The potential correlation between fatal intentional self-harm (suicide) and heatwaves and humidity


For a number of years now empirical evidence has suggested that the effects of human-caused climate change, and heat in particular, could have a significant impact on mental health. For example, previous links between heatwaves and temperature increase and suicide have been made in countries such as Australia, the US, Mexico, India, Finland as well as England and Wales.

However, new research investigating the relationship between suicide and heat on a global level has found that temperature alone may not be enough to understand the risks posed by a rapidly changing climate but that humidity also must be considered.

The new study, published today in Scientific Reports, conducted by an international team of academics, has found some startling key findings that could have much wider implications if the planet continues in the same climate trajectory.

The study, which analyzed data from 60 countries between 1979 and 2016, investigated the potential correlation between fatal intentional self-harm (suicide), heatwaves and humidity.

It looked at data from organizations such as the World Health Organization (WHO), the European Center for Medium-Range Weather Forecasts (ECMWF), and the UK’s Met. Office Hadley Center to study incidences of heatwaves, relative humidity compared to population and recorded incidents of suicide.

The investigation found a significant correlation between periods of intense humidity and the prevalence of suicide – whilst also finding an increased risk of fatal intentional self-harm in relation to humidity than that of heatwaves.

Additionally, the research also found that both younger age groups and women seemed to be more significantly affected by incidents of fatal self-harm with changes in both humidity and heatwaves, in comparison with the rest of the population, raising questions as to the unequal socio-economic impact of rising temperatures going forward.

The conceptualisation of the study was directed by Dr. Sonja Ayeb Karlsson, from the University of Sussex and United Nations University’s Institute for Environment and Human Security in Bonn, Germany, co-author and mental health-lead of The Lancet Countdown initiative tracking the connections between climate change and public health which new 2021 study warns about how we have reached ‘code red for a healthy future.”

The suicide, heat and humidity investigation was carried out in collaboration with co-author Fernando Florido Ngu and Prof. Ilan Kelman from University College London and University of Agder, Norway; and Dr. Jonathan Chambers from University of Geneva, Switzerland.

Dr. Sonja Ayeb Karlsson from the University of Sussex and UNU-EHS, who co-authored the journal article, said: “The study found interesting trends related to the increased suicide rates among particularly women and youth in relation to humidity.

Women and children are known to be suffering disproportionately from the impacts of climate change and extreme weather events due to social structures and power relations. We need to look further into these relations and the contextual reasons behind this in diverse geographical areas and social groups.”

Prof. Ilan Kelman from University College London and University of Agder, Norway said: “This study contributes to preventing suicide by understanding much better possible contributing factors. People suffer so much day-to-day and we hope that our study can contextualize the role of climate change, such as by explaining the importance of humidity alongside heat.”

Environmental factors are becoming increasingly important in psychiatry due to the fact that they can induce congenital defects, impair neurodevelopment, even trigger endogenous mental disorders as well arouse psychosomatic and neurological disorders (39). Climate can produce strong phenomena with a disastrous impact among human societies.

Disasters create a different kind of psychological and psychopathological distress compared to normal seasonal weather changes, as seen in tornados, floods, and droughts. Furthermore, other climatic events, usually overlooked in studies on mental health of exposed populations (e.g. ocean acidification, acid rain, superfog (40), glacier melting, biomass extinction), could also have a broader impact on mental health.

Psychiatry has only recently begun to deal with climate change, albeit specific literature concerning the climate events in relation to psychiatric disorders is still lacking and rather undefined (41–43).

Heat Waves

As a part of the Climate and Health Profile Report, both direct and indirect impact on health with regard to extreme heat and extreme precipitation were identified. Health risks caused by these factors has significantly risen in recent years (52). Heat weaves are spikes of high temperatures lasting some days that range outside the normal temperature for a specific season (20). This phenomenon is connected with climate change as they have increased in frequency and intensity. Moreover, the frequency and intensity of heat waves are considered extreme events linked to climate change, with a regional effect.

Physical health, mental health, human well-being, and heat waves appear to be specifically interconnected. Heat stress directly caused by heat waves has been associated with mood disorders, anxiety, and related consequences (53). People with mental illness were three times more likely to run the risk of death from a heat wave than those without mental illness (45).

During pregnancy, especially in the second and third trimester, exposure to heat waves has been showed to be related to a lower average birth weight and increase of incidence of preterm birth. Effects during childhood and adulthood comprised reduced schooling and economic activity, other than behavioral and motor problems and reduced IQ (45). Some evidence seems to hint a different vulnerability between genders.

The percentage of deaths were higher in women than men during the European heat wave. Negative outcomes of heat waves are also related to social factors. Women, young people, and people with low socioeconomic status have shown to be more vulnerable to anxiety and mood disorders related to disasters (54). Heat-related illnesses and waterborne diseases are also connected (52).

It could also be noted that people are outside more during the summer, and this could increase the opportunity for conflicts. In hot temperatures, increase in discomfort leads to increase feelings of hostility and aggressive thoughts and possibly actions. Hotter cities were more violent than cooler cities. The increase in heat-related violence are greater in hot summers and showed increased rates in hotter years (55).

Indeed, being exposed to extreme heat can lead to physical and psychological fatigue (53)—there is a clear association between warming temperatures and rising suicide rates, especially in an early summer ‘peak’ (56). Alcohol is likely to be involved in increasing aggression. Higher temperatures than usual, especially in June and July, were associated with an outbreak of aggressive crimes.

The co-occurrence of hot summer days and weekends is therefore a perfect mix, resulting in a massive increase in shootings (57). Preventive measures should be taken against extreme temperatures to protect health, planning targeted public health monitoring, expanding the availability of air conditioning and providing countermeasures for what is known as the “urban heat island” effect (52).


Global warming will lead to CCRWDs such as an increase in both intensity and global number of tropical cyclones (58, 59), frequency and severity of hurricanes, and flooding (60).


Floods are an overflow of water, usually from the ocean, submerging land areas. Flooding is one of the most frequent types of major disaster, leading to 53,000 deaths in the past decade (61). CCRWDs generate flooding in urban areas on the coasts, like those in Asian deltas rivers (62, 63). These events could potentially have a negative effect on the health of vulnerable populations.

Moreover, CCRWDs have a devastating impact on communities and the health of residents, for example by exposing people to toxins, precipitating population susceptibility, and creating crises for healthcare infrastructures (60). There are direct impacts in terms of morbidity and mortality related to water (such as drowning, electrocution, cardiovascular events, nonfatal injuries, and exacerbation of chronic illness); waterborne diseases (due to contamination of drinking water); infectious diseases (64); and psychiatric and mental health disorders.

The principal effect after flooding seems to be located in the mental health area, leading especially to PTSD. A direct correlation between the intensity of the disaster and the severity of the mental health effects was noted (65). After the acute emergency phase, a number of members of the affected populations are subjected to some level of psychological distress and mental health problems (61). As studied all around the world (i.e. India and Bangladesh (61); Dakota USA (66); Italy (67); Venezuela (68) etc.) floods bring mourning, displacement, and psychosocial stress due to loss of lives and belongings, as a direct outcome of the disaster or of its consequences.

All these are risk factors for PTSD, depression, and anxiety (69). With specific reference to flood victims, 20% had been diagnosed with depression, 28.3% with anxiety and 36% with PTSD (70). Consequences are still present well after the flood is over, due to the presence of mourning, economic problems, and behavioral problems in children (71, 72). Moreover, some cases show an increase in substance abuse, domestic violence, as the calamity exacerbates and precipitates previous existing people’s mental health problems (70, 73–75). Contradictory evidence was obtained regarding suicide following a flooding.

In addiction there are aspects of vulnerability such as poverty (60, 76), living in makeshift housing solutions, and the lack of access to healthcare. Flooding disrupts infrastructure, causing problems for the standard systems of care, including mental health care that could assist and mitigate the psychological outcome for victims.

Other susceptibility factors for mental illness related to these events include: women, the young or elderly age, having a disability, being part of an ethnic or linguistic minority, living in a household with a female head and having lower level of schooling. Among residents and relief workers, having limited resources or living in lower income countries are additional risk factors (77).

The disaster can only exacerbate any existing barriers in the access of mental health care in the population (78). The focus of many articles is on PTSD occurring immediately after the disaster, when vulnerable people are more at risk and fragile (61). This part of the population can develop mental health problems and mental disorders in the short-, medium- or long-term (78). In addition, even families not residing nearby the affected area shown high levels of post-traumatic stress, due to the fact that they still bear the charge of the disruption of community cohesion (76).

People who are affected by flooding could show remarkable resilience, however they still need organizations to support them, in order to recognize and cope with the distress, while also providing assistance to avoid any possible additional mental health problem or disorder arising from the situation.

Psychosocial resilience, due to its personal and collective components, makes it fundamental to mitigate the distress of flooding together with the social and health risks caused by the event. Restoring social cohesion of communities and families immediately after calamities is crucial in order to reduce suffering and promote effective recovery (73). Community resilience also has a preventive effect, as it prepares the population for future events while also helping them to cope with the current situation (76).


Tornadoes, hurricanes, and storms have all increased in intensity, frequency, and duration over recent decades. Data on tornadoes and mental health issues came from the latest kind of this disasters, such as hurricane Katrina in Florida and Louisiana in 2005 and Sandy in 2012 in Cuba, Jamaica, and Haiti (79–81).

There is still uncertainty regarding how much humanity is at fault for this increase (82). Based on information provided by the United Nations Development Program, nations like the United States, Japan, Australia, and New Zealand and other twenty-nine developing nations have been greatly exposed to hurricanes, cyclones, and typhoons (83). The damage suffered by health care infrastructure and the interruption of public health service due to hurricanes leads to an increase in serious illness, injuries, disability, and death (84).

As in extreme events, there are health issues that emerged or worsened after hurricanes due to psychological stress: increase in rates of cardiovascular diseases (85), prenatal maternal stress and depression, infants more likely to experience anxiety, fear and sadness, and less responsive to pleasant stimuli (86), lack of insurance possibly increasing chronic illnesses with no access to medical care during a disaster (87, 88), population exposures to contaminated floodwaters (88).

Many people experienced PTSD, stress, depression, anxiety (87), and suicide (79). Consequences of material damages lead to substance abuse (45, 89). The incidence of PTSD, that has been studied most extensively, was consistently associated with several factors. Severity of exposure and previous mental health problems has shown to be stable predictors of development of distress (84).

Other vulnerability factors are represented by: age, women, low education level, low socioeconomic status, being unemployed or disabled before hurricane disaster, and being single (54). People living in an affected area showed high levels of suicide and suicidal ideation, one in six people developed PTSD, and half of them developed an anxiety or mood disorder, including depression (45).

Additional consequences are the loss of social support, job insecurity, and loss of belongings, as well as disruption of medical health system, displacement, and relocation are related to the onset of psychological distress. Mental health disorders are often seen even one year after the disaster or event (90). A strategy for coping immediately after a hurricane is a successful evacuation of vulnerable areas by reducing the number of victims.

Displacement to shelters often results in separation from social support networks and creates a disruption in normal psychological processes, particularly familiarity, attachment and identity, and decrease in perceived social support in the months following the hurricane, which in turn has been shown to be associated with increased symptoms of general psychological distress. Being moved from one shelter to another is traumatic, compounded by the limited amount of healthcare services (91).


Historically, a natural drought lasts about a decade. Due to climate change, there will be droughts lasting around three decades, also known as “megadroughts”. From a current historical frequency of 12%, these events may increase up to 60% (92) due to possible changes in future anthropogenic greenhouse gas emissions and atmospheric concentrations measured in CO2-equivalents (93). A combination of high temperatures and low precipitation increases the frequency of drought over the world (94). A temperature fluctuation is correlated with agricultural loss by affecting crop productivity and yields.

This loss is linked to a decrease on economic growth leading to a long-term economic disadvantage and promoting political instability and conflicts (95). Farmers all over the world are more vulnerable to environment-induced mental health risks carried by drought. Long-term droughts and erratic rain fall have been associated with deterioration of economic conditions, reduced social functioning, and psychological distance to perception of negative climatic conditions (96).

The regulation and adjustment of emotion is disrupted by depression, demoralization, fatalism, passively resigning to fate, especially in women and adolescents or people with lower socioeconomic status, showing feelings of distress and helplessness (52, 53). Drought has been often connected to suicide (97–101), especially in older people (102).

In semi-arid and peri-urban areas, adaptive capacity is necessary to cope with an increased temperature and a reduced rainfall. In this case, vulnerability is seen as the degree to which people are susceptible by events that disrupt their lives—events that are beyond their control.

Resilient systems cope with extreme events in order to create a response that maintains essential function, identity, and structure, and the capacity for adaptation. Consequently, local communities strongly perceive the impact of climate change. Negative events stimulate feelings of alertness, constant monitoring of current and future events, mental distress, anxiety, depression, and suicide (103), as well as prolonged emotional stress, inevitably provoking high job insecurity (51) and other psychological issues (104).

Drought and migration are related through an assessment of crop yields (105). The landscape changes with periodical drought impact for example on the Turkana’s ability to gain access food, water, health, and security. These indigenous people are farmers that have grown dramatically in number in the last decades with a majority living below the poverty line.

Prolonged and more frequent droughts and unpredictable rainy seasons have exacerbated the difficult access to potable water or food. Consequently, changes in water availability, temperature or other environmental variables can have a truly devastating impact on their daily life. Many extreme weather events and famine lead to displacement of entire communities and forced migration, within and outside national borders, with onset on conflicts over natural resources (106).

In northern Ghana’s savanna (an arid zone with severe droughts), climate change exposes farmers to adverse climatic conditions that include low rainfall, forest fires, soil erosion, loss of soil fertility, poor harvest, and destruction of property and livestock. In this region, farmers with small plots are at a higher risk of suffering acute, sub-acute, and long-term problems caused by extreme climate change (51).


The term “wildfire” refers to large-scale fires, generally occurring in forests and jungles. These phenomena have involved Siberia, Central Africa, and the Amazon in the present times. The areas affected by the wildfire may be sparsely populated or nearby the city boundaries. The greatest concerns are those related to the climate. The effects on the ecosystem are devastating, as a forest’s carbon dioxide storage capacity is forever lost (107).

Once burnt, forest tend to become savannah, scarcely covered by deciduous trees or cultivation. Vast decline of forests will have reverberations for the world’s climate, as anticipated by many government administrations and institutions all over the world. A “business as usual” scenario will lead to a rise in temperature of about 4 degrees Celsius by 2100 and plants will have to find new strategies to survive.

In the past, a more local phenomenon has been described as bush-fire. These involved urban areas in proximity with bushlands or forests and could affect residential neighborhoods, suburbs or slums in different ways, albeit devastating for farmers and people in the area. Firestorms generated by bush-fires led to destruction and evacuation of residents. Wildfires have a heterogeneous impact on property damage, physical injuries, and mental health (108).

During the years following the disaster, an increase of mental health issues was observed, such as general mental health problems, post traumatic disorders (109), psychosomatic illness, and alcohol abuse (110). Effects can be delayed in onset and can persist over at least several years.

Proximal populations, not directly affected by the bush-fires, can also be involved (111). Studies performed in areas hit by Australian bush-fires observed that a year after the events 42% of the population exposed was classified as potential psychiatric cases (109). Californian wildfires also offer a rather dramatic picture, with 33% showing symptoms of major depression and 24% showing symptoms of PTSD. A similar effect was observed in Greek wildfires (112), with an increased somatization symptoms, depression, anxiety, hostility, and paranoia (113).

Post-disaster mental health issues observed were PTSD, physiological hyperarousal, chronic dissociation, sadness and depression, detachment (114) disorganized thinking and behavior, numbing or avoidance, poor concentration, and behavioral problems.

In the youth population, connections have been found between the young age and the experience of personal life threat (115). Children were also affected by bush-fires, showing posttraumatic phenomena such as anxiety disorders and panic attacks, problems sleeping, acute stress disorder, compulsively repetitive play, flashbacks, and psychotic disorders (116).

Wildfires could become quite frequent with climate change. Addressing such phenomena on a global scale will prove challenging. It is also difficult to predict how populations will react once the anthropogenic role of this type of event takes hold.

reference link :

More information: Fernando Florido Ngu et al, Correlating heatwaves and relative humidity with suicide (fatal intentional self-harm), Scientific Reports (2021). DOI: 10.1038/s41598-021-01448-3


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