The study regions differ in the annual number of days with snow cover on the ground, and the snow cover reduces the microbial diversity transferred indoors. A: Subjects included in the study were born in university hospitals in three cities in Finland (Oulu, Tampere, and Turku). B: Annual number of days with snow cover on the ground in the study regions during the study period (1994–2014). Statistical differences were analyzed using the pairwise Wilcoxon rank sum test with Bonferroni correction for multiple comparisons. C: Comparison of α diversity measures between doormat samples collected after at least 1 month of snow cover on the ground (February) and a snowless period (August) from the dwelling of 22 DIPP families. The statistical difference for several diversity measures was analyzed using the paired Student t test, and the P values were adjusted with Bonferroni correction for multiple comparisons. Adjusted P values (adj. P) are indicated in the graphs. Box-and-whisker plots show a horizontal line indicating median value, a box representing the interquartile range, and whiskers showing the 95% confidence interval. D: Demographic data of the study sites. NA, not available; T1D, type 1 diabetes.