Daily airborne pollen grains were collected in Trieste (1990Trieste ( -2004 using a Hirst-type trap. A large quantity of pollen grains was found, the mean annual total being 48 082 grains. Daily counts were particularly high between March and June, the highest values being recorded between April and May. A total of 123 different pollen taxa from 80 families were identified. Almost half of the total pollen content came from introduced trees and shrubs, one fifth from natural woodlands, and one quarter from herbaceous taxa. This composition reflects the ornamental and ruderal flora of the town, as well as the natural vegetation surrounding the urban area. The pollen richness of both mesophilous and thermophilous taxa reflects the complexity of this zone, considered a ''crossroad'' of species of different chorological types. Total daily pollen concentrations were positively correlated with temperatures only in the pre-peak period. The main taxa, except for Urticaceae, Pinus and Gramineae, displayed positive correlations with temperatures both in the main pollen season and in the pre-peak period. The annual pollen sum of Ostrya was negatively correlated with total rainfall of the previous summer, and that of Pinus was negatively correlated with the winter severity.

The aerobiological behaviour of Fagaceae in Trieste and the correlations with the meteorological parameters were examined. Airborne pollen grains of Castanea, Fagus and Quercus were collected from 1990 to 2003 using a Hirst type spore trap. The main pollen season (MPS) takes place in April and May for Quercus and Fagus, in June and July for Castanea. The highest values occur in year 1993 for Quercus, in 1998 for Castanea and in 1992 for Fagus. The Fagaceae content of the air is mainly due to Quercus and Castanea pollen, Fagus usually having a scarce pollen shedding in Trieste. The highest counts of Fagaceae pollen grains are found from late April to mid May and are mainly due to the pollen shedding of oaks. The cumulative counts vary over the years, with a mean value of 2.719 pollen grains, a lowest total of 1.341 in 2002 and a highest total of 4.704 in 1993. No positive nor negative long-term trends in pollen shedding are found. No cyclic variations were observed. Spearman's correlation was used to establish the relationship between the daily pollen counts and the daily meteorological data. Daily pollen concentrations present sometimes positive correlation with temperature, negative with rainfall and wind speed, and no correlation with humidity. Fagus and Quercus start dates result positively correlated between themselves. Significant correlations are found between the start of MPS and the mean and maximum temperature in March for Fagus and Quercus, and May for Castanea.