Spieksma. 1999. This study showed an overall similarity between airborne birch pollen grains and birch allergen. Small quantities of allergen were detected when no pollen was registered. The mechanism of allergen release in the paucimicronic sizes is not fully elucidated. The authors agree that there is a need for more accurate assessment of airborne allergen levels. They go on to state that until techniques for continuous monitoring are more widely available, pollen counts provide a sufficiently reliable surrogate.