The environmental consequences of climate change such as rising temperatures, more extreme weather events, and increased droughts and flooding are impacting human health and lives. 1, 2 Previous studies and reviews have documented the health impacts of climate change; however, they have focused on specific climate effects, 3, 4 health impacts, 5,6 countries, [7] [8] [9] or are no longer up to date. 10, 11 To guide future research and action to mitigate and adapt to the health impacts of climate change and its environmental consequences, we need a complete and thorough overview of the research already conducted. In this study, we aimed to develop such a synthesis of systematic reviews of health impacts of climate change. Our research objectives were to synthesize studies' characteristics such as geographical regions, years of publication, and authors' affiliations, to map the climate impacts, health outcomes, and combinations of these that have been studied, and to synthesize key findings.
We applied the Cochrane method for overviews of reviews. 12 This method is designed to systematically map the themes of studies on a topic and synthesize findings to achieve a broader overview of the available literature on the topic.
Our research questions were the following: 1) What is known about the relationship between climate change and health, as shown in previous systematic reviews? 2) What are the characteristics of these studies? We registered our plan (CRD42019145972 13 ) in PROSPERO, an international prospective register of systematic reviews and followed PRISMA 2020 14 to report our findings, as a reporting guideline for overviews is still in development. 15 Search strategy and selection criteria To identify relevant studies, we used a systematic search strategy. We included studies in this review if they 1) were systematic reviews of original research and 2) reported at least one health impact as it related (directly or indirectly) to climate change.
We defined a systematic review, based on Cochrane's definition, as a review of the literature in which one "attempts to identify, appraise and synthesize all the empirical evidence that meets pre-specified eligibility criteria to answer a specific research question [by] us[ing] explicit, systematic methods that are selected with a view aimed at minimizing bias, to produce more reliable findings to inform decision making." 16 We included systematic reviews of original research, with or without meta-analyses. We excluded narrative reviews, non-systematic literature reviews and systematic reviews of materials that were not original research (e.g., systematic reviews of guidelines.)
We based our definition of health impacts on the World Health Organization's (WHO) definition of health as, "a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity." 17 Therefore, health impacts included, among others, morbidity, mortality, new conditions, worsening/improving conditions, injuries, and psychological wellbeing. Climate change (or global warming) could be referred to directly or indirectly, for instance, by synthesizing the direct or indirect health effects of temperature rises or of natural conditions/disasters made more likely by climate change (e.g., floods, wildfires, temperature variability, droughts.) We included systematic reviews whose main focus was not the health impacts of climate change, providing they reported at least one result regarding health effects related to climate change (or consequences of climate change.)
On June 22, 2019, we retrieved systematic reviews regarding the health effects of climate change by searching the electronic databases Medline, CINAHL, Embase, Cochrane, Web of Science using a structured search (see Appendix 1 for final search strategy developed by a librarian.) We did not apply language restrictions. After removing duplicates, we imported references into Covidence. 18 Screening process
To select studies, we first screened titles and abstracts to eliminate articles that did not meet our inclusion criteria. Two trained analysts independently screened each article. A senior analyst resolved any conflict or disagreement. Because the topic was new to some team members, to ensure a high-quality screening process, the trained analysts then re-screened all included records and the senior analyst re-screened all excluded records. Two analysts then independently screened the full text of retained articles, again with a senior analyst resolving disagreements.
Next, we decided on key information that needed to be extracted from studies. We extracted the first author's name, year of publication, number of studies included, time frame (in years) of the studies included in the article, first author's institution's country affiliation, whether the systematic review included a meta-analysis, geographical focus, population focus, the climate impact(s) and the health outcome(s) as well as the main findings and limitations of each systematic review.
Two or more trained analysts (RR, CB, RN, LC, LPB, RAPR) independently extracted data, using Covidence and spreadsheet software (Google Sheets). For analysts who were new to evidence syntheses (CB, LPB, RAPR), the training process included extracting data repeatedly from the same articles to ensure accurate understanding, weekly group meetings to clarify understanding, and daily supervision by more senior team members (RR, RN, HOW). An additional trained analyst from the group or senior research team member resolved disagreements between individual judgments.
To summarize findings from previous reviews, we used a three-step procedure for coding and data mapping. First, to map articles according to climate impacts and health outcomes, two researchers (RR and LC) consulted the titles and abstracts of each article. We developed the categories for climate impacts separately from those for health outcomes and used a mixed approach to coding. We started with an inductive coding method, by identifying categories directly based on our data and followed up with a deductive approach to finalize categories by consulting previous conceptual frameworks of climate impacts and health outcomes. 1, 2, 19 The same two researchers independently coded each article according to their climate impact and health outcome. We then compared coding and resolved disagreements through discussion.
Next, still using spreadsheet software, we created a matrix to map articles according to their combination of climate impacts and health outcomes. Each health outcome occupied one row, whereas climate impacts each occupied one column. We placed each article in the matrix according to the combination(s) of their climate impact(s) and health outcome(s). For instance, if we coded an article as 'extreme weather' for climate and 'mental health' for health impact, we noted it in the cell at the intersection of these two codes. We calculated frequencies for each cell to identify frequent combinations and gaps in literature. Because one study could investigate more than one climate impact and health outcome, the frequency counts for each category could exceed the number of studies included in this review.
Finally, we summarized findings of the studies individually according to their combination of climate impacts and health outcomes. We re-read the Results and Discussion sections of each article as part of this step. We first wrote an individual summary for each study, then we collated the summaries of all studies exploring the same combination of categories to develop an overall summary of findings for each combination of categories.
We used a modified version of AMSTAR-2 to assess the quality of the included systematic reviews (Appendix 2). Since AMSTAR-2 was developed for syntheses of systematic reviews of randomized controlled trials, working with a team member with expertise in knowledge synthesis (AT), we adapted it to suit a research context that is not amenable to randomized controlled trials. We used items 5, 6, 10, 11, 12, 14, 15, 16 without modification and modified items 1 to 4, 7 to 9 and 13. 
A detailed table of all articles and their characteristics can be found in Appendix 3. Publication years ranged from 2007 to 2019 (year of data extraction), with the great majority of included articles (n = 69; 73%) published since 2015 ( Figure 2 ). A median of 30 studies had been included in the systematic reviews (mean = 60; SD = 49; range 7 to 722). Approximately one fifth of the systematic reviews included meta-analyses of their included studies (n = 18; 19%). The majority of included systematic reviews' first authors had affiliations in high-income countries, with the largest representations by continent in Europe (n = 30) and Australia (n = 24) ( Figure 3 ). Regarding the geographical focus of systematic reviews, most of the included studies (n = 68; 72%) had a global focus or no specified geographical limitations and therefore included studies published anywhere in the world. The remaining systematic reviews either targeted certain countries (n = 12) (1 for each Australia, Germany, Iran, India, Ethiopia, Malaysia, Nepal, New Zealand and 2 reviews focused on China and the United States), continents (n = 5) (3 focused on Europe and 2 on Asia), or regions according to geographical location (n = 6) (1 focused on Sub-Saharan Africa, 1 on Eastern Mediterranean countries, 1 on Tropical countries, and 3 focused on the Arctic), or according to the country's level of income (n = 3) (2 on low to middle income countries, 1 on high income countries).
Regarding specific populations of interest, most of the systematic reviews did not define a specific population of interest (n = 69; 73%). For the studies that specified a population of interest (n = 25; 26.6%), the most frequent populations were children (n = 7) and workers (n = 6), followed by vulnerable or susceptible populations more generally (n = 4), the elderly (n = 3), pregnant people (n = 2), people with disabilities or chronic illnesses (n = 2) and rural populations (n = 1).
We assessed studies for quality according to our revised AMSTAR-2. Out of 94 systematic reviews, the most commonly fully satisfied criterion was #1 (PICO components) with 81/94 (86%) of included systematic reviews fully satisfying this criterion. The next most commonlysatisfied criteria were #16 (potential sources of conflict of interest reported) (78/94 = 83% fully), #13 (account for limitations in individual studies) (70/94 = 75% fully and 2/94 = 2% partially), #7 (explain both inclusion and exclusion criteria) (64/94 = 68% fully and 19/94 = 20% partially), #8 (description of included studies in adequate detail) (36/94 = 38% fully and 41/94 = 44% partially), and #4 (use of a comprehensive literature search strategy) (0/94 = 0% fully and 80/94 = 85% partially). For criteria #11, #12, and #15, which only applied to reviews including metaanalyses, 17/18 (94%) fully satisfied criterion #11 (use of an appropriate methods for statistical combination of results), 12/18 (67%) fully satisfied criterion #12 (assessment of the potential impact of RoB in individual studies) (1/18 = 6% partially), and 11/18 (61%) fully satisfied criterion #15 (an adequate investigation of publication bias, small study bias). Full details are available in Appendix 4.
For both the climate impacts and health outcomes, systematic reviews could have a general or a specific focus. A general focus consisted of investigating the general impacts of climate change or multiple impacts simultaneously, whereas a specific focus targeted specifically only one climate impact or health outcome. When combining the climate impact to the health outcome, four combinations became apparent. Table 1 shows these four combinations with sample titles of systematic reviews within that combination. The most frequent combination (n = 52; 55%) consisted of studies investigating a specific climate impact on a specific health outcome (e.g., the impact of floods on mental health) and the least frequent combination (n = 5; 5%) consisted of studies exploring general or multiple climate impacts' effects on multiple health outcomes (e.g., health impacts of climate change.) Regarding climate impacts, we identified five mutually exclusive categories, with 13 publications targeting more than one category of climate impacts: 1) Meteorological (n = 71 papers) (e.g., temperature, heat waves, humidity, precipitation), 2) Extreme weather (n = 24) (e.g., waterrelated, floods, cyclones, hurricanes, drought), 3) Air quality (n = 7) (e.g., air pollution and wildfire smoke exposure), 4) General (n = 5), and 5) Other (n = 3). "General" climate impacts included articles that did not specify climate change impacts but stated general climate change as their focus. "Other" climate impacts included studies investigating other effects indirectly related to climate change (e.g., impact of environmental contaminants) or general environmental risk factors (e.g., environmental hazards, sanitation, and access to clean water.)
We identified ten categories to describe the health outcomes studied by the systematic reviews, and 29 publications targeted more than one category of health outcomes: 1) Infectious diseases (n = 41 papers) (vector-, food-and water-borne), 2) Mortality (n = 32), 3) Respiratory, cardiovascular, cardiopulmonary and neurological (n = 22), 4) Healthcare systems (n = 16) , 5) Mental health (n = 13), 6) Pregnancy and birth (n = 11), 7) Dietary (n = 9), 8) Skin and allergies (n = 9), 9) Occupational health and injuries (n = 6) and 10) Other health outcomes (n = 17) (e.g., sleep, arthritis, disability-adjusted life years, non-occupational injuries, etc.) Figure 4 depicts the combinations of climate impact and health outcome for each study, with Appendix 5 offering further details. The 5 most common combinations are studies investigating the 1) meteorological impacts on infectious diseases (n = 35), 2) mortality (n = 24) and 3) respiratory, cardiovascular, cardiopulmonary and neurological outcomes (n = 17), and 4) extreme weather events' impacts on infectious diseases (n = 14) and 5) meteorological impacts on health systems (n = 11). Figure 4 . Summary of the combination of climate impact and health outcome (frequencies). Note: The total frequency for one category of health outcome could exceed the number of publications included in this health outcome, since one publication could explore the health impact according to more than one climate factor (e.g., one publication could explore both the impact of extreme weather events and temperature on mental health.)
For studies investigating meteorological impacts on health, the three most common health outcomes studied were impacts on 1) infectious diseases (n = 35), 2) mortality (n = 24) and 3) respiratory, cardiovascular, cardiopulmonary and neurological outcomes (n = 17). Extreme weather event studies most commonly reported health outcomes related to 1) infectious diseases (n = 14), 2) mental health outcomes (n = 9) and 3) dietary outcomes (n = 6) and other health outcomes (e.g., injuries, sleep) (n = 6). Studies focused on the impact of air quality were less frequent and explored mostly health outcomes linked to 1) respiratory, cardiovascular, cardiopulmonary and neurological outcomes (n = 6), 2) mortality (n = 5) and 3) pregnancy and birth outcomes (n = 3). All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted September 30, 2020. . https://doi.org/10.1101/2020.09.29.20204123 doi: medRxiv preprint Meteorological factors' impact on all health outcomes are explored, although some health outcomes are more rarely explored (e.g., mental health and dietary outcomes). In contrast, the impact of extreme weather events and air quality on skin and allergies and occupational health are not explored and their impacts on respiratory, cardiovascular, cardiopulmonary and neurological outcomes, health systems and pregnancy outcomes are only rarely explored. The impacts of air quality on infectious diseases, dietary outcomes, skin and allergies, and occupational health and injuries are also not explored. Most health outcomes are most frequently explored according to the meteorological impacts, however, mental health outcomes and dietary outcomes are most frequently explored according to extreme weather events.
Most reviews suggest a deleterious impact of climate change on multiple adverse health outcomes, with some associations being explored and/or supported with consistent findings more often than others (see Table 2 for a summary of findings according to health outcomes). For instance, the association between meteorological factors, such as temperature and humidity, and vector-borne diseases is quite substantially supported by multiple reviews (n = 22) conducted in multiple geographic locations. In contrast, the association between wildfire smoke exposure and adverse birth outcomes is plausible, but the evidence from included reviews is still in its infancy stage because only a few reviews (n = 3) investigated this association and the findings are currently conflicting.
Most reviews concluded by calling for more research, noting the limitations observed among the studies included in their reviews, as well as limitations in their reviews themselves. These limitations included, amongst others, some systematic reviews having a small number of publications, 27,28 language restrictions such as including only papers in English, 20,23 arriving at conflicting evidence, 29 difficulty concluding a strong association due to the heterogeneity in methods and measurements or the limited equipment and access to quality data in certain contexts, 27, [30] [31] [32] and most studies included were conducted in high-income countries. 33, 34 Previous authors also discussed the important challenge related to exploring the relationship between climate change and health. Not only is it difficult to explore the potential causal relationship between climate change and health, mostly due to methodological challenges, but there are also a wide variety of complex causal factors that may interact to determine health outcomes. Therefore, the possible causal mechanisms underlying these associations were at times still unknown or uncertain and the impacts of some climate factors were different according to geographical location and specificities of the context. Nonetheless, some reviews offered potential explanations for the climate-health association, with the climate factor at times, having a direct impact on health (e.g., flooding causing injuries) and in other cases, having an indirect impact (e.g., flooding causing stress which in turn may cause adverse birth outcomes.) Table 2 . Summary of findings from systematic reviews according to health outcome and climate impact. Reviews that covered multiple climate impacts are listed in each relevant category. All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. Vector borne infectious diseases (n = 25)
22 Systematic reviews suggest that meteorological factors, such as temperature, precipitation, humidity, and wind, are associated with diverse vector-borne infectious diseases, including malaria and dengue. 6, 9, 20, 21, 30, 32, [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] This association was mostly proportional (e.g., higher temperature and increased rainfall associated with vector-borne diseases), although findings were at times conflicting, with some suggesting an inversely proportional association 9 (e.g., decreased rainfall) or no association at all 40 (e.g., with the human puumala hantavirus Infection.) Geographic location, seasonality and potential interaction with other climate-related factors may partly explain these inconsistencies. 9, 30 Temperature, humidity and rainfall were the most common and important meteorological factors reported by reviews and factors such as wind, air pressure and sunshine were reported less often.
There are limited and conflicting findings concerning the association of extreme weather events with vector-borne diseases. Some reviews suggest water-related extreme events 51 and flooding 3,32,52 are associated with an increased risk of vector-borne diseases, while drought is associated with a reduction of dengue incidence. 9 Other reviews focused specifically on Puerto Rico 43 and Australia 53 did not find an association between hurricanes and/or floods and mosquito-borne disease transmission.
Meteorolo gical 14 Reviews suggest that meteorological factors, such as temperature, precipitation, and humidity, are associated with diverse food-and water-borne infectious diseases, in particular, cholera, food poisoning, schistosomiasis, salmonella and E. coli. 8, 21, 32, 41, 45, 48, [54] [55] [56] [57] [58] [59] [60] [61] Overall, higher temperatures and humidity, 8, 41, 54, 58 along with lower precipitation 21,61 was associated with these infectious diseases (e.g., E. coli 58 ; bacterial gastrointestinal infections. 54 ) Directionality and strength of the association seemed to vary according to disease and pathogens, 59 seasons, and geographic region. 56 Extreme weather 10 Reviews suggest a proportional association between extreme water-related events, 47,51,62 such as flooding 3,41,52 and heavy rainfall 35 , and food-and waterborne diseases, including diarrhea, food contamination, cholera. 3, 32, 35, 41, 45, 47, 51, 52, 57, 62 Drought may also be proportionally associated with food-and water-borne disease, 24,35 but these associations are less consistent than those with water-related extreme events. 57 Other infectious diseases (n = 8) All rights reserved. No reuse allowed without permission.
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Reviews suggest an association of most meteorological factors, such as temperature and humidity, with various other infectious diseases, including meningitis, 27, 35 Ebola, 27 influenza, 32 and pediatric infectious diseases such as hand-foot-and-mouth disease. 4, 5, 31, 49, 55 This association was mostly proportional for meteorological factors such as temperature, 4,5,49 diurnal temperature range, 31 and humidity, 4,5,32 although some meteorological factors, such as air pressure 5 and lower temperatures 32, 49 were inversely proportional to these diseases. Some conflicting evidence is reported concerning the association with some meteorological factors, such as sunshine with hand-foot-and-mouth disease, 4, 5 and humidity and pediatric infectious diseases. 55 No association was found between some meteorological factors, such as precipitation, wind speed and sunshine with hand-foot-and-mouth disease. 4, 5 Mortality (n = 32)
Meteorolo gical 24 Reviews suggest that temperature (high, low, or diurnal range) was consistently associated with all-cause and cause-specific mortality. 20 67 very young children 49 and ageing populations. 28 Mortality seems to be the most frequent health outcome studied in association with heatwaves. 64 Inconsistent results are found concerning the association between heat and childhood mortality. 74 Due to limited evidence, this association was weaker in some geographical regions. 27, 71 Also, heat wave intensity (compared to duration) was more strongly associated with heat-related mortality. 75 Finally, although less studied, low temperature was also associated with mortality, 49,76 specifically respiratory, 63 stroke, 69 and cardiovascular mortality. 47, 66, 70 Extreme Weather 5
Reviews suggest an association between extreme weather events such as floods, 3 droughts, 24 cyclones 77 and other water-related events, 20,51 with direct (e.g., drowning) and indirect long-term mortality (e.g., due to malnutrition, environmental toxin exposure, armed conflict, etc.). 3, 24, 51, 77 Air quality 5 Reviews suggest an association between exposure to air pollution 20, 78 or wildfire smoke [79] [80] [81] and air pollution related-mortality, such as respiratory-specific mortality. There is currently limited evidence, but reviews suggest a potential association between wildfire smoke exposure and cardiovascular-specific mortality. [79] [80] [81] Respiratory, neurological, cardiovascular and cardiopulmonary (n = 22) All rights reserved. No reuse allowed without permission.
preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. 69 long-term neurological outcomes (due to heat strokes), 68 myocardial infarction, 34, 83 and childhood asthma and pediatric respiratory diseases. 26, 74 A review also suggests a beneficial association between heat and the shortening of a respiratory virus season. 45 Exposure to low temperature (cold), temperature drop, or diurnal temperature range was associated with cardiovascular and respiratory diseases, 31,63,66 stroke, 69 and myocardial infarctions. 34 Humidity (most often high humidity, but also lower humidity) and low temperatures were also associated with respiratory diseases in children, including childhood asthma. 26, 55, 82 Extreme Weather 1 A previous review suggests an association between drought and respiratory, cardiovascular and cardiopulmonary outcomes, most likely due to droughts leading to increased dust in the air. 24
Air quality 6 Reviews suggest a proportional association between exposure to air pollution 20, 21, 45 or wildfire smoke exposure [79] [80] [81] and respiratory outcomes, including asthma, chronic obstructive pulmonary disease, coughing, wheezing, and overall lung function. Although there is currently limited evidence, 79 reviews also suggest a potential association between air pollution or wildfire smoke exposure and cardiovascular outcomes. 45, 80, 81 Health systems (n = 16)
A previous review suggests that climate change in general puts a strain on public health resources, via population health issues and shows that using an integrated surveillance system may guide future adaptation to climate change. 84 Meteorolo gical 11 Previous reviews suggest an association between temperature change 31 extreme heat, aridity and cold temperatures and an increase in use of healthcare services (mostly linked to heat-related health impacts), such as an increase in emergency department visits, hospital admissions and use of ambulances. 20, 21, 27, 31, 34, 49, 64, 71, 74, 83, 85 Extreme weather 2 Reviews suggest that extreme weather events 33 and flooding 3 are associated with an increase in use of healthcare services (e.g., increased hospitalizations) and a compromised quality of care as extreme weather events may lead to power outages. 33 Air quality 2 Reviews suggest an association between wildfire smoke exposure and an increase in use of healthcare services, such as an increase in emergency department visits. 79, 81 Mental health (n = 13) All rights reserved. No reuse allowed without permission.
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Reviews suggest an association of most meteorological factors such as temperature increase, aridity, heat, and heat waves with mental health outcomes, including hospital admissions for mental health reasons, 21 suicide, 86 and exacerbation of pre-existing mental health conditions, difficulty sleeping, and fatigue. 83 No association was found between sunlight duration and suicide incidence. 86 Extreme weather 9
Most reviews reported a proportional association of extreme weather events, 45, 51, 87, 88 flooding, 3, 20, 89 and drought 24,25 with diverse mental health issues, including, psychological distress, post-traumatic stress disorder, anxiety, depression, psychotropic medication use, alcohol consumption. There was conflicting evidence regarding the association of floods with suicide, tobacco, alcohol and substance abuse. 89 No association was found between drought and suicide. 24
Air quality 1 A previous review suggests no association between wildfire smoke exposure and mental health, as measured by physician visits and hospitalizations for mental health reasons during wildfires. 80 Pregnancy and birth outcomes (n = 11)
Meteorolo gical 5 Reviews suggest that adverse birth outcomes are higher among people exposed to meteorological factors such as high temperature, heat, sunlight intensity, cold and humidity. 21,90-93 These outcomes include low birth weight, preterm birth, eclampsia and preeclampsia, hypertension and length of pregnancy. 21, [90] [91] [92] [93] The association between heat and adverse birth outcomes seems to have stronger support than the association with cold temperatures. 93 Extreme Weather 2 Reviews suggest an association of extreme weather events 87 and flooding 3 with adverse birth outcomes, such as low birth weight, preterm birth and preeclampsia. It is suggested that extreme weather events may indirectly affect birth outcomes via the pregnant person's well-being (e.g., stress and worry during pregnancy.) 3, 87 Air quality 3 There is limited and inconsistent evidence concerning the association between wildfire smoke exposure and adverse birth outcomes, but reviews suggest a potential proportional association between wildfire smoke exposure and lower birth weight. [79] [80] [81] Other 1
The association between environmental pollutants and adverse birth outcomes (i.e., preterm birth) remains unclear due to conflicting evidence. 29 Dietary (n = 9) General 1 A review suggests an association between climate change and obesity. 22 Meteorolo gical 4
Reviews suggest an association between meteorological factors, such as changes in temperature, heat and precipitation, with diverse dietary outcomes, including undernutrition, malnutrition and child stunting. 21, 23, 27, 71 This association may be All rights reserved. No reuse allowed without permission.
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The copyright holder for this this version posted September 30, 2020. . https://doi.org/10.1101/2020.09.29.20204123 doi: medRxiv preprint explained by the impact of meteorological factors, such as temperature increase and precipitation decrease, on crop production and food insecurity. 21, 71 Extreme Weather 6
Reviews suggest an association between extreme weather events, such as flooding and droughts, 24 and diverse dietary outcomes, including malnutrition and undernutrition in children and adults 21, 23, 35, 45, 47 via, amongst others, crops production and food insecurity (e.g., low food aid following flooding 21 ).
Other 1 A review suggests an association between certain environmental risk factors (e.g., sanitation, cooking fuels and food-borne mycotoxins), and childhood stunting, which could be aggravated by climate change. 94 Skin and allergies (n = 9) General 2 Reviews suggest a proportional association between climate change, in general, and skin and soft tissue infections (e.g., fatal vibrio vulnificus necrotizing) 95 and ragweed pollen allergies in Europe. 96 Meteorolo gical 7
Reviews suggest an association of meteorological factors, such as ultraviolet light exposure, temperature and humidity, with diverse skin and allergic diseases, including skin cancer, sunburn, acute urticaria, eczema and pediatric allergies and skin irritabilities. 27, 45, 47, 49, 55, 83, 97 Higher temperature and ultraviolet light exposure is proportionally associated with sunburn 83 and skin cancer, 45,97 while low humidity and low temperatures were associated with eczema and skin irritabilities in children. 49, 55 Occupational health and injuries (n = 6)
Meteorolo gical 6 Reviews suggest that heat is associated with adverse occupational health outcomes, including injuries, heat strain, dehydration and kidney diseases. [98] [99] [100] [101] [102] [103] The most frequent injuries consist of 'slips, trips, falls, wounds, lacerations and amputations.' 99 This association was found in many occupational settings, including agriculture, construction, transport and fishing, and seems to affect both outdoor and indoor workers. 98 This association may be explained by a combination of direct (e.g., dehydration) and indirect factors (e.g., impaired cognitive and physical performance.) 102 Other 1 A review suggests an association between environmental pollution (e.g., heavy metals, fertilizers, etc.) and occupational diseases, such as chronic kidney disease. 103 This association is suggested to be affected by increasing temperatures.
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The copyright holder for this this version posted September 30, 2020. . https://doi.org/10.1101/2020.09.29.20204123 doi: medRxiv preprint General 1 A review suggests an association between climate change in general and disability-adjusted life years, which is an indicator that quantifies 'the burden of disease attributable to climate change'. 104 Authors suggest that the cost of disability-adjusted life years could be high, especially in low to middle income countries.
Meteorolo gical 10 Reviews suggests an association between increasing temperatures and temperature changes, 31 and other various health outcomes, including acute gouty arthritis, 105 unintentional injuries, 106 diabetes, 63 genitourinary diseases, 31, 63 impaired sleep time and quality, 107 cataracts (indirectly associated via people spending more time outside and therefore increased exposure to ultraviolet light), 45, 47 heat stress, heat exhaustion and kidney failure, 83 and renal diseases, fever and electrolyte imbalance in children. 49, 74 Extreme weather 6
Reviews suggests an association between extreme weather events, 88 such as flooding, 3 cyclones, 77 hurricanes, 107 and drought, 24 and other various health outcomes including injuries (e.g., debris, diving in water that is shallower than expected), 3,24,77,88 impaired sleep, 107 esophageal cancer (likely linked to high salinity of water due to droughts), 24 and exacerbation of chronic illnesses. 3, 87 Air quality 1 There is limited evidence, but a systematic review suggests an association between wildfire smoke exposure and ophthalmic outcomes, such as eye irritation and cataracts. 79 
In this overview of systematic reviews, we aimed to develop an overview of systematic reviews of health impacts of climate change by mapping the characteristics and findings of studies exploring the relationship between climate change and health. We identified four key findings.
First, the most common climate impact studied by included publications consists of meteorological impacts (e.g., temperature, heat, precipitation and humidity), which aligns with findings from a previous scoping review on the health impacts of climate change in the Philippines. 7 Although this may not be surprising given that a key implication of climate change is the rise in temperature, this finding suggests we also need to undertake research focused on other climate impacts on health, such as the impact of droughts and wildfire smoke, to better prepare for the health crises that arise from these multiple climate-related impacts.
Second, systematic reviews primarily focus on physical health outcomes, such as infectious diseases, mortality, and respiratory, cardiopulmonary, cardiovascular and neurological outcomes, which also aligns with the country-specific previous scoping review. 7 Regarding mortality, we support Campbell and colleagues' 64 suggestion that we should expand our focus All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted September 30, 2020. . https://doi.org/10.1101/2020.09.29.20204123 doi: medRxiv preprint to include other types of health outcomes. This will allow us to better mitigate and adapt to the full range of threats of climate change.
It is unclear whether the distribution of frequencies of health outcomes reflects the actual burden of health impacts of climate change, or if the most frequently reported outcomes reflect a bias of Western definitions of health. The most commonly-studied health outcomes do not necessarily reflect the definition of health presented by the WHO as, "a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity." 17 This suggests that future studies should investigate in greater depth the impacts of climate change on mental and broader social well-being. Indeed, some reviews suggested that climate change impacts psychological and social well-being, via broader consequences, such as political instability, health system capacity, migration, and crime, 83,87 thus illustrating how our personal health is determined not only by biological and environmental factors but also by social and health systems.
Interestingly, the reviews that explored the mental health impacts of climate change were focused mostly on the direct impacts of experiencing extreme weather events. However, psychologists are also warning about indirect mental health impacts of climate change, which are becoming more prevalent for children and adults alike. 108, 109 Even people who do not experience direct climate impacts, such as extreme weather events, report experiencing disruptive negative emotions when thinking of the destruction of our environment or when worrying about one's uncertain future and the lack of actions being taken. To foster emotional resilience in the face of climate change, these mental health impacts of climate change need to be further explored. Humanity's ability to adapt to and mitigate climate change ultimately depends on our emotional capacity to face this threat.
Third, there is a notable geographic difference in the country affiliations of first authors, with three quarters of systematic reviews having been led by first authors affiliated to institutions in Europe, Australia, or North America. While perhaps unsurprising given the inequalities in research funding and institutions concentrated in Western countries, this is of critical importance given the significant health impacts that will be faced in other parts of the world. Research funding organizations should seek to provide more resources to authors in low-to middleincome countries to ensure their expertise and perspectives are better represented in the literature.
Fourth, overall, most reviews suggest an association between climate change and the deterioration of health in various ways, thus illustrating the interdependence of our health and well-being with the well-being of our environment. At times, climate change and its related environmental events may impact health directly (e.g., heat's impact on dehydration and exhaustion) and other times, it may impact it indirectly (e.g., via behaviour change due to heat.) The climate-health link has been the target of more research in recent years and it is also receiving increasing attention in both public health and climate communication literature. 110, 111 The health framing of climate communication also has implications for healthcare professionals 112 and policymakers, as these actors could play a key part in climate All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted September 30, 2020. . https://doi.org/10.1101/2020.09.29.20204123 doi: medRxiv preprint communication, adaptation, and mitigation. These key stakeholders' perspectives on the climate-health link, as well as their perceived role in climate adaptation and mitigation could be explored, 113 since research suggests that health professionals are important voices in climate communications 112 and especially since, ultimately, these adverse health outcomes will engender pressure on and cost to our health systems and health workers.
To the best of our knowledge, the current study provides the first broad overview of previous systematic reviews exploring the health impacts of climate change. Our review has three main strengths. First, by targeting systematic reviews, we achieve a higher-order summary of findings than what would have been possible by consulting individual original studies. Second, by synthesizing findings across all included studies and according to the combination of climate impact and health outcome, we offer a clear, detailed, and unique summary of the current state of evidence and knowledge gaps about how climate change may influence human health. This summary may be of use to researchers, policymakers, and communities. Third, we included studies published in all languages about any climate impact and any health outcome. In doing so, we provide a comprehensive and robust overview.
Our work has three main limitations. First, we were unable to access some full texts and therefore some studies were excluded, even though we deemed them potentially relevant after title and abstract inspection. Other potentially relevant systematic reviews may be missing due to unseen flaws in our systematic search. Second, due to the heterogeneity of the included systematic reviews and the relatively small proportion of studies reporting meta-analytic findings, we could not conduct meta-meta-analyses of findings across reviews. Future research is needed to quantify the climate and health links described in this review, as well as to investigate the causal relationship and other interacting factors. Third, due to limited resources, we did not assess overlap between the included reviews concerning the studies they included. Frequencies and findings should be interpreted with potential overlap in mind.
Overall, systematic reviews of the health impacts of climate change suggest an association between climate change and the deterioration of health in multiple ways, generally in the direction that climate change is associated with adverse human health outcomes. This is worrisome since these outcomes are predicted to rise in the near future, due to the temperature rise and increase in climate-change-related events such as extreme weather events and worsened air quality. Most studies included in this review focused on meteorological impacts of climate change on adverse physical health outcomes. Future studies could fill knowledge gaps by exploring other climate-related impacts and broader psychosocial health outcomes. Moreover, studies on health impacts of climate change have mostly been conducted by first authors affiliated with institutions in high-income countries. This inequity needs to be addressed, considering that the impacts of climate change are and will continue to predominantly impact lower-income countries. Finally, although most reviews also recommend more research to All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted September 30, 2020. . https://doi.org/10.1101/2020.09.29.20204123 doi: medRxiv preprint better understand and quantify these associations, to adapt to and mitigate climate change's impacts on health, it will also be important to unpack the 'what, how, and where' of these effects. Health effects of climate change are unlikely to be distributed equally or randomly through populations. It will be important to mitigate the changing climate's potential to exacerbate health inequities.
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The copyright holder for this this version posted September 30, 2020. Authors' Contributions RN, CF, ACT, HOW contributed to the design of the study. CB, RN, LPB, RAPR and HOW contributed to the systematic search of the literature and selection of studies. RR, HOW, LC conducted data analysis and interpretation. RR and HOW drafted the first version of the article with early revision by CB, LC and RN. All authors critically revised the article and approved the final version for submission for publication. RR and HOW had full access to all the data in the study and had final responsibility for the decision to submit for publication. All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (review adj1 ("selection criteria" OR "data extraction")).ti,ab Systematic review and meta-analysis combined 14 or/5-13
All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (review NEAR/2 ("selection criteria" OR "data extraction")):ti,ab All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. 18 TI (review N1 ("selection criteria" OR "data extraction")) All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted September 30, 2020. . https://doi.org/10.1101/2020.09.29.20204123 doi: medRxiv preprint
AB (review N1 ("selection criteria" OR "data extraction")) Systematic review and meta-analysis combined 20 S8 OR S9 OR S10 OR S11 OR S12 OR S13 OR S14 OR S15 OR S16 OR S17 OR S18 OR S19
Combination of concepts 21 S7 AND S20
All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. "Population" became "Population and/or location". "Intervention" became "Exposure". The "Comparator group" category was taken out.
A new section (#1.b)) was created, it includes "Definition of the exposure", "Definition of the outcome" and "Timeframe for follow up".
2 Did the report of the review contain an explicit statement that the review methods were established prior to the conduct of the review and did the report justify any significant deviations from the protocol?
To score "yes", a protocol must have been established before the review. There are no subcriterias, you can only score yes or no.
3 Did the review authors explain their selection of the study designs for inclusion in the review?
If the study designs are specified, you score "partial yes". They must be explained to score "yes". No specific study design is required.
4 Did the review authors use a comprehensive literature search strategy?
The "searched trial/study registries" category was taken out. Justified publication restrictions (e.g. language) moved from (partial yes) to (yes) 5 Did the review authors perform study selection in duplicate?
No modifications. 6 Did the review authors perform data extraction in duplicate?
No modifications.
Did the review authors provide a list of excluded studies and justify the exclusion?
The explanation of the inclusion and exclusion criteria is evaluated.
If there is only one out of the two, you score "partial yes". The two must be explained to score "yes".
8 Did the review authors describe the included studies in adequate detail?
"Populations" became "Populations and/or locations". "Interventions" became "Exposures". "Comparator groups" became "Comparator groups (if applicable)". "Populations and/or locations", "Exposures" and "Outcomes" must be described in details to score "yes" 9 Did the review authors use a satisfactory technique for assessing the risk of bias (RoB) in individual studies that were included in the review?
"RoB" became "limitations". Instead of assessing the RoB, the review authors must have used a satisfactory technique for assessing the limitations in individual studies that were included in the review.
All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted September 30, 2020. . https://doi.org/10.1101/2020.09.29.20204123 doi: medRxiv preprint 10 Did the review authors report on the sources of funding for the studies included in the review?
No modifications.
If meta-analysis was performed did the review authors use appropriate methods for statistical combination of results?
No modifications.
If meta-analysis was performed, did the review authors assess the potential impact of RoBin individual studies on the results of the meta-analysis or other evidence synthesis No modifications.
Did the review authors account for RoB in individual studies when interpreting/discussing the results of the review?
"RoB" became "limitations". Instead of accounting for RoB in individual studies, the review authors must have accounted for limitations when interpreting/ discussing the results of the review.
14 Did the review authors provide a satisfactory explanation for, and discussion of, any heterogeneity observed in the results of the review?
No modifications.
If they performed quantitative synthesis did the review authors carry out an adequate investigation of publication bias (small study bias) and discuss its likely impact on the results of the review?
No modifications. 16 Did the review authors report any potential sources of conflict of interest, including any funding they received for conducting the review?
No modifications.
All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. Appendix 4. Summary of quality assessment according to revised AMSTAR-2 items. (Y = yes, PY = partial yes, N = no, NA = non-applicable).
First author Year  1  1b  2  3  4  5  6  7  8  9  10 11 12 13 14 15 16
All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. 
All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted September 30, 2020. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted September 30, 2020. . https://doi.org/10.1101/2020.09.29.20204123 doi: medRxiv preprint