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Temperature Fluctuations During Hot Months Increase the Risk of Heat-Related Deaths

Global study finds unstable temperatures lead to higher heat-related mortality risk – another problem for climate crisis

Fluctuations in daily temperatures during the warmer months increase the risk of heat-related death, beyond the threat of rising temperatures due to global warming, a global study led by Monash University has found.

The study found there was a global increased risk of death of 290 per cent when there are large daily temperature variations, and as high as almost 2900 per cent in Central Europe.

The research, published in the Cell Press journal, The Innovation, is the first global study of the impact of temperature fluctuations (or unstable temperature) on heat-related mortality, as opposed to increased death rates due to increased temperatures alone.
Research partners from 36 countries used data on daily temperature and mortality in the warm season from the Multi-Country Multi-City (MCC) Collaborative Research Network to conduct the study.

The researchers, led by Professors Yuming Guo and Shanshan Li, Directors of the Monash Climate, Air Quality Research (CARE) Unit, researched the contribution of short-term temperature fluctuations to heat-related mortality between 1972 and 2018. It found there were more heat-related deaths on days where there were significant changes of temperature across a 24 hour period, compared to days when the temperature was stable.

Professor Guo said there were several ways both exposure to heat, and fluctuations in heat, may interact. Firstly, when exposed to heat, people expend more of their reserves on thermoregulation to respond to heat. This process involves elevations in heart rate and blood pressure, dilation of blood vessels leading to decreased blood pressure, and respiration to lose heat with the expired air.

Further, if the temperature suddenly changes in a short period of time, people may have difficulty with internal thermoregulation, resulting in inflammatory responses and coagulation abnormalities induced by heat stress. Sudden temperature changes may also lead to cardiovascular and respiratory diseases independent of heat exposure.

Professor Guo said: "We are living in a warmer and warmer environment that pervasively affects human life and undermines the years of gains in public health. Human-induced warming has been associated with an increase in frequency and intensity of hot days and increasing heat exposure leads to more sickness and death. Meanwhile, temperatures tend to be more and more unstable (variable) under climate change."

For the past two decades there has been an annual average of almost 490,000 heat-related excess deaths globally, and the heat-related excess death ratio increased from 0.83 per cent in 2000–2003 to 1.04 per cent in 2016–2019.

Professor Guo said that until now, few studies have examined whether temperature variability (or fluctuation) has an impact on heat-related deaths.

"For example, daily temperatures that fluctuate from 25 C to 45 C, and from 33 C to 37 C, represent the same mean temperature of 35 C – yet our study has shown they can have vastly different impacts on mortality," he said.

"The day with minimum and maximum temperatures of 25 C and 45 C has higher risk of heat-related deaths than a day with minimum and maximum temperatures of 33 C and 37 C.

"Assessing heat-mortality association without consideration of temperature variability may fail to recognise the most severe weather in relation to heat and implement effective early warning systems."

The authors call for urgent action "to raise public awareness of potential health risks of temperature variability".

"Targeted adaptation strategies against heat-related mortality burden should be implemented after taking the fluctuation of temperatures and geographical patterns into account," the paper said.

As a response to the increasing heat conditions, heat-health warning systems are developed in some parts of the world. For example, the WHO Regional Office for Europe developed the heat-health action plans (HHAPs) in 2008, covering 35 out of 53 member states of the WHO European Region by the end of 2018.

"We observed increased heat-related mortality risks with the increment of temperature variability across countries in Europe where HHAPs are implemented. More detailed and hierarchical warning system should be implemented to cope with increased health risk associated with both heat and fluctuated temperature," Professor Guo said.

Increased risk of heat-related mortality due to high daily temperature variations vs low daily temp variation – by region

  • Nth America 145%
  • Central America 895%
  • S America 263%
  • Nth Europe 1382%
  • Central Europe 2853%
  • Southern Europe 516%
  • Mid East Asia 126%
  • East Asia 163%
  • South East Asia 547%
  • Australia 95%
  • International/Global 290%

How do sudden temperature fluctuations impact health:

When exposed to heat, people expend more of their energy reserves on thermoregulation to respond to heat eg elevations in heart rate and blood pressure, vasodilatation to transfer heat to the skin, and respiration to lose heat with the expired air. 

This sort of adaptation to increased temperatures takes time – so, if the temperature suddenly drops, or rises, over a short period of time people may struggle to put these thermoregulatory strategies in place.

This can lead to - inflammatory response eg fever and coagulation abnormalities eg blood clots and stroke induced by heat stress. In addition, sudden temperature changes may also lead to non-heat related heart and lung diseases due to these inflammatory responses e.g., ischemic heart disease and pneumonia.

For people with underlying conditions (e.g., pre-existing illness, chronic diseases, and poor fitness level), sudden temperature changes may put extra pressure on the cardiovascular and respiratory systems, especially when temperature changes dramatically in a short period, can increase risk of heat‐related deaths. 

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