The method of the Italian network POLLnet for counting and evaluating the concentration of airborne particles in a daily sample

Airborne pollen monitoring is of concern for health purposes as well as for studies on climate changes and biodiversity. The reliability of data estimates depends on the accuracy and precision of pollen counts (Gottardini et al., 2008). An important aspect of the microscope analysis, heavily influencing this reliability, is the choice of the minimal surface to be read on a microscope slide. At the moment, operators seem to argue among 10%, 15% and 20% of the target surface. The Italian network POLLnet searched a criterion both on experimental and statistics basis. For this purpose, three microscope slides, with low, medium and high amounts of particles, were read by twenty experienced aerobiologists. Each slide was analyzed on 100% of target surface, as well as 10%, 15% and 20%. The statistic approach applied the Poisson distribution to search the minimal surface to examine in order to find at least 1 pollen grain if on the total target sample there are 5 grains, or 0.5 particles/m3 that is the threshold concentration for the detection of most of the monitored families. This study confirmed that the variability of data decreases as the examined area increases. In particular, when 10% of the target area is examined, up to 45% of the pollen species can escape the analysis; when the slides were examined on the wider areas (15% and 20%), this lost of taxa decreased up to 15%. Also, the repeatability and the reproducibility of the analysis varied significantly on the basis of the percentages of the target area examined. Moreover, the statistic approach indicated that 14% is the minimal area to examine to find at least 1 airborne particle on a total of 5, with a confidence of 50%. Even if the experimental data suggest that the ideal surface would be at least 20%, both the experimental and the statistical approach indicate that adopting as a minimum reading limit 15% of the sampling surface is still acceptable.