Impact of 2018 Japan floods on allergic rhinitis prescriptions

Climate change and natural disasters can impact allergic conditions. The 2018 Japan floods, which occurred in July 2018, were among the largest water-related disasters in Japan's history. We aimed to investigate the impact of flooding on prescription rates for allergic rhinitis.

This retrospective cohort study utilized data from the National Database of Health Insurance Claims from 1 year before and after the flood in the most severely affected region. Individuals with a victim code certified by local authorities were categorized into the victim group, whereas the others were classified into the non-victim group. A difference-in-differences analysis with a logistic regression model was employed to evaluate the impact of the disaster on prescription rates of corticosteroids or antihistamine nasal sprays. Cedar and cypress pollen (the major allergens causing seasonal rhinitis during spring in Japan) counts were measured using a rotary-type pollen collector.

Among 6,176,299 individuals included, 36,076 were identified as flood victims. An upward trend in prescriptions was observed during the cedar and cypress pollen season from February to April, and the pattern of higher prescriptions among disaster victims relative to non-victims continued throughout both the pollen and non-pollen seasons after the disaster. The difference-in-differences analysis indicated a significant increase in nasal spray prescription in disaster victims compared with non-victims, with adjusted odds ratios of 1.40 (95% confidence interval: 1.24-1.58) 3 months after the disaster and 1.72 (95% confidence interval: 1.56-1.95) 1 year after. Subgroup analyses showed that the prescription rates of nasal spray significantly increased across all age groups and in both males and females among disaster victims compared to non-victims.

This study demonstrated a long-term increase in prescriptions for allergic rhinitis among flood victims, underscoring the need to recognize natural disasters as potential contributors to the incidence of allergic rhinitis.