Alternaria spores at different heights from the ground

Alternaria tenuis (Alt) is one of the main allergens in pediatric age. In temperate climates, airborne Alt spores are detectable from May to November with peaks in late summer and autumn. Sensitized children display symptoms even in the absence of airborne Alt spores. Alt spore concentration, as well as pollen, is usually detected by fixed devices located on the roof of a building at a height of 10-20 m. The aim of the current study is to find out whether ground-level (50 cm) Alt spore concentrations are different from those at roof-top level, even during low-concentration periods.

Alt samples were taken simultaneously using a Hirst fixed volumetric collector (FVC) placed on a 15 m-high roof and by a portable volumetric collector (PVC). Firstly, the results of FVC and PVC, both placed on the roof-top, were compared to verify the correlation coefficient of the two samplers. Subsequently, the PVC was placed 50 cm above the ground in a courtyard (30 samplings) and in private green areas (50 samplings). The results were compared by statistical analysis (Student's t-test or K-S test).

The values of the 20 samples taken jointly in summer time (FVC 195 +/- 134 spores/m(3); PVC = 134 +/- 131 spores/m(3)) showed a good correlation between the two samplers (r = 0.850; P < 0.01), with a correction factor equal to 1.177. 1. Thirty samples obtained in summer and winter when the PVC was positioned in an enclosed courtyard directly below the FVC showed no significant difference (PVC, 181 +/- 194 spores/m(3); FVC, 152 +/- 145 spores/m(3); P = 0.221). 2. Fifty samples taken by PVC placed in private green areas in a low-concentration period, showed significantly higher concentrations than by FVC: PVC, 531 +/- 925 spores/m(3); FVC, 25 +/- 51 spores/m(3) (K-S test: P < 0.0001). In particular, 33 samples taken in winter when Alt counts by FVC were <10 spores/m(3) still demonstrated highly significant differences: PVC, 398 +/- 961 spores/m(3); FVC, 2.0 +/- 2 spores/m(3) (K-S test: P < 0.0001).

Our results lead to the conclusion that Alt spore concentration is significantly higher at ground level in the presence of vegetation, even when the spore concentration is very low (<10 spores/m(3)). These results further suggest that the individual's exposure to Alt, especially in the case of children, is underestimated by samples taken at roof-top level by FVC.