| Summary: | The objective of this study was to identify the risk factors for sensitization to Japanese cedar pollen (JCP) and house dust mite (HDM). The purpose was to clarify how children become sensitized to JCP and HDM and to evaluate the risk factors for sensitization to those antigens. A cohort of 267 children aged 6–11 years was surveyed using a self-administered questionnaire for symptoms and environmental factors such as passive smoking and the condition of the subject's home in 1994. A serum examination of JCP- and HDM-immunoglobulin E was also conducted every year from 1994 to 1997. A total of 35% of the children were positive to JCP in the first survey and a further 19% were positive in at least one of the subsequent three annual surveys. Twenty-seven percent of the children were sensitized to HDM in the first survey and a further 12% were sensitized in at least one of the subsequent three annual surveys. The incidence of JCP sensitization in any one year was proportional to the pollen count. No other environmental factors could be associated with the prevalence of either sensitization or the incidence of JCP sensitization, but HDM sensitization was more common in less well-ventilated houses (relative risk (RR)=3.42, 95% confidence interval (CI)=1.03-11.38) and among those who use kerosene stoves (RR=3.42, 95% CI=1.26-9.23). The susceptibility to an antigen varies continuously and cannot be divided in a dichotomous manner. The children who had already been sensitized when they were enrolled in this study were of such strong susceptibility to the antigen that they could be sensitized by the minimum exposure to the antigen regardless of the environmental factors. Those with weak susceptibility could be sensitized by an extraordinarily high exposure to the antigen or by environmental risk factors during school age.
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