What Causes Hay Fever?

Hay fever refers to an allergic reaction to pollen, the fine powder-like substance that flowers release as part of reproduction. These pollen particles act as allergens, which means they irritate the lining of the mouth, nose, eyes and throat, causing the symptoms of allergic reaction such as inflammation and swelling.

When a person has an allergic reaction, the body responds to a substance (allergen) as if it were a pathogen such as a virus or bacteria. In hay fever, the immune system launches an attack against pollen by releasing various chemicals in an attempt to prevent it “spreading,” like an infection would. It is the release of these chemicals (called histamines) that leads to the symptoms of hay fever such as itchy, red eyes and a runny or blocked nose.

Exactly what causes this reaction by the immune system is unclear, but several factors have been identified that can increase the risk of getting hay fever. These include the following:

  • The presence of another allergic condition such as asthma or eczema
  • A family history of hay fever
  • Previous exposure to tobacco smoke during childhood

In England, the majority of hay fever cases are caused by grass pollen, although the pollen released by weeds and trees can also trigger the condition. Studies have also shown that pollution such as exhaust fumes can worsen symptoms.

Around thirty different types of pollen can cause hay fever. Aside from grass, other types of pollen that can trigger this allergic reaction include the pollen from oak, ash, birch or cedar trees and weeds such as nettles, mugwort or dock.

In the UK, tree pollen is generally released between late March and mid-May; grass pollen between mid-May through to July and weed pollen between the end of June and through to September. However, the pollen count season does vary, with it sometimes beginning as early as January and not ending until as late as November.

Further Reading

Last Updated: Jun 12, 2023

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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