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What causes hay fever?

Laura Harcourt

Written byLaura Harcourt


Reviewed byHeeral Patel

two children playing in long grass in park setting.
Hay fever, or allergic rhinitis, is a widespread allergy, but what causes it? Find out more about hay fever causes and learn how to conquer hay fever symptoms.
It’s early spring or summer, your nose is stuffed up and your eyes are itching and streaming. Pretty sure it’s not a cold? It’s more likely to be hay fever, which affects up to 49% of the population in the UK.

Unlike a cold, hay fever symptoms can go on for months, so they can feel pretty irritating after a while. Pollen is often the culprit of these pesky symptoms, but did you know there are other hay fever triggers?

What is hay fever?

Hay fever, also known as allergic rhinitis, is an allergic reaction to allergens in the air, like pollen.

When you inhale pollen, or another allergen, your immune system can mistake it for a harmful substance. Instead of ignoring it, your body sees it as an invader and mounts a reaction to attack it.

One of the chemicals released during this reaction is histamine. Histamine makes blood vessels wider and triggers allergy symptoms like a blocked nose or streaming, itchy eyes – some of the most common symptoms of hay fever.2,

Common hay fever symptoms include:
  • sneezing 
  • blocked or runny nose 
  • watery, itchy eyes 
  • itchy mouth or tongue 
  • headaches 
  • tiredness 
  • wheezing
While it’s traditionally thought of as a summer problem, hay fever can actually start to affect people as early as February and the season continues until September.1 However, for some people, hay fever can be a year-round problem, depending on its cause.1

What causes hay fever symptoms?

In the UK, pollen is the most common cause of hay fever, especially during certain seasons.1 This is what’s also known as seasonal allergic rhinitis.

Tree pollen

The time of year of your symptoms flare up can give you an idea of which type of pollen you’re reacting to.
Tree pollen typically occurs from as early as February to as late as June and is thought to affect around 25% of people with hay fever.7,8

Trees like alder, oak, birch, horse chestnut, and hazel are the most likely to release pollen that can cause hay fever.7 However, other types of trees like pine, willow, and elm can also trigger hay fever symptoms.

Weed pollen

Weed pollen starts from late June to early September, depending on the type of weed. Certain types of weeds, like nettles, release really fine pollen that’s easily blown about in the wind and can cause hay fever symptoms.10

Other common weeds that can cause hay fever symptoms include ragweed, dock weed, mugwort, and ribwort.10

Grass pollen

Most people with hay fever are allergic to grass pollen, in fact it’s thought to affect 95% of those with hay fever in the UK. Grass pollen is generally in the air from May to July and is released from lawns, meadows, and ornamental grasses.11

In your garden, mowing your lawn regularly can help reduce the amount of grass pollen.11

Handpicked articleUK Pollen Count Calendar

Other hay fever triggers

In some cases, hay fever symptoms can happen all year round and some may mistake it for a common cold. This is known as perennial allergic rhinitis and can be triggered by environmental moulds and other indoor allergens.6

Perennial allergic rhinitis can be caused by:
  • Pet dander: if you have furry pets at home, it turns out that the proteins found in their skin flakes, urine and saliva can trigger allergic rhinitis.1 Under the microscope, these particles have a jagged shape and are very small, so they might linger for longer than other allergens, sticking to furniture, carpets and more.12
  • Mould spores: damp environments, like bathrooms, kitchens, basements, and any areas with water damage can encourage mould to grow.13  When inhaled, these mould spores can trigger hay fever like symptoms in some people.14
  • House dust mites: our homes are a haven for dust mites! These microscopic organisms live in dust, bedding, upholstery, and even carpets.15  Their presence in your home can trigger year-round allergy symptoms, particularly when you’re indoors.16
  • Occupational allergens: exposure to occupational allergens like chemicals, dust, fumes, or animals can trigger allergic rhinitis in those who are susceptible.17
  • Air pollution: vehicle exhaust, industrial emissions, and even cigarette smoke don’t cause allergies directly, but they can worsen existing allergic rhinitis and make you more sensitive to other allergies.18,19

Does the weather affect hay fever symptoms?

The weather and climate can really affect your hay fever symptoms as it affects how much pollen is produced and released into the air. The amount of rain, wind speed, sunshine levels, and the temperature can all contribute to the dispersal of pollen.

Rain reduces the amount of pollen in the air in a ‘wash-out’ effect, whereas more sunlight can increase pollen production.8 High winds can disperse pollen further, but light wind won’t move the pollen around as much.8

When it comes to temperature, trees need fairly low temperatures to release pollen, whereas grass prefers higher temperatures.8 If temperatures rise too high (28°c and above) then pollen levels can decrease and even be diminished completely if it’s too hot for several days in a row.8

Why has there been a rise in hay fever?

The increase in numbers of people suffering from hay fever isn’t clearly understood, but theories include our lifestyles, better hygiene, rising temperatures, more pollution, a higher pollen count, and even genetics.

A higher pollen count, combined with a rise in pollution mean that many of those with hay fever often find their symptoms get worse in cities.21  A historical 2003 report by the US Department of Agriculture found that ragweed growing in urban areas grew faster, flowered longer, and produced more pollen than in the countryside.22

In a more recent 2021 study, researchers found similar results when looking at pollen count between rural and urban areas in the UK.23

Climate change may also have an effect, as warmer temperatures lengthen growing seasons for trees, weeds, and grasses.24

How to limit your exposure to pollen

Limiting your exposure to pollen can help reduce your symptoms. Try these top tips:25
  • wear wraparound sunglasses outside the house 
  • shower and wash your hair when you come indoors 
  • change into clean clothes when you get home 
  • avoid drying your washing outside 
  • apply a barrier balm to your nostrils – these sticky balms trap pollen and may prevent it entering your system 
  • stay indoors when the pollen count is high

The final say

We know hay fever symptoms can be frustrating, especially when you want to enjoy the outdoors during the summer. Pollen is the main culprit, but there are other triggers too and their effects can often be felt year-round.

Hay fever is thought to be becoming more and more common as time goes on, but there are ways you can limit your exposure to pollen and help reduce your symptoms. In some cases, remedies or hay fever treatment like antihistamines can help ease your symptoms too, but speak to your GP first before you take them.

Handpicked content


1. Hay Fever | Allergy UK | National Charity [Internet]. Allergy UK | National Charity. 2020 [cited 2024 Mar 18]. Available from: https://www.allergyuk.org/types-of-allergies/hayfever/
2. NHS Choices. Hay fever [Internet]. 2024 [cited 2024 Mar 18]. Available from: https://www.nhs.uk/conditions/hay-fever/
3. Allergic Reaction | AAAAI [Internet]. Aaaai.org. 2023 [cited 2024 Mar 18]. Available from: https://www.aaaai.org/tools-for-the-public/conditions-library/allergies/allergic-reactions
4. Maeda K, Caldez MJ, Shizuo Akira. Innate immunity in allergy. Allergy [Internet]. 2019 Apr 14 [cited 2024 Mar 15];74(9):1660–74. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6790574/
5. Yamauchi K, Ogasawara M. The Role of Histamine in the Pathophysiology of Asthma and the Clinical Efficacy of Antihistamines in Asthma Therapy. International Journal of Molecular Sciences [Internet]. 2019 Apr 8 [cited 2024 Mar 15];20(7):1733–3. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6480561/
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7. Tree pollen season is nearly here | Allergy UK | National Charity [Internet]. Allergy UK | National Charity. 2023 [cited 2024 Mar 21]. Available from: https://www.allergyuk.org/news/tree-pollen-season/
8. Office M. When is hay fever season in the UK? [Internet]. Met Office. Met Office; 2019 [cited 2024 Mar 21]. Available from: https://www.metoffice.gov.uk/weather/warnings-and-advice/seasonal-advice/health-wellbeing/pollen/when-is-hayfever-season
9. Tree Pollen Allergy [Internet]. Asthma & Allergy Foundation of America. 2023 [cited 2024 Mar 22]. Available from: https://aafa.org/allergies/types-of-allergies/pollen-allergy/tree-pollen-allergy/
10. Weed Pollen | Allergy UK | National Charity [Internet]. Allergy UK | National Charity. 2022 [cited 2024 Mar 22]. Available from: https://www.allergyuk.org/news/weed-pollen/
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12. American Lung Association. Pet Dander [Internet]. Lung.org. ; 2023 [cited 2024 Mar 18]. Available from: https://www.lung.org/clean-air/indoor-air/indoor-air-pollutants/pet-dander
13. Basic Facts about Mold and Dampness [Internet]. 2024 [cited 2024 Mar 25]. Available from: https://www.cdc.gov/mold/faqs.htm
14. NHS Choices. Can damp and mould affect my health? [Internet]. 2024 [cited 2024 Mar 25]. Available from: https://www.nhs.uk/common-health-questions/lifestyle/can-damp-and-mould-affect-my-health/
15. Dust mites in your home [Internet]. Cambridge University Hospitals. 2020 [cited 2024 Mar 25]. Available from: https://www.cuh.nhs.uk/patient-information/dust-mites-in-your-home/
16. Aggarwal P, S Senthilkumaran. Dust Mite Allergy [Internet]. Nih.gov. StatPearls Publishing; 2023 [cited 2024 Mar 25]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK560718/
17. Anderson SE, Long C, Dotson GS. OCCUPATIONAL ALLERGY. European Medical Journal (Chelmsford, England) [Internet]. 2017 [cited 2024 Mar 25];2(2):65–71. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6454566/
18. Li S, Wu W, Wang G, Zhang X, Guo Q, Wang B, et al. Association between exposure to air pollution and risk of allergic rhinitis: A systematic review and meta-analysis. Environmental Research [Internet]. 2022 Apr 1 [cited 2024 Mar 25];205:112472–2. Available from: https://pubmed.ncbi.nlm.nih.gov/34863689/
19. Li CH, Sayeau K, Ellis AK. Air Pollution and Allergic Rhinitis: Role in Symptom Exacerbation and Strategies for Management. Journal of Asthma and Allergy [Internet]. 2020 Aug 1 [cited 2024 Mar 25];Volume 13:285–92. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7457822/
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24. Adams-Groom B, Selby K, Derrett S, Frisk CA, Catherine Helen Pashley, Satchwell J, et al. Pollen season trends as markers of climate change impact: Betula, Quercus and Poaceae. Science of The Total Environment [Internet]. 2022 Jul 1 [cited 2024 Mar 25];831:154882–2. Available from: https://www.sciencedirect.com/science/article/pii/S0048969722019751
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