Cold Summer Days Linked to Rising Depression Risk in Vulnerable Patients

Unusually cold summer days, not hot ones, appear to raise the risk of depression in people with a recent history of mental health problems, according to a case-crossover study conducted in Flanders, Belgium. The research, published in BMC Public Health, also found that extreme heat increased the risk of mental health events in a specific subgroup: vulnerable patients from lower socioeconomic backgrounds. The study drew on consultation records from Intego, a primary care database, covering the years 2012 to 2021.

The researchers focused on the warm half of the year, April to September, and examined four outcomes: a first registration of depression, a first registration of an anxiety disorder, a first antidepressant prescription, and a first benzodiazepine prescription. Daily maximum temperatures were adjusted for relative humidity and modelled with distributed lag non-linear models, allowing the effect of temperature to unfold over a window of up to 21 days after exposure. Associations were estimated with conditional Poisson regression, adjusted for weekday, month and year.

Moderate and extreme temperatures were defined using percentiles: the 1st and 5th percentiles for summertime cold and the 95th and 99th percentiles for summertime heat. The median daily maximum temperature in the study period was 20.6 degrees Celsius. The researchers split the population into two groups: a vulnerable group with a two-year history of mental health problems recorded in primary care, and a non-vulnerable group without such a history.

In total, they identified 34,166 events of any outcome in the vulnerable population and 133,866 events in the non-vulnerable population. In the non-vulnerable group, colder temperatures at the 1st percentile were associated with an overall increase in depression, with an incidence rate ratio of 2.42 (95% confidence interval 1.14 to 5.15). In the vulnerable group, the association with cold appeared later: colder temperatures at the 1st percentile were linked to depression at a 14-day lag, with an incidence rate ratio of 1.85 (95% confidence interval 1.06 to 3.25).

These are observational associations from a single region and do not establish that cold weather causes depression in every individual. Heat played a more selective role. Further analyses of patient subgroups revealed a positive association between extreme heat and any of the four mental health outcomes in the low socioeconomic status category of the vulnerable group.

At the 99th percentile of temperature, the incidence rate ratio for any outcome was 2.58 (95% confidence interval 1.03 to 6.47). The authors note that the role of the environment in depression and anxiety has been poorly studied in primary care settings, where most mental health problems are first diagnosed and treated. The findings suggest that both ends of the summer temperature distribution may matter for mental health, with cold posing a risk for depression across patient groups and extreme heat adding risk for socioeconomically disadvantaged patients already living with mental health problems.

Subject of Research: The association between spring and summer temperatures and depression, anxiety and psychopharmaceutical prescriptions in primary care patients in Flanders, Belgium

Article Title: The impact of temperature on anxiety and depression during spring and summer in Flanders, Belgium: a case-crossover study

Article References: Beerten, S. G., De Troeyer, K., Ali, E. A., Casas, L., Van Pottelbergh, G., & Vaes, B. (2026). The impact of temperature on anxiety and depression during spring and summer in Flanders, Belgium: a case-crossover study. BMC Public Health. https://doi.org/10.1186/s12889-026-29579-0

Image Credits: AI Generated

DOI: 10.1186/s12889-026-29579-0

Keywords: temperature, depression, anxiety, primary care, case-crossover study, Belgium, mental health, heat, cold, socioeconomic status, antidepressants, benzodiazepines