Impact of Spatial Aggregation Level on Environmental Epidemiology Analyses: A Case Study of Combined Heat and Ozone Effects on Cardiovascular Emergencies

Background: Spatial granularity plays a central role in the analysis of environmental hazards, yet its influence on health impact assessment remains overlooked. This study explicitly treats spatial aggregation level as a methodological variable and examines how different spatial aggregation strategies affect the association between high temperature, ozone, and out-of-hospital cardiovascular emergencies recorded by emergency medical services. Methods: A distribution thresholding approach is applied to both the environmental hazard and the health outcome. The analysis is conducted at three spatial levels: a fully aggregated region-wide level, population-based districts, and a combined strategy that cumulates district level results. The model estimates the Odds Ratio for each configuration. Results: The combined district-based strategy provides the most robust association, with an Odds Ratio of 1.13 (95% confidence interval 1.10 to 1.17). The region-wide and single district approaches show weaker or inconsistent significance. The findings indicate that the spatial level of analysis heavily impacts both the significance and the interpretability of the statistical results. Conclusions: The study demonstrates that the spatial structure of data strongly influences the detection of short-term health effects linked to environmental stressors. This contributes to the geomatics field by explicitly isolating spatial aggregation as an analytical dimension, demonstrating how spatial aggregation choices and explicit consideration of the Modifiable Areal Unit Problem can enhance methodological accuracy, support clearer spatial reasoning, and guide the development of more reliable territorial health indicators.