Airborne pollen is a natural aerobiological particle that varies seasonally: cypress and plane trees in winter–spring, grasses in May–June, and olive trees in April–May (especially in southern Spain). Concentrations are measured in grains/m³ using Hirst-type volumetric traps.
In Spain
Spain has one of Europe's greatest pollen diversities. Grasses are the most widespread allergen (affecting ~34% of allergy sufferers), followed by olive (south) and cypress (Mediterranean). Levels are affected by rain (cleans the air), wind (disperses pollen), and climate change (advancing and lengthening seasons).