We have examined data collected from counts of airborne Betula (birch) pollen, over the last 8 to 16 years, for six cities in Hokkaido: Hakodate, Sapporo, Iwamizawa, Asahikawa, Obihiro and Kitami. From these data we analyzed the observed trends from the annual pollen count totals. Cyclic alternate rhythms of high counts and low counts were observed from the pollen data for all six cities. However, within the overall periods observed, we noticed that two-year cyclic rhythms were interrupted by 4 year periods comprising three years with low counts where, theoretically, high counts were to be expected, and only one year with high counts where low counts had been predicted. The observed cyclic rhythms did not continue for an indefinite period, however, and phase shifts in the cyclic rhythms were noted after an extended period of 10-14 years in the five cities where the observation periods were 10 years or more. The basic rhythms observed for airborne birch pollen counts were more or less synchronous for all six cities. Five year moving average of annual birch pollen counts in the five cities, with observation periods extending over 10 years or more, showed a gradual increase. From these data we concluded that longer monitoring terms for airborne birch pollen may provide more informative data, from which to develop improved counter-measures against a potential increase in the numbers of patients with pollinosis symptoms.

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