Editing IPCC:AR6/WGII/Chapter-7 (section)

==== 7.2.4.1 Heat- and Cold-Related Mortality and Morbidity ====

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E ''xtreme heat events and extreme temperature have well-documented, observed impacts on health, mortality (very high confidence'' '') and morbidity (high confidence).'' AR5 described the thermoregulatory mechanisms and responses, including acclimatisation, linking heat, cold and health, and these have been further confirmed by recent studies and reviews (e.g., [[#Giorgini--2017|Giorgini et al., 2017]] ; [[#Ikaheimo--2018|Ikaheimo, 2018]] ; [[#McGregor--2015|McGregor et al., 2015]] ; [[#Stewart--2017|Stewart et al., 2017]] ; [[#Schuster--2017|Schuster et al., 2017]] ; [[#Zhang--2018b|Zhang et al., 2018b]] ). The health impacts of heat manifest clearly in periods of extreme heat often codified as heatwaves. For example, heatwaves across Europe (2003), Russia (2010), India (2015) and Japan (2018) resulted in significant death tolls and hospitalisations ( [[#McGregor--2017|McGregor et al., 2017]] ; [[#Hayashida--2019|Hayashida et al., 2019]] ). Heat continues to pose a significant health risk due to increases in exposure, changes in the size and spatial distribution of the human population, mounting vulnerability and an increase in extreme heat events ( ''high confidence'' ) ( [[#Harrington--2017|Harrington et al., 2017]] ; [[#Liu--2017|Liu et al., 2017]] ; [[#Mishra--2017|Mishra et al., 2017]] ; [[#Rohat--2019a|Rohat et al., 2019a]] ; [[#Rohat--2019b|Rohat et al., 2019b]] ; [[#Rohat--2019c|Rohat et al., 2019c]] ; [[#Watts--2019|Watts et al., 2019]] ). Some regions are already experiencing heat stress conditions approaching the upper limits of labour productivity and human survivability ( ''high confidence'' ). These include the Persian Gulf and adjacent land areas, parts of the Indus River Valley, eastern coastal India, Pakistan, northwestern India, the shores of the Red Sea, the Gulf of California, the southern Gulf of Mexico and coastal Venezuela and Guyana ( [[#Krakauer--2020|Krakauer et al., 2020]] ; [[#Li--2020|Li et al., 2020]] ; [[#Raymond--2020|Raymond et al., 2020]] ; [[#Saeed--2021|Saeed et al., 2021]] ; [[#Xu--2020|Xu et al., 2020]] ).

Under a variety of methods, estimates of the world’s population exposed to extreme heat indicate very large and growing numbers and an increase since pre-industrial times. For example, Li et al. (2020) estimate that globally, 1.28 billion people each year experience heatwave conditions similar to that of the lethal Chicago 1995 event, compared with 0.99 billion people that would be similarly exposed under a pre-industrial climate. Further, for the 150 most populated cities of the world, a 500% increase in the exposure to extreme heat events occurred over the 1980–2017 period ( [[#Li--2021|Li et al., 2021]] ), while for the 1986–2005 period, the total exposure to dangerous heat in Africa’s 173 largest cities was 4.2 billion person-days yr –1 ( [[#Rohat--2019a|Rohat et al., 2019a]] ). Globally the present exposure to heatwave events is estimated to be 14.8 billion person-days yr –1 , with the greatest cumulative exposures measured in person-days occurring across southern Asia (7.19 billion), sub-Saharan Africa (1.43 billion), and north Africa and the Middle East (1.33 billion) ( [[#Jones--2018|Jones et al., 2018]] ).

The country level percentage of mortality attributable to non-optimum temperature (heat and cold) has been found to range from 3.4 to 11% ( [[#Gasparrini--2015|Gasparrini et al., 2015]] ; [[#Zhang--2019b|Zhang et al., 2019b]] ). Heat as a health risk factor has largely been overlooked in low- and middle-income countries ( [[#Campbell--2018|Campbell et al., 2018]] ; [[#Green--2019|Green et al., 2019]] ; [[#Dimitrova--2021|Dimitrova et al., 2021]] ). For 2019, the GBD report estimates the burden of DALYs attributable to low temperature was 2.2 times greater than the burden attributable to high temperature. However, this global figure obscures important regional variations. Countries with a high sociodemographic index—mainly mid-latitude high-income temperate to cool climate countries— were found to have a cold-related burden 15.4 times greater than the heat-related burden, while for warm lower-income regions, such as south Asia and sub-Saharan Africa, the heat-related burden was estimated to be 1.7 times and 3.6 times greater, respectively ( [[#Murray--2020|Murray et al., 2020]] ). For countries where data availability permits, there is evidence that extreme heat (and extreme cold) leads to higher rates of premature deaths ( [[#Armstrong--2017|Armstrong et al., 2017]] ; [[#Cheng--2018|Cheng et al., 2018]] ; [[#Costa--2017|Costa et al., 2017]] ).

Rapid changes and variability in temperatures are observed to increase heat-related health and mortality risks, the outcomes varying across temperate and tropical regions ( [[#Guo--2016|Guo et al., 2016]] ; [[#Cheng--2019|Cheng et al., 2019]] ; [[#Kim--2019a|Kim et al., 2019a]] ; [[#Tian--2019|Tian et al., 2019]] ; [[#Zhang--2018b|Zhang et al., 2018b]] ; [[#Zhao--2019|Zhao et al., 2019]] ).

''Several lines of evidence point to a possible decrease in population sensitivity to heat, albeit mainly for high-income countries (high confidence), arising from the implementation of heat warning systems, increased awareness and improved quality of life.'' ( [[#Sheridan--2018|Sheridan and Allen, 2018]] ). Evidence suggests a general decrease in the impact of heat on daily mortality ( [[#Diaz--2018|Diaz et al., 2018]] ; [[#Kinney--2018|Kinney, 2018]] ; [[#Miron--2015|Miron et al., 2015]] ), a decline in the relative risk attributable to heat ( [[#Åström--2018|Åström et al., 2018]] ; [[#Barreca--2016|Barreca et al., 2016]] ; [[#Petkova--2014|Petkova et al., 2014]] ) and an increase in the minimum mortality temperature (MMT) ( [[#Åström--2018|Åström et al., 2018]] ; [[#Folkerts--2020|Folkerts et al., 2020]] ; [[#Follos--2021|Follos et al., 2021]] ; [[#Chung--2018|Chung et al., 2018]] ; [[#Todd--2015|Todd and Valleron, 2015]] ; [[#Yin--2019|Yin et al., 2019]] ). It is difficult to draw conclusions regarding trends in heat sensitivity for low- to middle-income countries and specific vulnerable groups as these are under-represented in the literature ( [[#Sheridan--2018|Sheridan and Allen, 2018]] ). Trends in heat sensitivity are generally believed to be scale and situation dependent, but there is considerable variability in changes in heat sensitivity as measured by trends in heat-related mortality or MMT ( [[#Follos--2021|Follos et al., 2021]] ; [[#Kim--2019a|Kim et al., 2019a]] ; [[#Lee--2021|Lee et al., 2021]] ), with notable variability across different population groups ( [[#Lu--2021|Lu et al., 2021]] ).

Temperature interacts with heat-sensitive physiological mechanisms via multiple pathways to affect health. In the worst cases, these lead to organ failure and death ( [[#Mora--2017a|Mora et al., 2017a]] ; [[#Mora--2017b|Mora et al., 2017b]] ). Excess deaths during extreme heat events occur predominantly in older individuals and are overwhelmingly cardiovascular in origin ''(very high confidence)'' . A higher occurrence of CVD mortality in association with prolonged period of low temperatures has been well documented globally ( [[#Giorgini--2017|Giorgini et al., 2017]] ; [[#Stewart--2017|Stewart et al., 2017]] ); however, there is growing evidence that cardiovascular deaths are more related to heat events than cold spells ( [[#Chen--2019|Chen et al., 2019]] ; [[#Liu--2015a|Liu et al., 2015a]] ; [[#Bunker--2016|Bunker et al., 2016]] ). While there is strong association between ambient temperature and cardiovascular events globally, there are complex interactions and modulators of individual response ( [[#Wang--2017|Wang et al., 2017]] b). Further, some CVD morbidity sub-groups such as myocardial infarction (MI) and stroke hospitalisation display temperature sensitivity while others do not ( [[#Bao--2019|Bao et al., 2019]] ; [[#Sun--2018|Sun et al., 2018]] ; [[#Wang--2016|Wang et al., 2016]] ). Although older adults have inherent sensitivities to temperature-related health impacts ( [[#Bunker--2016|Bunker et al., 2016]] ; [[#Phung--2016|Phung et al., 2016]] ), children can also be affected by extreme heat ( [[#Xu--2014|Xu et al., 2014]] ). Cardiovascular capacity or health is also a critical determinant of individual health outcomes ( [[#Schuster--2017|Schuster et al., 2017]] ). Medications to treat CVDs, such as diuretics and beta-blockers, may impair resilience to heat stress ( [[#Stewart--2017|Stewart et al., 2017]] ). Other mediating factors in the causal pathway range from alcohol consumption ( [[#Cusack--2011|Cusack et al., 2011]] ; [[#Epstein--2019|Epstein and Yanovich, 2019]] ) and obesity ( [[#Speakman--2018|Speakman, 2018]] ) to pre-existing conditions, such as diabetes and hyperlipidaemia, and urban characteristics ( [[#Chen--2019|Chen et al., 2019]] ; [[#Sera--2019|Sera et al., 2019]] ).

Under extreme heat conditions, increases in hospitalisations have been observed for fluid disorders, renal failure, urinary tract infections, septicaemia, general heat stroke as well as unintentional injuries ( [[#Borg--2017|Borg et al., 2017]] ; [[#Phung--2017|Phung et al., 2017]] ; [[#Goggins--2017|Goggins and Chan, 2017]] ; [[#Hayashida--2019|Hayashida et al., 2019]] ; [[#Hopp--2018|Hopp et al., 2018]] ; [[#Ito--2018|Ito et al., 2018]] ; [[#Kampe--2016|Kampe et al., 2016]] ; [[#McTavish--2018|McTavish et al., 2018]] ; [[#Ponjoan--2017|Ponjoan et al., 2017]] ; [[#van%20Loenhout--2018|van Loenhout et al., 2018]] ). Hospitalisations and mortality due to respiratory disorders also occur during heat events with the interactive role of air quality being important for some locations but not others ( [[#Krug--2019|Krug et al., 2019]] ; [[#Pascal--2021|Pascal et al., 2021]] ; [[#Patel--2019|Patel et al., 2019]] ). Increased levels of heat-related hospitalisation also manifest in elevated levels of emergency service calls ( [[#Cheng--2016|Cheng et al., 2016]] ; [[#Guo--2017|Guo, 2017]] ; [[#Papadakis--2018|Papadakis et al., 2018]] ; [[#Williams--2020|Williams et al., 2020]] ).

Heat- and cold-related health outcomes vary by location ( [[#Dialesandro--2021|Dialesandro et al., 2021]] ; [[#Hu--2019|Hu et al., 2019]] ; [[#Phung--2016|Phung et al., 2016]] ), suggesting outcomes are highly moderated by socioeconomic, occupational and other non-climatic determinants of individual health and socioeconomic vulnerability ( [[#Åström--2020|Åström et al., 2020]] ; [[#McGregor--2017|McGregor et al., 2017]] ; [[#McGregor--2017|McGregor et al., 2017]] ; [[#Schuster--2017|Schuster et al., 2017]] ; [[#Benmarhnia--2015|Benmarhnia et al., 2015]] ; [[#Watts--2019|Watts et al., 2019]] ) ( ''high confidence'' ). For example, access to air conditioning is an important determinant of heat-related health outcomes for some locations ( [[#Guirguis--2018|Guirguis et al., 2018]] ; [[#Ostro--2010|Ostro et al., 2010]] ). Although there is a paucity of global level studies of the effectiveness of air conditioning for reducing heat-related mortality, a recent assessment indicates increases in air conditioning explains only part of the observed reduction in heat-related excess deaths, amounting to 16.7% in Canada, 20.0% in Japan, 14.3% in Spain and 16.7% in the US ( [[#Sera--2020|Sera et al., 2020]] ).

Significant effects of heat exposure are evident in sport and work settings with exertional heat illness leading to death and injury ( [[#Adams--2020|Adams and Jardine, 2020]] ). Although most studies of heat-related sports injuries refer to high-income countries, these point to an increasing number of heat injuries with widening participation in sport and an increasing frequency of extreme heat events. The highest rates of exertional heat illness are reported for endurance type events (running, cycling and adventure races), American football and athletics ( [[#Gamage--2020|Gamage et al., 2020]] ; [[#Grundstein--2017|Grundstein et al., 2017]] ; [[#Kerr--2020|Kerr et al., 2020]] ; [[#McMahon--2021|McMahon et al., 2021]] ; [[#Yeargin--2019|Yeargin et al., 2019]] ). The health, safety and productivity consequences of working in extreme heat are widespread ( [[#Ma--2019|Ma et al., 2019]] ; [[#Morabito--2021|Morabito et al., 2021]] ; [[#Kjellstrom--2019|Kjellstrom et al., 2019]] ; [[#Orlov--2020|Orlov et al., 2020]] ; [[#Smith--2021|Smith et al., 2021]] ; [[#Vanos--2019|Vanos et al., 2019]] ; [[#Varghese--2020|Varghese et al., 2020]] ; [[#Williams--2020|Williams et al., 2020]] ). Occupational heat strain in outdoor workers manifests as dehydration, mild reduction in kidney function, fatigue, dizziness, confusion, reduced brain function, loss of concentration and discomfort ( [[#Al-Bouwarthan--2020|Al-Bouwarthan et al., 2020]] ; [[#Boonruksa--2020|Boonruksa et al., 2020]] ; [[#Habibi--2021|Habibi et al., 2021]] ; [[#Levi--2018|Levi et al., 2018]] ; [[#Venugopal--2021|Venugopal et al., 2021]] ; [[#Xiang--2014|Xiang et al., 2014]] ). In the case of armed forces, a global review of the available literature points to a slightly higher incidence of heat stroke in men compared to women but a higher proportion of heat intolerance and greater risk of exertional heat illness amongst women ( [[#Alele--2020|Alele et al., 2020]] ). There is also some evidence that for healthcare workers, the risk of occupational heat stress grew during the COVID-19 pandemic due to the need to wear personal protective equipment ( [[#Foster--2020|Foster et al., 2020]] ; [[#Lee--2020|Lee et al., 2020]] ; [[#Messeri--2021|Messeri et al., 2021]] ). Based on a systematic review of the literature, one study estimates global costs from heat-related lost work time were USD 280 billion in 1995 and USD 311 billion in 2010, with low- and middle-income countries and countries with warmer climates experiencing greater losses as a proportion of gross domestic project (GDP) ( [[#Borg--2021|Borg et al., 2021]] ). Other global level assessments note an increase in the potential hours of work lost due to heat over the 2000–2018 period; in 2018, 133.6 billion potential work hours were lost, amounting to 45 billion hours more than in 2000 ( [[#Watts--2019|Watts et al., 2019]] ). For China, heat-related productivity losses have been estimated at 9.9 billion hours in 2019, equivalent to 0.5% of the total national work hours for that year, with Guangdong province, one of the warmest regions in China, accounting for almost a quarter of the losses ( [[#Cai--2021|Cai et al., 2021]] ).

Wide ranging knowledge regarding the specific detection of heat- and cold-related mortality/morbidity and its attribution to observed climate change is lacking ''.'' Although there has been an observed increase in winter-season temperatures for a number of regions, to date there is variable evidence for a consequential reduction in winter mortality and susceptibility to cold over time due to milder winters; some countries demonstrate decreasing trends, while other countries show stable or even increasing trends in cold-attributable mortality fractions over time (e.g., [[#Arbuthnott--2020|Arbuthnott et al. (2020)]] ; [[#Åström--2013|Åström et al. (2013)]] ; [[#Diaz--2019|Diaz et al. (2019)]] ; [[#Hajat--2017|Hajat (2017)]] ; Hanigan et al. (2021); [[#Lee--2018b|Lee et al. (2018b)]] ). While there is a burgeoning literature on the attribution of extreme heat events to climate change (e.g., [[#Vautard--2020|Vautard et al. (2020)]] ), the number of studies that assess the extent to which observed changes in heat-related mortality may be attributable to climate change is small ( [[#Ebi--2020|Ebi et al., 2020]] ). During the 2003 European heatwave, anthropogenic climate change increased the risk of heat-related mortality by approximately 70% and 20% for London and Paris, respectively ( [[#Mitchell--2016|Mitchell et al., 2016]] ). For the severe heat event across Egypt in 2015, the impact on human discomfort was 69% (±17%) more likely due to anthropogenic climate change ( [[#Mitchell--2016|Mitchell, 2016]] ), and for Stockholm, Sweden, it has been estimated that mortality due to temperature extremes for 1980 to 2009 was double what would have occurred without climate change ( [[#Åström--2013|Åström et al., 2013]] ). To date there has only been one multi-country attempt to quantify the heat-related human health impacts that have already occurred due to climate change. Based on an analysis of 732 locations spanning 43 countries for the 1991–2018 period, the study found that on average 37.0% (inter-quartile range 20.5–76.3%) of warm-season heat-related deaths can be attributed to anthropogenic climate change, equivalent to an average mortality rate of 2.2/100,000 (median: 1.67/100,000; interquartile range: 1.08–2.34/100,000). Regions with a high attributed percentage (&gt; 50%) include southern and western Asia (Iran and Kuwait), Southeast Asia (Philippines and Thailand) and several countries in Central and South America. Those with lower values (&lt; 35%) include Western Europe (the Netherlands, Germany and Switzerland), eastern Europe (Moldova, the Czech Republic and Romania), southern Europe (Greece, Italy, Portugal and Spain), North America (USA) and eastern Asia (China, Japan and South Korea) ( [[#Vicedo-Cabrera--2021|Vicedo-Cabrera et al., 2021]] ). Due to data restrictions, some of the poorest and most susceptible regions to climate change and increases in heat exposure, such as west and east Africa ( [[#Asefi-Najafabady--2018|Asefi-Najafabady et al., 2018]] ; [[#Sylla--2018|Sylla et al., 2018]] ) and south Asia, could not be included in the analysis ( [[#Mitchell--2021|Mitchell, 2021]] ).

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