Ambient Air Pollution and Asthma-Coded Hospital Admission Rates in Children Aged 0–3 Years in Spain: A Municipality-Year Urban–Rural Ecological Study

Background/Objectives: Asthma-coded hospital admissions in very young children represent a clinically relevant but diagnostically complex marker of severe wheezing/asthma-like morbidity. Methods: We conducted a nationwide ecological municipality-year study of children aged 0–3 years in Spain during 2020–2022 using hospital discharge records from the Spanish Minimum Basic Data Set (MBDS/RAE-CMBD). Exposure and outcome were analyzed at the municipality-year level: the exposure metric was the contemporaneous annual mean municipality-level concentration of PM2.5, PM10, NO2, SO2, O3, and CO, and the outcome was the annual number of asthma-coded admissions, with the population aged 0–3 years as offset. Results: The regression panel comprised 20,746 municipality-year observations, 7251 municipalities, and 3231 admissions; 93.7% of municipality-years had zero admissions. In unipollutant Poisson models adjusted for calendar year, rural–urban status, temperature, and relative humidity, incidence rate ratios per interquartile-range increase were 1.54 (95% CI 1.28–1.85) for PM2.5 and 1.89 (95% CI 1.59–2.26) for PM10. Negative-binomial and province-fixed-effect sensitivity models attenuated but did not remove the particulate-matter signal. The multipollutant model was unstable because of strong collinearity between particulate fractions and was interpreted only as an exploratory sensitivity analysis. Conclusions: These findings should be interpreted as ecological, population-level associations between annual ambient particulate pollution and pediatric respiratory hospital burden, not as individual-level causal risk estimates.