Background
The author writes that "in recent decades a large number of epidemiological studies investigating the change of prevalence of hay fever showed an increase in the occurrence of this disease," but he notes that "other studies carried out in the 1990s yielded contradictory results in this regard," adding that "recent studies show that no further increase in asthma, hay fever and atopic sensitization in adolescents and adults has been observed during the 1990s and the beginning of the new century."

What was done
Frei analyzed time series of pollen counts and pollen season lengths in order to identify their trends, based on epidemiological data for hay fever in Switzerland for the period 1926 to the present and pollen data from 1969 to the present.

What was learned
The data analyses indicated that "pollen exposure has been decreasing in Basel since the beginning of the 1990s, whereas the rate of the hay fever prevalence in Switzerland remained approximately unchanged in this period but with a slight tendency to decrease."

What it means
Although many people have claimed that rising temperatures and CO2 concentrations lead to more pollen and more hay fever (Wayne et al., 2002), the analysis of Frei suggests that such is not the case in Switzerland, as was also supported by the earlier work of Frei and Gassner (2008).

References
Frei, T. and Gassner, E. 2008. Trends in prevalence of allergic rhinitis and correlation with pollen counts in Switzerland. International Journal of Biometeorology 52: 841-847.

Wayne, P., Foster, S., Connolly, J., Bazzaz, F. and Epstein, P. 2002. Production of allergenic pollen by ragweed (Ambrosia artemisiifolia L.) is increased in CO2-enriched atmospheres. Annals of Allergy, Asthma, and Immunology 88: 279-282.