The effect of indoor exposure to nitrogen dioxide on respiratory health was studied over a period of 2 yr in a population of nonsmoking Dutch children 6 to 12 yr of age. Lung function was measured at the schools, and information on respiratory symptoms was collected from a self-administered questionnaire completed by the parents of the children. Nitrogen dioxide was measured in the homes of all children with Palmes' diffusion tubes. In addition, information on smoking and dampness in the home was collected by questionnaire. There was no relationship between exposure to nitrogen dioxide in the home and respiratory symptoms. Respiratory symptoms were found to be associated with exposure to tobacco smoke and home dampness. There was a weak, negative association between maximal midexpiratory flow (MMEF) and exposure to nitrogen dioxide. FEV1, peak expiratory flow, and MMEF were all negatively associated with exposure to tobacco smoke. Home dampness was not associated with pulmonary function. Lung function growth, measured over a period of 2 yr, was not consistently associated with any of the indoor exposure variables. The development of respiratory symptoms over time was not associated with indoor exposure to nitrogen dioxide. There was a significant association between exposure to environmental tobacco smoke in the home and the development of wheeze. There was also a significant association between home dampness and the development of cough.

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Lung growth was studied in 420 Dutch children aged 6-11 yrs. Forced vital capacity (FVC), forced expiratory volume in one second (FEV1), peak expiratory flow (PEF) and maximal mid-expiratory flow (MMEF) were measured four times over a 2.5 yr period with a rolling-seal spirometer. In boys, pulmonary function increased with approximately the same velocity at all ages studied. In girls, however, the growth velocities of FVC and FEV1 increased markedly at age 10 yrs, and growth velocities of PEF and MMEF had increased already at age 9 yrs. The minimum pulmonary function growth velocity could not be determined from the available data in boys. In girls, the minimum pulmonary function growth velocities preceded the minimum height growth velocity at the onset of the pubertal growth spurt. All lung function growth rates were significantly associated with the growth rate of height. In girls, the growth rate of FVC was also associated with the weight growth rate. There was also some association between the growth rates of PEF and MMEF and age. In boys, there was a negative association between age and the growth rates of FVC and FEV1, after adjustment for the growth rate of height.

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Several suggestions have been made to enhance study designs to strengthen weak associations. Among these are reduction of non-differential misclassification, and restriction of studies to populations with low underlying risk of disease, due to low exposures to other known determinants of the health effect under investigation. To some extent, these suggestions have been put to use in environmental epidemiology, and in this paper, some prospects and problems associated with these attempts are discussed using examples from our own research.