- This study is one of the first to consider relative humidity and suicide, rather than just heat.
- Country-level results vary, so heat–humidity does not always increase suicide rates.
- Many challenges exist, such as proper terminology and including all important factors.
Extensive concerns exist about human-caused climate change impacting mental health and well-being. Today, we published new research in the journal Scientific Reports examining heat–humidity and self-harm rates, one of the first studies to account for relative humidity rather than heat alone.
Our results show the importance of considering humidity as the world warms, rather than just air temperature. In fact, increasing relative humidity correlated better with reported suicide rates than heatwaves alone. Women are affected more than men, and people aged 5 to 14 years and 15 to 24 years are the most affected out of all age groups.
We used as much country-level data as we could find for heatwaves, relative humidity, population, and recorded incidents of suicide or of fatal intentional self-harm. The best combined data set overall covered 60 countries from 1979 to 2016, all via publicly available data from organizations such as the World Health Organization, the European Centre for Medium-Range Weather Forecasts, and the United Kingdom’s Met Office Hadley Centre.
Suicide vs. Fatal Intentional Self-Harm
Difficulties always emerge in defining, understanding, and analyzing such numbers, as well as in linking specific calculations to wider contexts. One major challenge here is identifying and differentiating between suicide and fatal intentional self-harm. Suicide is highly stigmatized and is not always admitted to be reported and recorded.
Meanwhile, the International Classification of Diseases has been moving toward “intentional self-harm” as a category, which can then be divided into “fatal” and “nonfatal.” Intentionality, though, is not easy to tabulate consistently, because survivors might not admit it, and people who kill themselves might not always indicate, or even have been clear about, their intention. Then, connections between self-harm and mental health and well-being are not always clear or straightforward.
Debates continue within clinical and theoretical psychology and psychiatry, disease classifiers, general medicine and health practitioners, and global mental health specialists, among others. We did not, and could not, set out to provide insight into these disagreements or discussions, instead deliberately choosing to focus on the numbers as reported in the public data.
Differences Between Men and Women
The calculated differences between men and women might be as much cultural as physiological. In many places, men are reluctant to seek mental health care, reducing their diagnosis rate. Women’s diagnosis rates can also be undercalculated when they are deemed to be hysterical or irrational rather than given needed mental health support. Other differences in rates exist because temperature responses of men's and women’s bodies differ.
In addition, no standard definition is accepted for “heatwave,” while humidity has numerous measurements. Again, we had no scope for resolving debates among meteorologists, climatologists, statisticians, and many others. Our heatwave definition is “four or more days of temperature that exceed the 99th percentile of temperature” within our data set. Relative humidity is directly measured as the amount of water vapor in the air compared to the maximum possible for the air’s temperature.
Major confounders beyond definitions interfere with connecting the data sets. Heat–humidity combinations might not affect people immediately, instead producing a time lag between experiencing environmental conditions and implementing self-harm. Cumulative effects are hard to determine, such as trying self-harm after several successive heatwaves within a timeframe, rather than a single experience.
And an individual can only die once. They might have intended to attempt suicide, but beforehand died from heat–humidity during the heatwave.
These difficulties require detailed further investigation, particularly as the results are mixed. Some countries show increasing rates of suicide with worse heat–humidity, some show decreasing rates, and some show no significant correlation. We could not discern definitive patterns in the countries falling into each category.
These results confirm that factors beyond simple increases in heat–humidity impact suicide and fatal intentional self-harm, so climate change should never be considered in isolation. A major concern in climate change and health research has long been assuming that climate change is the dominant impact on health, an approach related to “environmental determinism.” Presuming that a specific environmental change must lead to a specific health outcome has significant problems, affirmed by our work.
Certainly, killing oneself and various modes of self-harm occur due to many, complicated, underlying factors. Attribution to a single issue — or even a single class of determinants such as neurological biochemistry, clinical diagnoses, or social circumstances — is unlikely to withstand scrutiny.
Role of Health Systems
Health systems have a huge role to play in prevention and interventions. Where training for mental health and well-being is poor, where self-harm and suicide attempts lead to ostracization, or where health resources are inadequate, fewer people are likely to receive help. These situations affect both averting problems and reporting incidences.
Finally, increasing evidence indicates that doom and gloom about climate change — narratives of hopelessness and unavoidable destruction — are negatively affecting mental health and well-being. Terms such as solastalgia, eco-anxiety, eco-grief, climate anxiety, and climate grief might implicate doomsday storylines about climate change more than climate change’s actual impacts.
We hope that our study provides a small step forward in science with a giant impetus toward refining and improving the data and analyses. We need realistic understandings of mental health and well-being, especially preventing self-harm, under all circumstances, not just with respect to those linked to climate change.
Florido Ngu, F., Kelman, I., Chambers, J., Ayeb-Karlsson S. 2021. “Correlating heatwaves and relative humidity with suicide (fatal intentional self-harm).” Scientific Reports, 11: 22175.
Kelman, I., Ayeb-Karlsson, S., Rose-Clarke, K., Prost, A., Ronneberg, E., Wheeler, N., Watts, N.. (2021). “A review of mental health and wellbeing under climate change in small island developing states (SIDS).” Environmental Research Letters, vol. 16, article 033007.
Romanello, M. et al. (2021). “The 2021 Report of The Lancet Countdown on Health and Climate Change.” The Lancet, forthcoming.
Voyatzis-Bouillard, D. and I. Kelman. “Do Climate Change Interventions Impact the Determinants of Health for Pacific Island Peoples?” The Contemporary Pacific, vol. 33, no. 2, forthcoming.