The pollens that cause hay fever vary from person to person and from region to region; generally speaking, the tiny, hardly visible pollens of Anemophily (wind-pollinated) plants are the predominant culprits. Pollens of entomophily (insect-pollinated) plants are too large to remain airborne and pose no risk. Examples of plants commonly responsible for hay fever include:

• Trees: Such as birch (Betula), alder (Alnus), hazel (Corylus), hornbeam (Carpinus), horse chestnut (Aesculus), willow (Salix), poplar (Populus), plane (Platanus), linden/lime (Tilia) and olive (Olea). In northern latitudes birch is considered to be the most important allergenic tree pollen, with an estimated 15–20% of hay fever sufferers sensitive to birch pollen grains. Olive pollen is most predominant in Mediterranean regions.
• Grasses (Family Poaceae): Especially ryegrass (Lolium sp.) and Timothy-grass (Phleum pratense). An estimated 90% of hay fever sufferers are allergic to grass pollen.
• Weeds: Ragweed (Ambrosia), plantain (Plantago), nettle/parietaria (Urticaceae), mugwort (Artemisia), Fat hen (Chenopodium) and sorrel/dock (Rumex)
• Drugs: Prednisolone