A Literature Review of the Impacts of Heat Stress on Human Health across Africa

Abstract

Heat stress-related illness attributed to the changing climate, particularly the more frequent extreme high temperatures, is becoming a theme of public concern, especially in the most vulnerable regions, such as the African continent. Knowledge of the existing research directions and gaps on heat stress and human health is vital for informing future strategic research foci capable of influencing policy development, planning, adaptation, and mitigation efforts. In this regard, a bibliometric analysis was conducted, with an emphasis on Africa, to assess regional research contributions to heat stress impacts on human health. The goals of the study were to review publication growth and patterns of the scientific publications and to identify key players (especially collaborating institutions and countries) and the evolution of research themes on the African continent, while paying attention to global trends and emergent hot topics and methodology of heat stress research. Using the Web of Science (WoS) and Scopus core collection databases, a structured keyword search was undertaken, which yielded 463 and 58 research publications from around the world and Africa, respectively. The retrieved scientific documents, published between 1968 and 2020, were analyzed and visualized using a bibliometric analysis technique and the VOSviewer software tool. The results indicate low statistics and slow scientific growth in publication output, with the highest peak having been reached in 2018, resulting in 13 scientific publications. While global research collaborations are successfully reflected in the literature, there is a considerable gap in understanding heat stress and related collaborations between African countries and international institutions. The review study has identified key opportunities that can benefit Africa through the expansion of the scope of heat stress and human health research on the continent. These opportunities can be achieved by closing the following research gaps: (1) vulnerability assessments within demographic classes, such as the elderly, (2) personal exposure and associated risks, (3) Urban Heat Island (UHI) evaluation for urban environments, and (4) heat adaptation research, which will enable informed and targeted preventive actions that will limit future heat health impacts. The authors opine that the pursuit of such studies will be most impactful if the current knowledge gaps are bridged through transdisciplinary research supported by local, regional, and international collaborators.

1. Introduction

Climate change poses a fundamental threat to a wide range of sustainable development matters, particularly relating to health equity, food security, employment, gender equality, education, and housing, among others [1,2]. Adding to the existing threats are the immediate and long-term effects of climate change on the primary determinants of health [3,4]. Likewise, the ongoing socio-economic challenges have been found to worsen the interdisciplinary facets of extreme heat vulnerabilities stemming from the changing climate. As widely observed, the disproportional impacts of climate change on human health stem primarily from exposure to high temperatures coupled with weather extremes (i.e., heat), as well as the increased production of certain air pollutants and aeroallergens [5,6]. According to McMichael [5], potential health effects of climate change can be divided into direct and indirect effects, depending on whether they occur predominantly via the impacts of climate change and variability upon human biology or are mediated by climate-induced changes in other biological and biogeochemical systems. According to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), climate change will mostly exacerbate existing health burdens by 2050, and populations currently most affected by climate-related illnesses will also be at most significant risk in the future [7]. Regions exhibiting the poorest health infrastructure will be the least able to adapt, prepare, and respond to the variety of increased health risks likely in a changing climate [8,9]

Heat stress, a well-established phenomenon from extreme heat exposure, is projected to intensify in the future, owing to global warming. For example, it is predicted that temperatures will increase by 1.4 to 5.8 °C globally [10]. In Africa, the rise in temperature is projected to range between 4 and 6 °C in the subtropics and between 3 and 5 °C in the tropics by the end of the century [11]. This temperature increase is expected to exacerbate the number of extreme weather events, including heatwaves [12], which are already occurring more frequently, are more intense, and lasting longer [13]. The anticipated increase in global temperature, together with other environmental factors, such as the rise in humidity [14], will significantly increase heat health risks for the most vulnerable [4], particularly those who reside in low-income countries, such as in Africa, where coping or adaptation measures are limited ([15]. Africa, in particular, is considered to be the most vulnerable continent, with underrepresented extreme heat and heatwave exposure and corresponding heat health risks due to limited access to data [9]. Consequently, it is projected that heat-related extreme events will be more severe and more frequent in Africa than anywhere else in the world [15]. In addition, Africa’s vulnerability is partially attributed to the current problems of multidimensional poverty, poor governments, and political instability, which restricts the capacity of certain countries to meet the challenges raised by climate change [16]. Being located in hot regions and showing high rates of urban population growth, African cities, in particular, will likely face significantly increased exposure to dangerous heat in the coming decades [17]. Furthermore, even under strict climate action, an increasing number of urban residents will be vulnerable to high levels of heatwaves in Africa [18].

The Urban Heat Island (UHI) phenomena and compounding factors, such as the aging population, suggest that heat health risks will be pertinent, threatening the progress towards Sustainable Development Goal 3 (“Good Health and Wellbeing”) [19]. Globally, the effects of heat stress are already being felt by people of all ages. However, there are segments of the most vulnerable population—the elderly, children under the age of five, the chronically ill, the socially isolated, and high-risk occupational groups—that ought to be of concern to human health stakeholders. It is no surprise that a number of reported deaths globally, that is, approximately 70,000 and 500 in Europe in 2003 [20] and 2006 [21], respectively, 225 in North America in 2006 [22], and 55,000 in Eastern Europe and western Russia in 2010 [23], were mainly from the abovementioned vulnerable groups. Additionally, hospital admissions have been increasing due to accumulated metabolic stress on human bodies, which triggers a variety of heat stress conditions, such as heat stroke and, in some cases, exacerbating underlying medical conditions, including respiratory and cardiovascular diseases, diabetes Mellitus, and renal disease [24,25,26].

Heat can impact people’s lives in many ways. Apart from heat health-related risk threats, high temperature-induced heat stress is increasingly becoming an impediment to socio-economic activities. Workers engaged in strenuous labor, mainly in humid and poorly ventilated environments, including those regularly exposed to hot working conditions, such as those in construction or the agricultural sector (including the fisheries and forestry sectors), are likely to suffer from heat stress risks [27,28]. Over the years, it has been proven in occupational heat exposure research that heat exposure does not only threaten the health of workers, consequentially leading to a high mortality rate (see Gun [29] and Vaidyanathan et al. [30]), but also diminishes performance and work capacity [31,32]. According to Kjelltrom et al. [33], this is a challenge for working populations in mining, agriculture, construction, quarries, and outdoor services in countries like South Africa that experience hot seasons each year. Therefore, the effects of heat stress have become most relevant, especially where there are inadequate preventive and control measures. This is particularly true in sub-Saharan Africa. Equally, in school environments, precautions should be taken in protecting schoolchildren from increased indoor and outdoor heat exposure since they are considered one of the groups susceptible to heat stress. This is because this heat stress-susceptible group is still developing physiologically and has limited capacity to cool their bodies, as has been demonstrated in several studies [34,35,36,37]. Furthermore, some of the distinctive features of urban environments (i.e., UHI), especially in cities, exposes many of the inhabitants to extreme heat, which carries health burdens beyond heat-related illnesses that are projected to increase and will affect many in the future due to rapid urbanization coupled with high population growth [38,39]. Undoubtedly, the anthropogenic nature of the world we live in and the projected one of the future will exacerbate the effects of heat even more and beyond the direct and indirect negative health impacts, thereby affecting key socio-economic sectors required to achieve the Sustainable Development Goals.

In response to the increasing potential threat of heat exposure and the need to reduce the risk of harm, many countries, particularly developed countries, have drafted heat action plans as per the World Health Organization (WHO) recommendation [40]. In Africa, countries such as South Africa [41] are in the process. Such plans, which are an important part of mitigation and adaptation planning [42], will ensure preparedness and timely responses by the affected communities in order to minimize heat exposure risks. However, for such plans to be developed and implemented effectively, knowledge of heat exposure risks is required. This information is limited and still emerging in low- and middle-income countries. Such inexhaustive heat stress scholarship has adverse implications on the policy developments needed to formulate adaptation measures that will influence public health decisions and the perceptions of heat health risks in Africa. Awareness of what research has been conducted and what gaps exist in the existing published literature will pave the way for strategic research and mainstreaming actions in heat health research and development. It is against this background that this present study aims to conduct a scientific mapping and analysis on heat health research studies in Africa to decipher thematic trends, patterns, and the nature of linkages (collaboration networks) inherent in the heat health scientific domain. This study, therefore, will (a) have an important scientific contribution through an exposition of the evolution of heat health research themes and methodology, and hence, help to build the heat health body of research, and (b) make a practical contribution in terms of keeping the community of practice abreast with current hot topics and the future direction of heat health research in support for policy decision-making and the development of relevant heat health interventions that are needed.

2. Materials and Methods

The current review study on heat stress was carried out by following a stepwise procedure often used in bibliometric analysis studies, summarized in Figure 1. The preparation of the bibliometric analysis involved firstly defining the research questions within the topic of heat stress and human health. Components of the research questions were used to decide on the geographic region covered as well as the time period for the study. A number of keywords and search topics were identified and used to retrieve published documents on heat stress and human health. In particular, the Web of Science (WoS) and Scopus databases were used to retrieve various document types on the research topic of heat stress and human health. The selected sources are core collection databases that provide global coverage of peer-reviewed research output, such as scientific articles, books, and conference proceedings across various disciplines.

The keywords used in the search of documents were defined in a way that limited the risk, thereby allowing the retrieval of the most published documents on the research topic of heat stress and human health. An example of research conducted in both Scopus and WoS was set by entering the search topics, as follows, on each row of the databases’ web portals: “heat stress” AND “vulnerability” AND “Africa”. A complete set of search topics used in the current review study is given in

Table 1. Summary of search topics used in Scopus and Web of Science collection databases to search and retrieve heat stress-related scientific documents published in Africa.

Search Topic (1st Row)	Areal Restriction (2nd Row)	Areal Restriction (3rd Row)
“Heat stress”	[AND] “vulnerability”	[AND] “Africa”
“Heat stress”	[AND] “human health”	[AND] “Africa”
“Heat stress”	[AND] “impacts”	[AND] “Africa”
“Heat stress”	[AND] “mortality”	[AND] “Africa”
“Heat stress”	[AND] “hospitalization”	[AND] “Africa”
“Heat stress”	[AND] “heat exhaustion”	[AND] “Africa”
“Heat stress”	[AND] “dehydration”	[AND] “Africa”
“Heat stress”	[AND] “thermal comfort”	[AND] “Africa”
“Heat stress”	[AND] “heat stroke”	[AND] “Africa”

, with the area restricted to only Africa and an open-ended search for the global scale option. The retrieved documents contained primary information, such as authors, title, keywords, abstract text, countries, institutions, journals, and cited references, among others. After duplicated documents were removed during pre-processing, a total of 463 and 58 documents, were considered for the global and continental scales, respectively, spanning from 1968–2020 and including articles, book chapters, and conference proceedings. See Appendix A for a list of 58 documents retrieved from Africa, with a summary of respective research themes. A summary catalog of the information retrieved is given in Figure 2.

The data analysis of the retrieved documents on heat stress and human health was based on bibliometric mapping methods, and thus, we used the Bibliometrix R tool established by Aria and Cuccurullo [44]. A set of analyses covering, for instance, aspects of the annual publication growth and trends, collaborations between countries and institutions, citations and keywords frequency, and so forth, were conducted for both the global and continental data. The analyzed information was exported to the VOSviewer visualization software [45] to create network visualization maps. The interpretation of the network maps was based on the cluster size and link strength provided by VOSviewer.

3. Results

3.1. Scientific Mapping of Heat Stress Research

3.1.1. Trends in the Scientific Publications in Heat Stress and Human Health Research

Figure 3 illustrates the trends and patterns of the time series of scientific publications for Africa, contextualizing those published globally. Years that do not appear on the timeline had zero publication output for both the globe and the African continent. Based on the dataset, Hall [46] issued the first publication in Africa in 1968. Since then, one to three publications have been published on a sporadic basis, with an average annual percentage growth rate of about 3.1%, thus indicating a marginally growing publication output trend. However, publication productivity peaked in 2018 in Africa with 13 articles, and subsequently declined by almost 50% between 2019 and 2020. Globally, there was a gradual increase in the number of publications between 2010 and 2012, increasing from 14 to 20 publications. Noticeably, since 2013, there was a high growth trend in the number of publications, increasing from about 40 to 48 articles by the year 2020, a large proportion of which has been published in the last eight years. In 2019, the global publications reached a high of 59 articles. The global annual percentage growth is 7.7%—nearly twice the annual percentage growth rate for Africa.

3.1.2. The Most Productive Countries

Various countries, both in Africa and globally, have contributed to the research on human heat stress. However, the heat stress scientific domain has not received equal attention in Africa as compared to most developed countries. Figure 4 illustrates the top 10 active countries globally;

Table 2. The top five leading countries based on the affiliation of the corresponding author. SCP—Single Country Publications, MCP—Multiple Country Publications.

Country	Articles	SCP	MCP	Total Citations
South Africa	8	8	0	46
Germany	4	3	1	271
United Kingdom	4	4	0	31
United States of America	4	4	0	76
New Zealand	3	2	1	84

illustrates the top 5 countries leading in human heat stress research in Africa. The identified leading countries are ranked based on the affiliation of the respective author’s country. As depicted in Table 2, South Africa is the most productive country in Africa, significantly dominating with 8 publications, all published through Single Country Publications (SCPs), that is, all authors are from the same country and reflect intra-country collaboration. These articles have received a total of 45 citations from the literature. Germany, the United Kingdom (UK), and the United States rank second with 4 publications each, all through SCPs, with exceptions in Germany. For Germany, 3 articles were published under an SCP, whereas 1 was published under Multiple Country Publications (MCP) and has been cited the most (271 citations) in the literature. In MCP publications, authors are from various countries and represent inter-country, or worldwide, collaboration. Generally, most of the published articles in Africa resulted from SCPs, with less than 13% of the publications resulting from MCPs. From the global perspective (see Figure 4), the United States has the highest number of publications, the majority (95%) of which are published through an SCP. Germany again takes the second lead with 34 scientific publications, followed by China, Australia, and the United Kingdom with 26, 23, and 16 combined articles, respectively.

3.1.3. Journals with Top Manuscript as per Citations

The 58 scientific publications retrieved and analyzed in the current review study were published in various journals. The top 10 journals that have published heat stress and human health-related research in Africa are summarized in descending order in

Table 3. Top 10 manuscripts per citation based on heat stress and human health research in Africa. TC—Total Citations, TCPY—Total Citations Per Year.

Journal Name	TC	TCPY
Proceedings of the National Academy of Sciences of the USA	174	19.33
Journal of Climate	152	11.69
Climate Research	98	5.76
Environmental Research Letters	55	13.75
Global Health Action	48	4.00
Journal of Travel Medicine	46	3.54
American Journal of Industrial Medicine	40	2.22
Proceedings of the National Academy of Sciences of the USA	38	4.22
Environmental Health Global Access Science Source	31	3.44
Environmental Research Letters	30	4.29

. The ranking of the journals is based on the total citations of the published manuscripts. Consequently, approximately 17% of the articles were published in the top 10 journals given in Table 3. The published articles are scattered in the human heat stress-related field of research in Africa. The top journal publisher observed in this review is the Proceedings of the National Academy of Sciences of the USA, which resulted in 174 citations of the published manuscript, followed by the Journal of Climate, Climate Research, and Environmental Research Letters with 152, 98, and 55 total citations per manuscript, respectively. The Environmental Health Global Access Science Source and Environmental Research Letters are the lowest in the top 10 journals, with 31 and 30 citations per manuscript, respectively.

3.1.4. Country Collaboration Network

In this analysis, the VOSviewer software [45,47] was used to construct and visualize the bibliometric collaboration network between countries that have published scientific documents on heat stress and human health-related research, both globally and across Africa. The analysis results are presented in Figure 5 (global) and Figure 6 (Africa). The countries are assigned into various clusters, as can be seen from the VOSviewer network, with the scale of the cluster reflecting a given country’s close collaboration with other countries. Most of the countries seen in the network fall in the top 10 leading countries illustrated in Figure 4. The network mapping yielded seven main clusters, as shown in Figure 5, for the global scientific analysis. The red cluster contains the most countries (5), with Brazil, France, and Greece having collaborated with 6 countries each, followed by Russia and Portugal with 5 and 2 associated links, respectively. The green cluster comes second, depicting the collaborations of 4 countries, where Canada has 4 associated links with other countries, followed by the United Kingdom [3], China [3], and Australia [2]. The remaining five clusters consist of 2 countries per group. For instance, Germany and Austria, in the blue cluster, are composed of 11 and 1 collaboration links; India and Italy (yellow) with 3 and 1 links; the Netherlands and Belgium (pink cluster) with 2 and 1 links; the United States and Japan (light blue) with 10 and 1 links, and the Czech Republic and Korea (orange) with 3 and 1 collaboration links, respectively.

Based on the results shown in Figure 6, countries in Africa offer less collaboration with domestic and international countries. As shown in Figure 6, the network depicts only the countries that have collaborated with others during the study period. The network classifies the countries into three clusters. The purple cluster shows the main country collaborations, with 4 countries having collaborated among themselves, as well as with Germany in the blue cluster. Collaborations between these countries suggest similar research interests within the field of heat stress and human health research. The green cluster had a strong collaboration with Australia, Sweden, and New Zealand, also suggesting common scientific priorities in heat stress and human health research subject matter.

3.1.5. Institution Collaboration Network

Similar to the country collaboration mapping, the VOSviewer analysis was used to visualize the global and Africa collaboration network according to the researcher’s institutions. Portrayed in Figure 7a are 5 institution collaboration clusters suggesting strong collaboration in scientific publications among global institutions. The largest and densest cluster is the red cluster, consisting mainly of university institutions in Australia, China, Japan, and the United States. Linked to these universities are two non-academic institutions, namely the National Cancer Institution (NCI), a United States federal government’s principal agency for cancer research and training, as well as the Research Centre for Artificial Intelligence (Deutsches Forschungszentrum für Künstliche Intelligenz) in Germany. The blue cluster connects with the red cluster through Yale and Columbia Universities, the two dominant universities in the United States. The latter academic institutions connect to the yellow cluster through the Arizona State University and the University of Washington, both in the United States. The National Centre for Atmospheric Science in the United Kingdom and Beijing Normal University in China completes the yellow cluster collaboration. The University of Colorado in Boulder, USA, links the green cluster, which is the second dominating cluster in the institution collaboration network mapping. With the exception of Khon Kaen University in southeastern Asia and the University of Kelaniya, the green cluster is dominated by research institutions in (1) Mexico (Instituto Nacional de Cardiología Ignacio Chávez), (2) Paraguay (Centro de Hemodialisis Hospital), (3) India (Madras Medical Mission Hospital), (4) Australia (George Institute for Global Health) and lastly, (5) the Cooperative Institute for Research in Environmental Sciences (CIRES) in the United States. The relatively small cluster in the global mapping is the purple cluster that links to the red dominating cluster through the Ludwig Maximilian University of Munich in Germany.

There are 6 clusters showing institution collaboration among the African institutions, and institutions from outside Africa are shown in Figure 7b. Institution collaborations in Africa were dominated by institutions from the United Kingdom and Asian countries. In the red cluster are the Medical Research Institute (IMIM) and the Spanish Consortium for Research, both located in Spain. In addition, the University of Athens in Greece, Lazio Regional Health Services in Italy, l’Institut Pasteur de Tunis in Tunisia, and the Universidade de Lisboa in Portugal are linked to each other in the red cluster. Institutions from outside Africa connect with African institutions through the Federal University of Technology Owerri in Nigeria, a country located in the southeast of West Africa. The teal cluster is the smallest in comparison to other clusters in the institution network in Africa; it connects with the purple cluster through Tlhoeko Environmental Consultants in Maseru, Lesotho, and also connects with the blue cluster through the University of Chinese Academy of Sciences in Huairou District, China. In the center of the blue cluster is the Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences linked to Basel University in Switzerland, the University of Oxford in the United Kingdom, and Paris-Saclay University in France. The latter two universities are connected to the Royal Centre for Defence Medicine (RCDM), which connects with the United Kingdom-based institutions in the yellow cluster, including Birmingham Heartlands Hospital (BHH) and the Academic Department of Military Rehabilitation. The purple cluster comprises the institution in South Africa, including North-West University, the University of Pretoria, and the South African Medical Research Council (SAMRC).

3.1.6. Keywords Co-Occurrence Analysis

The VOSviewer software was used to create a density visualization of the keywords’ co-occurrences from heat stress and human health research at a global and continental (Africa) scale. The results for the global scale analysis are depicted in Figure 8. The keywords were assigned to specific clusters, where each cluster in the density visualization map is accompanied by a specific color corresponding to the density of the cluster. As noted in Figure 8 and

Table 4. Classification of keyword co-occurrences based on heat stress and human health data collected at a global scale.

Cluster	No. of Documents	Most Frequent Words
Red	44	Adaptive management, air pollution, climate change, environmental exposure, environmental health, extreme, event, health hazards, health risks, heatwave, high temperature, morbidity, mortality, risk assessment, urban health, thermal stress
Green	23	Adult, 80 and over, male, female, young adult, middle-aged, newborn and preschool, extreme heat, cause of death
Blue	14	Heat shock response, heat stress, oxidative stress, gene expression, genetics, animals
Yellow	13	Age factors, demography, risk factors, socio-economic factors, thermal exposure, vulnerable population
Purple	6	Heat injury, heat stress disorder, hot temperature, humans

, the keywords were assigned to five main density clusters. The keywords in the red density cluster appear more frequently, referred to in approximately 44 documents. This suggests that most researchers have paid uttermost attention to words such as “climate change”, “environmental exposure”, “environmental health”, “extreme”, “event”, “health hazards”, “health risks”, “heatwave”, “high temperature”, “morbidity”, and “mortality”, among others, when conducting human heat stress research globally. More frequent keywords relating to a vulnerable population (see Table 4) are also observed in the green density cluster, attracting more attention to researchers, as they were referred to in approximately 23 scientific articles published in human heat stress-related research. The scientific community has also shown moderate interest in keywords in the blue and yellow density clusters, which include “heat shock response”, “heat stress”, and “oxidative stress”, as well as “age factors”, “demography”, “risk factors”, and “socio-economic factors”, among others, respectively. On the contrary, the keywords in the purple density cluster appear less frequently, suggesting that researchers are paying minimal attention to aspects of heat stress with inherent impacts, such as heat injury, heat stress disorder, and changing temperatures, and their effects on humans. The differences of keyword co-occurrence results inherent in the clusters could be attributed to the geographical differences, the design (content and research questions) and the methodology of a scientific topic, or the priority research interests of institutions and countries.

Analysis of keyword co-occurrences based on heat stress and human health research conducted in Africa resulted in 6 clusters, each identifying a particular sub-group or theme, as shown in Figure 9 and

Table 5. Classification of keyword co-occurrences by clusters based on the continental scientific data.

Cluster (Sub-Field)	No. of Documents	Selected Top Words (No. of Links)	Remarks
Red (health hazards)	23	Mortality (58); health hazard (55); public health (53); risk factor (50); diarrhea (40) morbidity (36); malaria (35); disease transmission (32); mosquito (30)	Most researchers (i.e., 23 documents) have paid utmost attention to the theme of health hazard in heat stress and human health research in Africa.
Green (vulnerability groups)	13	Male (52); adult (51); female (50); middle-aged (46); adolescent (47); child (40); young adult (37)	The scientific community (13 articles) has paid high attention to the most vulnerable groups affected by heat stress-related illness in Africa.
Blue (impacts)	12	Heat stress (67); injury (35); heatstroke (29); thermal exposure (29); heat exhaustion (28); dehydration (22)	The scientific community has paid moderate attention to the inherent impacts of heat stress-related illness in Africa.
Yellow (climate change)	9	Climate change (62); global warming (40); temperature effect (40); humidity (38); climate effect (33); climate modelling (25); thermal stress (25)	Heat stress is attributed to climate change and variability; hence, the theme draws modest attention when addressing issues relating to heat stress in Africa
Purple (areas)	9	Africa (66); Middle East (40); Europe (42); Asia (34)	Approximately 9 documents paid attention to the identified areas in heat stress and human health research, with Africa leading.
Teal (temperature variability)	4	Humans (69); heat (55); environmental temperature (47); hot temperature (42)	Changes in temperature have a direct impact on humans. However, the theme draws minimal attention when addressing aspects of heat stress in Africa

. The theme or sub-group in each keyword co-occurrence-based cluster was classified based on the words involved. In Table 5, Column 2 indicates the number of scientific articles in each cluster, whereas Column 3 depicts the associated frequency of the keywords. Consequently, the main red cluster, with 23 documents, is composed of terms that address the “health hazards” subject group, the top words being “mortality”, with 58 association links, followed by “health hazard” (55), “public health” (53), “risk factor” (50), “diarrhea” (40), and “morbidity” (36). The high number of documents in the red cluster suggests that scientists have paid uttermost attention to the words in heat stress and human health research in the African continent. The vulnerability groups of heat stress are identified in the green cluster, with 13 documents paying more attention to human heat stress research in Africa. Identified vulnerability groups include “male”, with 52 associated links, “adult” (51), “female” (50), “middle-aged” (46), “adolescent” (47), “child” (40), and “young adult” (37).

The blue cluster keywords cover the general impacts of heat stress, and these appear in 12 documents. The identified impacts include “heat stress” (67), “injury” (35), “heat stroke” (29), “thermal exposure” (29), “heat exhaustion” (28), and “dehydration” (22). Climate change is identified in the yellow cluster as the main subject group, covering words such as “climate change” with 62 associated links, followed by “global warming” (40), “temperature effect” (40), and “humidity” (38), among others. Approximately nine scientific articles paid attention to climate change in human heat stress research in Africa during the analyzed period. The purple cluster identified study areas considered in human heat stress research, including Africa, with the highest number of association links (62), followed by the Middle East (40), Europe (42), and Asia (34). The highest number of association links in Africa confirms the continent’s vulnerability to heat stress attributed to the changing climate. The teal is the cluster that least addresses issues of the changing temperature. Only 4 documents paid attention to the sub-group in Africa, focusing on terms such as “heat” (55), “environmental temperature” (47), and “hot temperature” (42), and their impacts on humans (69).

3.1.7. Network of Themes

To visualize the evolution of research themes in heat stress and human health research, Cobo et al. [48]’s method was used to create two-dimensional thematic maps based on the most notable keywords and key research domains, also known as themes, shown in Figure 10. The quadrant themes consist of positive and negative values along the x- and y-axes, and they are characterized by their centrality and density rank values. The size of the spheres is proportional to the number of occurrences of keywords associated with each theme. The upper right quadrant indicates the motor themes, also referred to as the “hot topics”; the upper left quadrant displays very specialized themes; the lower right quadrant corresponds to the general transversal and basic themes; and lastly, the lower left quadrant consists of emerging or disappearing themes.

Based on the global research analysis (Figure 10), the themes “climate change”, “heat stress”, “extreme heat”, “mortality”, “extreme heatwave”, “urban heat island”, “climate”, “adaptation”, and “elderly” in the lower right quadrant are the most basic, transversal, and relevant to the heat stress and human health research field. The motor themes in the upper right quadrant reflect high density and strong centrality; these are “oxidative stress”, “hyperthermia”, “mitochondria”, “vulnerability”, “heat”, and “public health”, for the global context. The position of themes in this quadrant suggests that they are well developed and integral to current research developments in human health and heat stress subject matter. Themes in the lower left quadrant, such as “diseases”, “conservation”, “global warming”, “heat adaptation”, and “hydration”, have high density but low centrality and are, therefore, of marginal relevance to the heat stress and human health subject in the global perspective. In addition, themes such as “gene expression”, “stress response”, “virulence”, “build environment”, “urban design”, and “microclimate” are considered to be emerging or disappearing, with low density and centrality.

Based on Figure 11 for the African context, the themes in the lower right quadrant are the most basic, transversal, and relevant to the heat stress and human health research field. These themes include “heat stress”, “developing countries”, “Ebola”, “climate change”, “mortality”, and “health”. Similar to the global context, themes in this quadrant are weakly developed yet have excellent development potential and are integral to heat stress and human health research. The well-developed and integral themes (i.e., motor themes) include “humidity”, “temperature”, and “decaying muscles”, situated in the upper right quadrant of Figure 11. The specialized themes, displayed in the upper left quadrant, are “Ebola virus” and “hemorrhagic fever”, whereas “cortisol”, “dehydration”, “climate change”, and “genotype” in the lower left quadrant are considered to be either emerging or disappearing themes, with low density and centrality.

In general, significant research themes are observed in the thematic map for Africa, as shown in Figure 11. These include those appearing in the upper left quadrant, which are weakly developed and marginal, with low centrality and low density. Such themes have well-developed internal ties but unimportant external ties. Researchers in Africa are paying close attention to motor themes in the upper right quadrant. These are at the core of research developments in heat stress and human health research in the African continent.

4. Discussion

Heat stress and human health research has progressed well and is generally now embedded within the sub-area of study within the field of climate change and human health. Interest in the subject matter has been growing steadily since the early 1990s, following widespread attention on the human health consequences of climate change from scholars worldwide [49]. This is mirrored in the growing trend in the number of publications, as reported in this review study. Such an increasing trend is positively correlated with the increase in excessive heat and heatwave-induced mortality and morbidity incidences reported in many countries, both globally and across Africa. While severe heatwave events were recorded as early as 1950 [50], research publications on the subject remained limited globally, with relatively fewer publications prior to 2007. However, following significant heatwave events in various cities in the United States (i.e., the 1966 St Louis heatwave, the 1980 St Louis and Kansas City heatwave, the 1993 Philadelphia heatwave, and the 1995 Chicago heatwave [51]); in Europe (the 2003 European-wide heatwaves in England, Wales, France, Italy, and Portugal) [52]; as well as the 2009 southeastern Australia heatwave [53], there was a slight growth in the number of publications due to the need to understand the relationships between extreme heat exposure and related health impacts.

Notwithstanding this global interest, research in Africa is still nascent. Heat health scholarship remains comparatively sparse and fragmented, as illustrated by the number of publication outputs and the subsequent declining trend in the scientific publications observed in the past two years (2019 and 2020). Heat health-related outcomes are dependent on the local context [54]. However, the large volume of global scientific publications and the wealth of knowledge stemming from extensive research power from wealthier countries (i.e., the United States and Germany) does not necessarily reflect or address the heat health burden in African countries. This warrants the need for African countries to improve their research output and conduct research studies that will capture local conditions and release publications that will influence policy development to build resilience and mitigate unintended consequences of heat health risks in the continent.

The observed low scientific publication productivity might be attributed to various challenges, including a lack of funding, data availability, and the necessary expertise African researchers are confronted with [55,56]. On the contrary, these challenges are not usually found in developed countries. Hence, this research revealed that the United States is the most prolific country in heat health research, based on the publication output. This outcome is consistent with findings in other research fields, including sports entrepreneurship [57], climate change and infectious diseases [58], and medical data mining research [59]. Germany, which collaborates with the United States and two African countries, Nigeria and Burkina Faso, ranks second in publication output, followed by China. The close collaboration suggests that these countries share common scientific priorities and are better positioned to influence the course and direction of heat health research. The researcher’s institutions affiliation was dominated by academic institutions mostly residing in the United States, including the University of Yale, and Columbia, Arizona State, and Washington Universities. This is followed by German institutions, including the Ludwig Maximilian University of Munich and the Research Centre for Artificial Intelligence (Deutsches Forschungszentrum für Künstliche Intelligenz). Likewise, the leading productive authors come from Germany and the United States, with 17 and 7 publications, respectively.

The country collaboration network analysis presented, in this review, indicated insufficient international research collaboration, yet this is highly needed in the African continent and it revealed the lack of cooperation among African countries. It is worrisome for neighboring nation-states who are more prone to issues that are unlikely to occur in isolation. This is a cause for concern since African countries, in most instances, face common cumulative challenges (i.e., a lack of or inaccessible data and funding) and development agenda needs. However, instead of working together in collaboration, they often work in silos, which subsequently affects the quality of research and limits research output, as demonstrated in this study. Most importantly, it impedes knowledge generation that can be fed into policy development processes. As research has shown, collaboration among researchers, students, and countries, in general, can help improve public health interventions, knowledge sharing and expertise, and can ultimately foster research development within countries and across regions [60]. Furthermore, it can be valuable in exchanging and accelerating heat health policy formulations critical for mitigation and adaptation strategies [42].

The perception that society’s challenges are nuanced and interdependent suggests the need for strong collaborations and more interdisciplinary and transdisciplinary partnerships [61,62]. Transdisciplinary partnership seeks to leverage diverse approaches and experiences to integrate and transcend disciplinary bounds to advance scientific knowledge that will tackle pressing social, environmental, and health problems and, more importantly, reduce disparities [62]. Inputs from various stakeholders provide complementary insights and ensure the success and sustainability of efforts to reduce vulnerability to heat stress and mitigate adverse heat-related health outcomes [60]. A good example is the development of a heat health policy for the city of Adelaide in Australia [63], where multi-stakeholder consultations and involvement from different agencies led to the successful development and implementation of the adaptation strategy in response to the societal threats of heatwaves in that region. Akompab et al. [63] concluded that other regions of the world could adopt the same approach applied in their study to guide heatwave response plans. The current study revealed that South Africa had the biggest research contribution in Africa, and this has been reflected in the progress that is being made in the formulation of a national heat health plan called for by the National Climate Change Response Policy White Paper [41]. The reviewed literature on this topic did not reveal much development in other countries in Africa, suggesting the association of heat- health is currently an under-prioritized policy issue in Africa [64]. Furthermore, the lack of research productivity and collaboration at both the local and global levels exacerbates the status quo. Even though policy development might be influenced by numerous governance and institutional arrangements, as averred in the study by Akompab et al. [63], there is a high probability that if South African researchers had partnered more with local, continental collaborators and international partners, the development of the heat health plan in South Africa would have been at an advanced stage.

In terms of the keyword co-occurrence analysis, the red cluster, which is the largest in both the density visualization chart and the keyword co-occurrence analysis mapping, revealed Africa’s research progress and focus, as defined by popular words like “health hazards”, “public health”, “risk factor", “evaluation”, and “morbidity”, which dominated a relatively large portion of publication production in this area. The thematic analysis further revealed emerging themes appearing in the right lower quadrant, covering specialized themes such as “urban planning”, “thermal stress”, “land surface temperature”, “build in environment”, “urban design”, and “stress response”, which are currently at the center of heat health research at the global level. The emerging themes in Africa are “dehydration” and the “cortisol” hormone, which has been found to increase with heat exposure [65]. While research themes have progressed dramatically at the global scale, capturing the current developments and advancements in heat health research, as evidenced by the diverse thematic areas in Figure 9, they are at a rather slow uptake in Africa. Basic themes, such as “vulnerability” (within population groups such as the elderly), “heat exposure”, “exploration of effects of UHI on health”, and “adaptation to extreme heat” are not represented under research themes in Africa. On the one hand, the results revealed the research gaps in Africa on this topic, and on the other, they provided new avenues for future research that need to be pursued by African researchers if the continent is to win the battle against climate changes and protect its citizens from associated heat health risks.

5. Conclusions

In this study, a bibliometric analysis of scientific publications in heat stress and human health research was conducted, with emphasis on the African continent. The study reviewed 463 global and 58 African scientific publications retrieved from the WoS and Scopus core databases, all released between 1968 and 2020. In this study, it is noted that while research on heat human health has gained momentum over the years, particularly on the global scale, the research within the heat health scientific domain has remained comparatively sparse and fragmented across Africa. The low research productivity in heat stress and human health subject field could be attributed to various factors, including a lack of funding, limitations to data accessibility and availability, unequal power dynamics, as well limited skilled human resources to undertake such research work on the continent. In addition, there are significant gaps in terms of heat stress and human health research in Africa compared to the global perspective. These research gaps provide opportunities for prospective researchers to carry out human health and heat stress-related research in Africa, focusing on numerous developmental areas such as (1) vulnerability assessments within and across demographic classes, including the elderly; (2) personal exposure and associated risks; (3), the UHI evaluation for urban environments; and (4) heat adaptation research, which will enable informed and targeted preventive actions that will limit future heat health impacts in the most vulnerable communities. While there have been many successful global research collaborations, such partnerships are very minimal among African countries, institutions, and researchers. There is a need for international organizations to ally with African researchers to strengthen research that will address heat health risks across the continent, subsequently influencing heat health policy developments and decision-making. An initiative such as the Global Heat Health Information Network (hereafter GHHIN), spearheaded by the WMO/WHO, could be used to bridge the gap in Africa and beyond borders. This is because the GHHIN advocates for the creation of alliances and interdisciplinary capacity-building and cooperation, including the sharing of resources for individual and administrative preparation, interinstitutional and interdisciplinary communication, and collaborations. In general, this review study contributes to building a body of research on heat stress and human health in Africa, thereby providing the necessary support in mitigating future impacts of heat stress-related illness under the changing climate.