Measurement

The questionnaire used in this study was the Japanese version of the American Thoracic Society and Division of Lung Diseases, National Heart, Lung, and Blood Institute questionnaire, or ATS-DLD questionnaire, for children. The questionnaire has been adopted by the former Japanese Ministry of the Environment, or the Japan Environment Agency. We also added supplementary questions on parental history of asthma and feeding patterns from the age of zero to three months.