Background
From 1926 to 1991, hay fever prevalence in Switzerland rose from just under 1% of the country's population to just over 14%; but from 1991 to 2000, it simply fluctuated about a mean value on the order of 15%. In addition, the authors write that "several 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," citing the work of Braun-Fahrlander et al. (2004) and Grize et al. (2006).

What was done
To see to what extent this capping of the historical increase in the prevalence of these maladies in Switzerland might have been caused by changes in pollen production, Frei and Gassner analyzed daily average concentrations of birch and grass pollen monitored by the country's National Pollen Network, at Basel for the period 1969-2007, at Locarno for the period 1989-2007, and at Zurich for the period 1982-2007.

What was learned
The two researchers report that "the pollen exposure has been decreasing in Basel since the beginning of the 1990s," that "in Locarno, most of the pollen species also show a decreasing trend," while in Zurich, "most of the pollen types have been increasing," although they add that "some of the pollen counts of this station (grass, stinging nettle, mugwort and ragweed) have been decreasing in the period 1982-2007." In addition, they state that the length of the pollen season has recently been tending "to get shorter."

What it means
In discussing their findings, Frei and Gassner write that "parallel to the increasing hay fever rate, the pollen amounts of birch and grass were increasing from 1969 to 1990," but that "subsequently, the pollen of these plant species decreased from 1991 to 2007," which they say "is more or less consistent with the changes of the hay fever rate that no longer increased during this period and even showed a tendency to decrease slightly." Hence, we find that during this latter period of rising atmospheric CO2 concentrations, there has been no corresponding increase in the pollen production of grass and birch trees - and ragweed - contrary to what climate alarmists are always claiming should occur in a CO2-accreting atmosphere.

References
Braun-Fahrlander, C., Gassner, M., Grize, L., Takken-Sahli, K., Neu, U., Stricker, T., Varonier, H.S., Wuthrich, B., Sennhauser, F.H., SCARPOL Team. 2004. No further increase in asthma, hay fever and atopic sensitization in adolescents living in Switzerland. European Respiratory Journal 23: 407-413.

Grize, L., Gassner, M., Wuthrich, B., Bringolf-Isler, B., Takken-Sahli, K., Sennhauser, F.H., Stricker, T., Eigenmann, P.A., Braun-Fahrlander, C., SCARPOL Team. 2006. Trends in prevalence of asthma, allergic rhinitis and atopic dermatitis in 5-7-year old Swiss children from 1992 to 2001. Allergy 61: 556-562.