Negrini et al. 1992. The study was conducted in a effort to
establish a threshold-value for Parietaria pollen. It
was found that the onset of mild symptoms occurred at about 15 grains/m3,
moderate symptoms at about 40 grains/m3, and severe symptoms at
>80 grains/m3. The authors
found a significant correlation between weekly pollen count and symptom scores.