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Editor,—In their study on the effect of various indoor pollutants on peak flow variability, Fielder and colleagues1 should have considered several methodological issues before concluding that environmental tobacco smoke increases airway variability.
Low parental social class is known to be associated with a high prevalence of both parental smoking and respiratory diseases including asthma. Hence, social class is likely to be an important confounding variable, and could easily have accounted for the 5.5% variability in peak flow detected. If parents in the city centre were generally of lower social class than those in the village, it would also have explained higher peak flow variability in children attending school at the city centre. The authors should have included parental social class as a predictor variable in their multiple logistic regression model.
As children spend a considerable proportion of their time at school, and the peak flow measurements were carried out at school, peak flow variability could have been significantly affected by the school air quality. Hence, the air quality should have been measured at the four schools and taken into account in the multiple logistic regression model.
True peak flow variations due to airways variability and apparent variations due to imperfect techniques in the use of peak flow meters both contribute to variations in peak flow measurements. As the winter measurements took place before the summer measurements, the higher peak flow variations in winter could also be explained by increased experience in the use of peak flow meters. Similarly, the lower coefficients of variation in older children may be explained by their relatively better techniques in the use of peak flow meters. Hence, the expected associations between peak flow variations with age and season should not necessarily give confidence in the results showing that poorer lung function was associated with passive smoking.
Dr Fielder comments:
Wai-Ching Leung raises some interesting points. For social class to confound the relation between the prevalence of asthma and parental smoking it must be related to both. The literature suggests that the prevalence of asthma may not follow a social gradient,1-1 although the severity of symptoms and admissions to hospital are often associated with deprivation.1-2 1-3 Our findings are in agreement with other studies showing that passive smoking has an adverse effect on respiratory function, and therefore it seems reasonable to conclude that some of the variability in peak flow measurement could be attributable to passive smoking. Inclusion of social class may have improved the amount of variability predicted by the model.
The sources of indoor air pollutants were recorded by questionnaire and assumed to remain constant for the study period. Outdoor air quality was measured during the study using monitors placed at each school. The association of peak flow measurements with fluctuations in atmospheric particulates and gases were studied in a time series analysis, to be reported in a separate paper.
The children and staff were trained in the technique of peak flow measurement by a respiratory nurse in the autumn term before the study started. During the study, the teachers supervised each recording session in the morning to observe the correct technique. Members of the study team visited each school at least once a week to check that the protocol was followed. Measurement of peak flow is not difficult and we did not observe any differences in technique with age or experience.
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