Comparison of pollen indoor/outdoor ratios during off-peak and peak periods in Japan

Outdoor pollen particles can invade indoor spaces through various pathways, causing hay fever. This study aimed to investigate the effect of pollen diameter and shape on indoor invasion ratio (indoor/outdoor [I/O] ratio) during off-peak and peak pollen periods as well as the pollen types that easily invade indoor spaces. The measurement site was located in an apartment in Tokyo, Japan. Daily pollen sampling was performed from November 2021 to May 2022. The I/O ratios were analyzed for six pollen diameter ranges (0–2.5 μm, 2.5−10 μm, 10−20 μm, 20−30 μm, 30−40 μm, and >40 μm), four pollen shapes (monomer, ruptured, agglomerate, and unknown pollen), and 14 pollen types. The results showed that the total I/O ratio per day during the peak pollen period (0−2.8) was larger than during the off-peak period (0−1.2). The highest I/O ratio values (mean: 0.68, median: 0.47) were observed for pollen >40 μm in diameter during the off-peak period. Regarding their shape, the highest I/O ratio values (mean: 0.70, median: 0.28) were observed for ruptured pollen during the peak period. The most abundant pollen types were Corynespora (off-peak period) and Japanese cedar (peak period) indoors.