Climate change has emerged as a significant issue of concern for the entire global community (Shukla, 2013). Indeed, the greatest challenges facing our global leaders, which have substantial implications for global public health, are to retain the global temperature from rising by 1.5˚C and to restore biodiversity loss (Atwoli et al., 2021).

Those who are most vulnerable (e.g., children, older adults, deprived communities, and those with pre-existing health conditions) are likely to be worse impacted by the climate crisis and extreme weather events (Atwoli et al., 2021). Moreover, those with poorer mental health are often most affected (Shukla, 2013), with existing mental health service provision under-resourced to respond to increased needs (GBD 2019 Mental Disorders Collaborators, 2022). Given mental health conditions are one of the top ten leading causes of disease burden globally (GBD 2019 Mental Disorders Collaborators, 2022), reducing the prevalence of mental ill-health is a significant public health priority (Chisholm et al., 2016; Trautman et al., 2016).

Exposure to extreme weather-related events (i.e., flooding, extreme cold or heat, air pollution) can lead to short and long-term adverse mental health outcomes through direct (e.g., post-traumatic stress) and indirect pathways (e.g., increases in alcohol consumption and drug abuse, and anxiety associated with climate change concerns) (Cianconi et al., 2020).

While the impact of days of extreme heat on physical health outcomes has been well documented, there is less good evidence about the impact of periods of extreme heat on mental health. Therefore, Nori-Sarma et al (2022) in the linked paper set out to explore the relationship between warm-season temperatures and rates of emergency department (ED) visits for a diverse range of mental health outcomes among adults living in the United States (US).

Methods

This study used a case-crossover design where the study subjects are selected from cases who have experienced an event. Each case acts as his/her own control making it possible to study the effects of intermittent exposure on the outcome. In this study, the exposure was extreme heat during warm season months (May to September) and the outcome was medical claims for emergency department (ED) visits with a mental health diagnosis. The study addressed a clearly focused issue, and the use of case-crossover design was an appropriate way of exploring this topic.

The sample size for this study was large and included adults (18 years or older) living in 2,775 US counties. Data on age, sex, county of residence, admission data and principal diagnosis code (based on ICD-9 until 2015, or ICD-10 after 2015) were extracted from an established and reliable system (i.e., OptumLabs Data Warehouse database).

The cases were defined precisely where the authors applied the Agency for Healthcare Research and Quality’s Clinical Classifications Software scheme to ICD-9 and ICD-10 principal diagnosis codes at discharge.

The exposure was clearly defined and accurately assessed ambient temperature using a validated spatiotemporal model. This model was used to calculate a population weighted average of daily maximum temperature for each day in each county. Extreme heat was defined as ambient temperature at the 95th percentile of the county-specific temperature distribution.

Conditional logistic regression models were used for the statistical analysis to determine the incidence rate ratio (IRR) and 95% Confidence Intervals (CI) for the relationship between daily temperature and incidence rates of ED visits. An IRR is the incidence rate in the exposed group divided by the incidence rate in the unexposed group and is interpreted as an odds ratio.

Results

Data on 3,496,762 emergency department (ED) visits among adults with health insurance residing in the US (n = 2 243 395; 56.8% female and 43.2% male; mean [SD] age 51.0 [18.8] years) were identified. Days of extreme heat were associated with an IRR of 1.08 (95% CI, 1.07 to 1.09) for ED visits for any mental health condition compared with days of optimal temperature (i.e., temperature percentile with the minimum ED visits across each county).

ED visits for substance use disorders were the most common, followed by ED visits for anxiety, stress-related, and somatoform disorders; mood disorders; schizophrenia, schizotypal, and delusional disorders; self-harm; and childhood-onset behavioural disorders.

The researchers reported no heterogeneity across age groups, however, there were increased rates of ED visits for mental health amongst males (IRR, 1.10; 95% CI, 1.08 to 1.12) compared with females (IRR, 1.06; 95% CI, 1.05 to 1.08), and IRRs were higher in the Northeast, Midwest, and Northwest regions of the US.

Conclusions

The authors found that extreme heat was associated with higher rates of emergency department (ED) visits for any mental health condition and for specific mental health conditions, including anxiety, stress-related, and somatoform disorders and mood disorders. As both the frequency and severity of extreme weather events may increase over the long-term due to the impacts of climate change, the findings have implications for the preparedness of mental health services in their response to the climate challenge.

Strengths and limitations

The sample included in this study was a large population of adults living across the US and therefore expands the evidence-base beyond local and regional findings. While studies in other contexts with different climates (e.g., Western Europe) are limited, these findings are nonetheless consistent with research that found that increasing ambient temperatures were associated with increased mental health hospitalisations in Switzerland (Bundo et al., 2021).

The analysis used routine data to link and map relationships between environmental changes and health events using a case-crossover design. By using emergency department (ED) visits as a clinical marker for mental health problems and the degree of stress on health systems, the authors were able to assess the highest cost impacts for both individuals and health systems.

Key limitations included that data on ethnicity, race, SES, and access to air conditioning was not available for inclusion in the analysis. Additionally, only ED medical claims data for those with health insurance were available, and this population might be healthier and more economically able to cope with the adverse impact of extreme heat than the general population. Exploring the intersection between individual-level characteristics and markers and ED visits for mental health during periods of extreme heat would expand our understanding of the impacts of climate change and extreme weather events on health inequalities.

Implications for practice

This study highlights the inter-relationship between planetary health and human health and expands the evidence-base by exploring how those with mental health difficulties are most impacted by extreme heat. This has implications for policy and practice where preventative and responsive measures can be implemented to support personal and public mental health outcomes.

The authors of this study concluded that their findings:

…could aid clinicians who provide mental health services in preparing for increases in health service needs when high ambient temperature is anticipated.

This will require significant investment and scaling up of mental health provision where there are existing shortages in access to services and resource deficiencies in responding to mental health needs globally (GBD 2019 Mental Disorders Collaborators, 2022). In addition, the provision of adaptive coping strategies by mental health professionals and age-appropriate information in educational settings may support the general population in coping with the stresses associated with climate change (Ma et al., 2022; Mah et al., 2020).

As well as improving the preparedness of health services to cope with the adverse health impact of extreme heat, there is also need for more proactive and coordinated policy interventions by governments and the global health community to mitigate the impacts of climate change. For example, nature-based solutions (e.g., green roofs, urban greening) in urban areas can mitigate the effects of rising urban temperatures (e.g., McConnell et al., 2022). Such investments should be viewed as critical preventative policy and infrastructure interventions to offset the health risks associated with extreme weather events such as prolonged heatwaves.

Statement of interests

Trish Darcy is a Research Associate with the Department of Health Sciences, University of York. Peter Coventry is part-funded by the NIHR Yorkshire and Humber ARC. The views expressed are those of the authors, and not necessarily those of the NIHR or the Department of Health and Social Care.

Links

Primary paper

Nori-Sarma A, Sun S, Sun Y, Spangler KR, Oblath R, Galea S, Gradus JL, Wellenius GA (2022) Association between ambient heat and risk of emergency department visits for mental health among US adults, 2010 to 2019. JAMA Psychiatry. 2022 Feb 23.

Other references

Atwoli L, Baqui AH, Benfield T, Bosurgi R, Godlee F, Hancocks S, Horton R, Laybourn-Langton L, Monteiro CA, Norman I, Patrick K (2021) Call for emergency action to limit global temperature increases, restore biodiversity, and protect health: Wealthy nations must do much more, much faster. Nutrition Reviews. 2021 Nov;79(11):1183-5.

Bundo M, de Schrijver E, Federspiel A, Toreti A, Xoplaki E, Luterbacher J, Franco OH, Müller T, Vicedo-Cabrera AM (2021) Ambient temperature and mental health hospitalizations in Bern, Switzerland: A 45-year time-series study. PloS one. 2021 Oct 12;16(10): e0258302.

Chisholm D, Sweeny K, Sheehan P, Rasmussen B, Smit F, Cuijpers P, Saxena S (2016) Scaling-up treatment of depression and anxiety: a global return on investment analysis. The Lancet Psychiatry. 2016 May 1;3(5):415-24.

Cianconi P, Betrò S, Janiri L (2020) The impact of climate change on mental health: a systematic descriptive review. Frontiers in psychiatry. 2020:74.

GBD 2019 Mental Disorders Collaborators (2022) Global, regional, and national burden of 12 mental disorders in 204 countries and territories, 1990–2019: a systematic analysis for the Global Burden of Disease Study 2019. The Lancet Psychiatry. 2022 Jan 10.

Mah AY, Chapman DA, Markowitz EM, Lickel B (2020) Coping with climate change: Three insights for research, intervention, and communication to promote adaptive coping to climate change. Journal of anxiety disorders. 2020 Oct 1; 75:102282.

Ma T, Moore J, Cleary A (2022) Climate change impacts on the mental health and wellbeing of young people: A scoping review of risk and protective factors. Social Science & Medicine. 2022 Mar 18:114888.

McConnell K, Braneon CV, Glenn E, Stamler N, Mallen E, Johnson DP, Pandya R, Abramowitz J, Fernandez G, Rosenzweig C. (2022) A quasi-experimental approach for evaluating the heat mitigation effects of green roofs in Chicago, Illinois. Sustainable Cities and Society. 2022 Jan 1; 76:103376.

Shukla J (2013) Extreme weather events and mental health: Tackling the psychosocial challenge. International Scholarly Research Notices. 2013;2013.

Trautmann S, Rehm J, Wittchen HU (2016) The economic costs of mental disorders: Do our societies react appropriately to the burden of mental disorders?. EMBO reports. 2016 Sep;17(9):1245-9.

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