Extreme temperature and infant deaths in rural Spain, 1920-1950: a case-crossover analysis

Infant mortality in southern Europe historically peaked in both winter and summer, a pattern commonly linked to temperature extremes and seasonal changes in childcare and disease environments. We use individual-level death registers from 15 rural villages in Zaragoza, Spain, from 1920 to 1950, linked to daily temperature records from nearby meteorological stations, to examine short-term associations between extreme heat, extreme cold, and deaths in the first year of life. We apply a time-stratified case-crossover design restricted to infant deaths, comparing temperature exposure on each death day with matched control days from the same month, year, and day of the week. This design controls by construction for seasonality, long-term trends, day-of-week patterns, and time-invariant individual, familial, and village-level characteristics. Extreme heat is associated with a higher short-term incidence rate of infant death, with the strongest associations observed on the day of death and in the immediately preceding days. Associations with extreme cold are smaller and less consistent. Because complete population denominators are unavailable, the design does not estimate absolute infant mortality rates. It does, however, estimate relative changes in the incidence rate of infant death associated with short-term exposure to extreme temperatures.