Causes of hay fever 

Hay fever is an allergic reaction to pollen. When these tiny particles come into contact with the cells that line your mouth, nose, eyes and throat, they irritate them and trigger an allergic reaction.

Allergic reaction

When you have an allergic reaction, your body overreacts to something it perceives as a threat. In hay fever, the allergen (the substance you're allergic to) is pollen. Your immune system (the body’s natural defence) responds as if it were being attacked by a virus.

Your immune system will release a number of chemicals designed to prevent the spread of what it wrongly perceives as an infection. These chemicals cause the symptoms of the allergic reaction, such as watering eyes and a runny nose.

Risk factors

It's unclear what causes the immune system to react in this way, but there are several factors that can increase your risk of developing hay fever. They include:

  • having asthma or another allergic condition, such as eczema
  • having a family history of hay fever
  • being exposed to tobacco smoke and diesel exhaust particles during early childhood

Most people with hay fever are allergic to grass pollen, but it can also be caused by trees and weeds. Research suggests that pollution, such as cigarette smoke or car exhaust fumes, can make allergies worse.


There are around 30 types of pollen that could cause your hay fever. The pollen that causes hay fever can come from a number of sources, including:

  • grass – in the UK, about 9 out of 10 (90%) of people with hay fever are allergic to Timothy and Rye grass pollen
  • trees – about 1 in 4 (25%) of people with hay fever in the UK are allergic to pollen from trees, including oak, ash, cedar and birch (people with an allergy to birch often also experience an allergic reaction to apples, peaches, plums and cherries because these types of fruit contain a similar protein to birch pollen)
  • weeds – such as dock, mugwort and nettles; being allergic to weeds is less common and tends to occur during the autumn 

It's possible to be allergic to more than one type of pollen.

When is there most pollen?

Different trees and plants produce their pollen at different times of the year. Depending on which type of pollen you're allergic to, you may experience hay fever symptoms at different times.

In the UK, the pollen count season is usually separated into three periods:

  • tree pollen – late March to mid-May
  • grass pollen – mid-May to July
  • weed pollen – end of June to September

However, the pollen count season can sometimes begin as early as January or end in November. For example, depending on the weather conditions, sometimes there can be an "early spring" or a "long summer".

The effect of the weather

The amount of sunshine, rain or wind affects how much pollen plants release and how much it's spread around. On humid and windy days, pollen spreads easily. On rainy days, pollen may be cleared from the air, causing pollen levels to fall.

During their pollen season, plants release pollen early in the morning. As the day gets warmer and more flowers open, pollen levels rise. On sunny days, the pollen count is highest in the early evening.


Some people can also develop a hay fever-like allergy to fungi. Fungi can release tiny particles called spores, which can be inhaled in the same way as pollen.

Fungi spores are most widespread during periods of wet or damp weather. Fungi can also grow indoors in damp areas, such as under washing machines or sinks.

Page last reviewed: 27/10/2015

Next review due: 27/10/2017