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Re: Personal exposure of children to nitrogen dioxide

Editor,

We read with interest a recently published study on personal exposure of asthmatic children to nitrogen dioxide (NO2), relative to concentrations in outdoor air.[1] In their results, authors didn't find: 1. "…significant correlation… between each child's weekly mean personal exposures and mean outdoor concentrations for the corresponding periods"; 2. "…marked evidence of seasonality" on personal exposure . They concluded: "…at low concentrations, changes in NO2 in outdoor air…contribute little to variations in personal exposure to the pollutant." We think that these conclusions cannot be drawn from the method used to evaluate outdoor concentrations. Besides, we report different findings on seasonal trend at higher levels of personal exposure. We performed a study to evaluate the annual distribution of personal exposure to NO2 in school children of Novara, a small city in north west Italy (about 110.000 inhabitants) and to study determinants of this exposure. NO2 exposure was measured using passive samplers (Palme's tubes) in 310 school children aged 5-14 years. The children wore the tubes 5 days a week, in each season of the year. The possible differences in personal measurements were assessed by ANOVA and Tuckey tests. Information about the sources of potential exposure was collected by a questionnaire. The relative risk for these variables was estimated with a multiple regression model (Logit). The annual average of 6,200 measurements was 42.7 mg/m3 with a significant difference between seasons, and higher values in winter. The only factor associated with increased personal exposure was to live along busy streets, only for children of maternal school. Even if designs of two studies are different, at this point it is possible to make some considerations. Firstly, at higher levels of NO2 exposure respect to those reported by Linaker et al,[1] the seasonal changes of concentration in outdoor air contribute significantly to variations in intra-individual exposure. Besides, the role of risk factors present at home, respect to child-child differences, is not clear. We suppose that our result depends on actual habit of children to spend, everyday, a lot of hours in many different occupations away from home, reducing the individual differences caused by domestic sources of NO2. Secondly, we think that only one measurement station used by Linaker et al.[1] is inappropriate to evaluate the real impact of outdoor concentrations on personal exposure, because outdoor levels of pollutants are, in our and in other studies,[2] [3] related to traffic density in each street.