Short-Term Effect of Air Pollutants on Sudden Unexpected Death in Infancy: A Case-Crossover Study Based on the French National Registry.

BACKGROUND: Sudden unexpected death in infancy (SUDI) remains poorly understood, and the potential contribution of air pollution exposure is largely unexplored. Given its known impact on infant health, air pollution may influence the risk of SUDI, particularly through short-term effects. OBJECTIVES: We aimed to evaluate the short-term effect of exposure to ambient air pollutants on SUDI in France. METHODS: Using data from the French national SUDI registry (2015-2022) and modelled air pollution data, we assigned daily concentrations of particulate matter (PM10 and PM2.5, μg/m3), nitrogen dioxide (NO2, μg/m3) and ozone (O3, μg/m3) at each infant's residence. We conducted a time-stratified case-crossover analysis comparing single- and multi-pollutant exposure on the day of death with matched control days, considering lagged effects up to 6 days. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using conditional logistic regression, adjusted for public holiday, influenza-like syndrome and daily mean temperature. We assessed the potential effect of several effect modifiers, such as biological sex, age at death, season, urban or rural residence, social deprivation and sleeping position at discovery. RESULTS: A total of 1078 SUDI cases were included. Exposure to PM was consistently associated with higher odds of SUDI, with a lag effect of 3 to 5 days before death and during the last week of life. NO2 exposure presented a trend of association. Higher effects were observed for PM and NO2 with cumulative exposure, especially for PM and consistent with the multi-pollutant model. We observed seasonal heterogeneity, with higher ORs in the spring for PM and O3. CONCLUSIONS: Our findings support a plausible short-term trigger effect of air pollution on SUDI. These results highlight the need to integrate environmental determinants, particularly for NO2 and PM, into SUDI prevention strategies.