Extreme heat is one of the growing weather-related hazards affecting human health and well-being [1]. Most of its effects, which have, over the years, gained significant momentum, are attributed to elevated temperatures resulting from climate change and variability ^[2][3][2,3]. Notably, higher than normal temperatures, coupled with high relative humidity, often cause a sensation of discomfort and sometimes heat stress ^[4][5][4,5]. Heat stress in the present context is defined as the combination of air temperature, radiation, air movements, moisture content, amount of clothing worn, as well as behaviors that induce a physiological inability of the body to maintain its temperature within the heat tolerance limit of 35–37 °C [6]. Regardless of age, gender, or health status, all persons are at risk of heat-related illnesses, including heat stress [7]. However, the vulnerability increases typically in older people, those who are chronically ill, low-income earners, pregnant women, socially isolated individuals, slum and informal settlement dwellers, people working in exposed environments, as well as children under the age of five years ^[8][9][10][8,9,10]. In addition, athletes and tourists who often cannot fathom the danger of hot weather conditions are also at risk of heat-related illnesses ^[11][12][11,12]. Similarly, people living in urban areas are confronted with a substantial rise in heat stress as compared to those living in the rural–urban fringe or rural areas, since the air temperature in such areas is often higher than in the surrounding due to the urban heat island (UHI) phenomena ^[13][14][13,14]. Demographic growth and ever-increasing urbanization trends suggest the likelihood of many people being placed at increased risk in the future ^{[15][16][17][18]}[15,16,17,18].

Prolonged exposure to extreme heat, predominantly during acute heatwaves, can cause a varying degree of medical severities, including oedema, rash, cramps, exhaustion, and heat stroke, which can cause shock, brain damage, as well as internal organ failure, consequently leading to coma ^[19][20][19,20]. Some underlying and pre-existing medical conditions, e.g., asthma, chronic obstructive pulmonary disease, cardiovascular disease, hypertensive diseases, chronic renal failure, diabetes mellitus, mental disorder, and dementia, are exacerbated by extreme heat exposure ^{[1][21][22][23]}[1,21,22,23]. Various studies have alluded that exposure to extreme heat and heatwaves conditions are likely to increase mortality and morbidity ^[24][25][26][24,25,26]. These effects are demonstrated by the deaths of tens of thousands of people in Europe ^[27][28][27,28], China ^[29][30][29,30], the United States of America [31], and other parts of the world [32].

Global model projections indicate that extreme heat and heatwave events are likely to increase in frequency, duration, and intensity well into the future, owing to climate change and variability [33]. As established across various studies, these weather phenomena will be accompanied by heat-related morbidity and mortality ^{[34][35][36][37]}[34,35,36,37], to the peril of society. Based on these findings, it is apparent that heat stress is a universal problem even in Africa, where information relating to heat-related illnesses and attributed deaths are not as readily available [32]. This is a cause for concern for the continent, particularly in South Africa, where the observed local climatic factors show an increase in the average temperature of approximately 1.5 times the observed global average of 0.65 °C [38]. Future model projections are also showing a plausible increase in this region [39]. Trends over the past 40 years show a rise of 0.02 °C yr⁻¹ with considerable regional variation [40]. The rate of warming is pronounced in the western region and towards the northeastern parts of South Africa [41]. When further assessing historical data, it is noted that warmer temperatures have dominated in the winter season during 1995–2010, relative to the 1979–1994 period [42]. Moreover, there has been a significant increase in the number of days and nights with relatively high temperatures [41]. Model projections under the A2 scenario indicate that the intense heat and the number of hot days and nights are expected to increase in the near future with adverse implications on human health [43]. Some of the growing evidence of the increase in heat stress exposure in this region was revealed in two local studies; one based on the analysis of heat stress in East London using weather station observation data in the Eastern Cape Province [44] and the second study based on an outdoor and indoor experimental study in the Greater Giyani local municipality in rural Limpopo [45]. For a more comprehensive perspective, Garland et al. [43] provided a model-based regional overview of heat stress conditions over Africa using apparent temperature (AT) and projected an increase in the number of hot days in the region.

As reported in the literature, South Africa is projected to experience hotter temperatures and more frequent and intense heat stress identified by the National Climate Change and Health Adaptation Plan: 2014–2019 as one of the nine environmental risks impacted by climate change [46]. To improve public health response and protect the South African population from extreme heat exposure, the government’s National Climate Change Response Policy (NCCRP) called for the development and implementation of heat-health action plans that must factor population vulnerability and adapt to local conditions [47]. These efforts are in line with the recommendations made by the World Health Organization (WHO) for countries to develop their heat action plans aimed at (1) minimizing increased mortality and morbidity associated with extreme heat and heatwave events, (2) promote careful preparation measures across relevant sectors, including public and medical professional awareness, and (3) mobilize resources to mitigate the health impacts of heat [48]. Berry et al. [49] alluded that identifying and mapping heat stress is one of the key elements in developing and implementing effective and informed heat health plans. Hence, this study investigates climatological patterns and long-term trends of heat stress conditions in South Africa based on meteorological data from 51 weather stations across South Africa using the Steadman’s apparent temperature (hereafter AT) heat index. Steadman’s apparent temperature is a temperature-like combined measure of all the climatic variables that affect human comfort and performance [50].

2.  Long-Term Trends of Heat Stress for South Africa

Like many countries, South Africa is in the process of developing a heat health plan, as called for by the NCCRP [47]. The current study evaluated heat stress conditions and long-term trends based on 51 selected stations throughout South Africa’s nine provinces using weather station data spanning 1997 to 2020. This evaluation is one of the first attempts to use observation data to characterize heat stress on a national scale in South Africa. Such an assessment provides an important basis for identifying vulnerable areas that need to be prioritized for further research and development of appropriate heat adaptation strategies that are informed by local factors [49]. The study revealed regions in South Africa that are susceptible to heat stress. These cover areas predominantly at low altitude regions in the northwestern region of MP and KZN provinces, the northern part of the LP as well as southwestern and southern coastline regions of the WC and NC provinces, and parts of the EC province. Additionally, the potentially hottest towns with ATmed values that lie in the danger category (i.e., 39–50 °C) were identified, including Patensie-EC (41 °C), Pietermaritzburg-KZN (39 °C), Hoedspruit-LP (39 °C), Skukuza (45 °C), and Komatidraai (43 °C). These relatively high values have a significant likelihood of causing heat-related illnesses and, in the worst-case scenario, mortality due to elevated heat exposure. For example, in the studies by Baccini et al. ^[51][76] and Almeid et al. ^[52][77], a change of 1 °C was linked to a rise in the mortality rate. These results are in accordance with one local study investigating the association of heat stress and mortality rate in three cities in South Africa ^[53][78]. The study by Wichmann, 2017 ^[53][78] established an increase of 0.9% in mortality per 1 °C increase in AT. Not far from these observations are the findings reported in Azongo et al. ^[54][79] and Egondi et al. ^[55][80], where direct correlations between high temperatures and mortality were reported in Kenya and Ghana, respectively. Consistent findings in studies by Almeid et al. ^[52][77], Azongo et al. ^[54][79], Baccini et al. ^[51][76], Egondi et al. ^[55][80], and Wichmann ^[53][78] indicate that high mortality rates were particularly prevalent among the elderly, who currently account for 8.7% of the South African population ^[56][81], half of whom are living in poverty ^[57][82] and are therefore less able to adopt adequate adaptation strategies to mitigate the adverse effects of heat stress. Furthermore, high heat stress conditions are a source of concern for 66% of the South African population residing in densely populated urban areas where a large proportion of the dwellings are found to be mostly informal and, in most cases, are characterized by overcrowding, poor housing conditions, lack of basic infrastructure, and poverty ^[10][58][59][10,83,84]. As supported by Tran et al. ^[60][85], socio-demographic factors and access to resources are likely to exacerbate vulnerabilities to heat stress conditions in large proportions of the South African population, particularly those most vulnerable. In line with the study by Orimoloye et al. [44], this study investigated trends in AT. However, 23 years of trend analysis might not provide meaningful results (e.g., 30 years). There were increasing trends detected in almost all the weather stations (88%) across the nine provinces, with most of the observed positive trends (47%) being statistically significant at 5% statistical level. The results are consistent with observations of significantly greater warming in the western region and towards the northeastern and eastern parts of South Africa, as per Kruger and Shongwe [41]. Heat stress is expected to become more common and severe due to climate change on a global scale [33]; similar observations have been made for several African countries, including South Africa, Sahelian Africa, and the Northern and Central African countries ^[43][61][43,86], where extreme heat has become an increasing threat to human life.