Comtois and Gagnon. 1988. The authors compared airborne pollen levels with patient symptom scores in Montreal over a two year period. Their goal was to establish clinical thresholds for pollen concentrations which triggered symptoms. Their data suggest that concentrations of 8-25 grains/m³ for tree pollen, 4-12 grains/m³ for grass pollen and 1-3 grains/m³ for ragweed pollen were clinical thresholds.