):e2020EF001924. doi:https://doi.org/10.1029/2020EF001924


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