This is simplified information about pollen allergies, also called rhinitis, pollinosis, seasonal allergies and hayfever – there are more resources available at the bottom of the page for further reading for those who are interested in knowing more.

Pollen is a fine dust containing lots of microspores, these vary in shape depending on the plant they came from, most pollen spores are symmetrical, often spherical or oval in shape. The outer layer of most pollen particles are tough so they can last well in the environment and often have an irregular surface which makes them ‘sticky’, attaching to plants and animals. These molecules are very small so the pollen of plants that rely on wind pollination can travel massive distances.

Pollen Size

Generally wind pollinated plants make smaller, lighter pollen so that it can travel great distances. Insect pollinated plants are brightly coloured and the pollen doesn’t have to be so small. Some plants are self pollinated, which means that pollen only has to transfer from one flower on the plant to another. These plants are less likely to cause allergic reactions unless you are in close contact with them, so are more likely to cause problems in people who plant or harvest them.

Lots of air filtration fans include information about PM2.5 and PM10s. PM stands for Pollen Molecule; the number is the size in micrometers. PM2.5 are very fine pollens and PM10s are larger. They are both capable of causing allergic rhinitis and asthma symptoms.

Pollen Seasons

The most common pollens are grouped into three categories, tree pollen, grass pollen and weed pollen. These categories can help us determine general pollen seasons for these pollen types.

Tree pollen season starts late February to June
Grass pollen season runs from April to September
Weed pollen season runs from June to September

These are generalisations, pollen season is different for most species and there is a lot of overlap, so without allergy testing it can be quite difficult to determine which pollen is causing a problem.

Note information is for the Western hemisphere – countries in the Southern hemisphere have the opposite seasons, so the pollen seasons for those countries will be different.

Pollens and Allergens

The table below shows which pollens contain which allergens, note that the groups of plants are generalised, for example there are many species of oak trees, many have been found to contain profilin proteins in their pollens, but not all species have been studied and not all species will have the exact set of allergens in their pollens.

Table is sorted alphabetically by common name, trees are in green, weeds in orange, grasses in blue and other types are in yellow.



Information in this table is from multiple resources – please visit the Pollen Allergy Index for information on more than 30 plant pollens.

You can also download a pollen allergens chart from my KOFI page for just 25p!

How can pollen affect us?

Asthma – There are two ways pollen can affect asthma. In non-allergic asthma, smaller pollen particles can be inhaled and can irritate the respiratory in the same way that pollution and other irritants like chemicals can trigger symptoms. In allergic asthma the immune system is overreactive to pollen particles and causes an IgE response which can include swelling in the respiratory system.

Nasal Symptoms – Pollen allergens cause symptoms at the point of contact. The most common place for them to cause irritation is in the nasal passages causing itching and swelling.

Rhinoconjunctivitis - The eye is another common point of contact as the surface has to stay moist to function. Pollen can cause itching and swelling of the tissues around the eyes.

What makes pollen counts worse?

Pollen and the Weather – Dry, mild and sunny weather makes pollen counts worse. Rain, especially on those days where it rains all day, can reduce the pollen count in the air as do extremely windy days.

Thunderstorms can cause an extreme exacerbation of asthma symptoms in people who suffer from both hayfever and asthma, this is because unusual downdrafts collect pollen from the ground then updrafts of air lift pollen particles into the clouds. The larger pollen particles are damaged, causing smaller particles to be released into the air. These smaller particles are able to get more easily into the lower parts of the lung and can make asthma symptoms worse.

Pollen counts start to rise in the morning with the sun, are at their peak in the middle of the day and then very slowly start to decrease as the day continues.

Pollen and Pollution – Air pollution affects the pollen count in a few different ways

1 – Pollutants can bind to pollen making them more irritating in the lings, exacerbating asthma symptoms
2 – Pollutants can damage the cell wall of pollens, releasing the allergens more easily into the environment
3 – NO2, a common traffic linked pollutant increases the allergenicity of certain pollens, most notably the Bet v 1 allergen which is found in many trees

Pollen and the Landscape – The pollen count is generally lower by the sea due to the wind blowing particles away from the beach. Pollen in valleys are generally worse as it is harder for wind to disperse pollution and pollen in the air, they tend to sit for longer in the air in comparison to flat or hilly locations.

What can I do to relieve symptoms?

Which antihistamines are the best? The most commonly used symptom relief for hayfever and rhinitis are antihistamines. They block histamine from causing symptoms. They are a well-researched group of drugs which are suitable for most people to take daily through pollen season. There are two types. First generation antihistamines have the side effect of making you drowsy, these are recommended for people who struggle to sleep with their symptoms. The second generation work just as well but don’t have any side effects. It is important to take antihistamine at the start of the day as already having it in your system makes it easier to counter the histamine in your body when it is produced.

Physical Barriers – Avoiding going out when the pollen count is exceptionally high, as well as wearing large wraparound glasses or sunglasses that prevent pollen from entering and irritating the eyes, can help lessen symptoms. Putting Vaseline inside the nose traps pollen. When pollen levels are high, keep windows and doors closed. When you come in from the outside, change your clothes and take a shower to get rid of the pollen on your body.

Pollen Immunotherapy – Immunotherapy is when a small amount of an allergen is given on a regular basis to desensitise individuals to the symptoms of rhinitis. An increased amount is given every time with the aim of building up their tolerance to pollen. It is not commonly offered in the UK for allergic rhinitis but can be undertaken privately. It is not a quick process, taking anywhere from 3 to 5 years and can be costly.

Pollen Holidays – a holiday in coastal areas or places with low flora, like Iceland can offer some relief from pollen. The University of Worcester has a list of low grass pollen holidays and low tree pollen holidays (see links at the bottom of the page).

What is Pollen Food Allergy Syndrome?

This was previously called Oral Allergy Syndrome (OAS), the term was coined to define the symptoms. It has now been termed Pollen Food Allergy Syndrome (PFAS) more definitively as it mostly affects people who already suffer from pollen allergies and seasonal rhinitis due to cross reactivity.

In PFAS the body mistakes the shape of a protein that it is already sensitive to (a plant pollen) to a similarly shaped protein in certain fruits, vegetables, spice, nuts and herbs.

The key allergen in PFAS is the pollen associated with the group of foods you are having symptoms from eating. It is most commonly associated with birch tree pollen, the allergens are called Bet v 1-like proteins.

Worcester Pollen Forecast (UK)
London Allergy & Immunology Centre (UK)
Pollen.com (US)
Pollen Library (US)
Asthma & Allergy Foundation of America - Pollen
NHS – Hayfever (UK)
Healthline – Pollen
American College of Allergy, Asthma & Immunology (US)
Australian Society of Clinical Immunology & Allergy
The Met Office (UK)
University of Birmingham - How is pollen, pollution and climate change affecting our health?
NHS Antihistamines
UK Health Security Agency, thunderstorm asthma and public health
Allergy UK - Immunotherapy
Allergy UK - Pollen Food Syndrome

Uncovering Severe Patient Group With Pollen-Related Extrarespiratory Allergic Symptoms: A Year-Long Diary Survey in Japan, 2024

Air pollutants contribute to epithelial barrier dysfunction and allergic diseases, 2023

Hay fever: Allergen-specific immunotherapy (desensitization) in the treatment of allergies, 2020

Interaction Between Air Pollutants and Pollen Grains: The Role on the Rising Trend in Allergy, 2018

Let me know if you found any of these interesting or useful. If you spot an article or research that you think is interesting you can message me or tag me on Facebook or Twitter - links at the bottom of the page.