Month of birth and outdoor temperature after birth predict childhood atopic diseases in Finland.

BACKGROUND

Previous evidence on season-of-birth differences in atopic diseases is partially mixed, and the etiology behind them is not well understood. For example, outdoor temperature may be an important modifier of the association but has been previously neglected.

METHODS

We assess how the month of birth is associated with medication use for atopic diseases at ages 0-15 and how outdoor temperatures after birth modify these associations for 0.55 million Finns born during 1995-2004. We used Finnish register data on purchases of medications used for allergic rhinitis, eczema, asthma, and food allergies. We predicted month-of-birth associations with medication use using extensive controls for observed and unobserved confounders and assessed effect modification by 3-month mean outdoor temperature after birth.

RESULTS

Approximately half of the children had at least one purchase of medication used for atopic diseases. For children born in spring or summer, the probability of medication use was moderately lower than for children born in the autumn or winter. Among children born in the autumn or winter, exposure to the coldest outdoor temperatures in the first 3 months of life was associated with a nearly 10-percentage-point increase in the risk for medication use compared with the warmest temperatures.

CONCLUSIONS

Behind the moderate overall associations between month of birth and childhood atopic diseases, there was notable variation by environmental conditions after birth, with cold weather after birth being particularly harmful. Future studies should assess what specific exposures do the outdoor temperatures affect, and in turn how they affect the development of atopic diseases.