Association of the incidence of atopic dermatitis until 3 years old with birth month and with sunshine duration and humidity in the first 6 months of life: Japan Environment and Children’s Study

OBJECTIVE: To compare the incidence of atopic dermatitis in children aged from 6 months to 3 years across birth seasons and climate conditions. DESIGN: Cohort study. SETTING: Fifteen regional centres across Japan. PARTICIPANTS: A total of 100 304 children born from 2011 to 2014. EXPOSURE: Birth month, and mean sunshine duration (short/long) and humidity (high/low) in the first 6 months of life. PRIMARY OUTCOME MEASURE: Incidence of atopic dermatitis. RESULTS: The highest incidence of atopic dermatitis was in children born in the months of October to December. The lowest incidence of atopic dermatitis was in the months of April to June and in periods with a long duration of sunshine and high humidity. Low humidity was significantly associated with a higher incidence of atopic dermatitis. However, this significant difference disappeared when the birth season and parental history of allergic disease were considered in multivariate analysis. CONCLUSIONS: In Japan, being born in the late autumn to early winter months is associated with a risk of developing atopic dermatitis until the age of 3 years. Sunshine duration and humidity from birth to 6 months of age are not associated with the incidence of atopic dermatitis.