Health impacts of extreme heat in medically at-risk populations: a space-time stratified case-crossover analysis in Belgium.

BACKGROUND: The impact of heat on a vulnerable population, particularly those with pre-existing chronic diseases, is a growing public health concern. However, the risks for those with specific conditions and their variation across geographic, demographic, and socioeconomic (SES) status remain underexplored. This study aimed to investigate the association of extreme heat with morbidity and mortality among individuals with pre-existing chronic diseases in Flanders, Belgium. METHODOLOGY: We analysed 14 years (2005-2019) of general practitioners (GPs) data from Flanders, northern Belgium, assessing both morbidity and mortality. Morbidity was defined as GP-recorded general and heat-related illnesses among individuals with pre-existing chronic diseases, while mortality included overall mortality and mortality among people with pre-existing chronic diseases. A space-time-stratified case-crossover design was employed, with a distributed lag non-linear model (DLNM) applied in quasi-Poisson regression. Various subgroup analyses were conducted to identify the most at-risk population. We quantified the relative risk (RR) at the 99th percentile of the daily minimum temperature relative to the minimum morbidity/mortality temperature (MMT). RESULTS: We found a strong association of heat with morbidity and mortality. A substantially increased risk of morbidity was observed among individuals with pre-existing heart failure (RR = 2.79 [95% CI: 1.84-4.24]) and a high risk of mortality was found among those with pre-existing hypertension (RR = 2.01 [95% CI: 1.23-3.30]). We also observed a rise in heat-related morbidity risks for individuals with pre-existing Chronic Obstructive Pulmonary Disease (COPD) (RR = 2.09 [95% CI: 1.53-2.85]), hypertension (RR = 1.37 [95% CI: 1.08-1.74), chronic kidney disease (CKD) (RR = 1.74 [95% CI: 1.25-2.42]), and chronic mental health disorders (RR = 1.41 [95% CI: 1.06-1.89]). There was an increased risk of overall mortality (RR = 1.29 [95% CI: 1.02-1.62]) and mortality with pre-existing COPD (RR = 1.80 [95% CI: 1.19-2.73]). Urban populations and low-SES groups had increased heat-related risks for some health outcomes, and the highest vulnerability was observed in those aged 85+ for several chronic diseases. CONCLUSION: Our findings indicated that chronic diseases are associated with higher vulnerability to adverse health effects due to extreme heat. We observed notable variations across geographic, demographic, and socioeconomic subgroups, emphasizing the need for targeted public health strategies.