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, the authors did not find:

“ . . .significant correlation . . . between each child's weekly mean personal exposures and mean outdoor concentrations for the corresponding periods”;

“ . . .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 a seasonal trend at higher concentrations 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. Exposure to NO₂ was measured with passive samplers (Palmes' tubes) in 310 school children aged 5–14 years. The children wore the tubes for 5 days a week, in each season of the year.

The possible differences in personal measurements were assessed by analysis of variance (ANOVA) and Tukey's 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 6200 measurements was 42.7 μg/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, and then only for children from playschool (3–6 years). Even if the designs of two studies are different, at this point it is possible to make some comparisons.

Firstly, at higher concentrations of NO₂ exposure, as in those reported by Linaker et al,1 the seasonal changes in concentration in outdoor air contribute significantly to variations in exposure within individual people. On the other hand, the role of risk factors present at home, and the differences between children are not clear. We think that our results depend on the habits of most children to spend many hours every day 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, as used by Linaker et al,1 is inappropriate to evaluate the real impact of outdoor concentrations on personal exposure, because outdoor concentrations of pollutants are, in our and in other studies,2 3 related to traffic density in each street.