Analysis: Rising temperatures and wider climatic changes are altering pollen seasons and human exposure to airborne allergens
For many hay fever sufferers, June is the month to endure. Once the main grass pollen season begins to fade, it can be tempting to think that the worst is over for another year. Unfortunately, the pollen season has one final sting in its tail. One of the pollens we increasingly encounter towards the latter part of summer comes from a plant almost everyone in Ireland will recognise: the common stinging nettle.
We associate nettles with the painful sting they deliver when we brush against them, but they can affect us in another way. They produce large quantities of tiny pollen grains which are released into the atmosphere and carried by the wind.
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From RTÉ Radio 1's Today with David McCullagh, Dr. Iseult Sheehan from Allergy Ireland on how climate change is making the pollen season last longer
For some people, breathing in this pollen can contribute to allergic symptoms including sneezing, nasal congestion and itchy or watery eyes. Research has confirmed that nettle pollen can cause IgE-mediated allergic rhinitis and contains proteins capable of acting as allergens, although nettle is generally considered less allergenic than major pollen types such as grass and birch.
The familiar sting from touching a nettle and an allergy to its pollen are two completely different processes. The sting results from the plant's specialised hairs interacting with the skin, while pollen allergy occurs when the immune system of a sensitised person reacts to proteins associated with pollen.
There isn't really one pollen or hay fever season
We frequently talk about the pollen season, as though it were one event that arrives in spring and disappears after the grasses finish flowering. In reality, it is more like a relay race.
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From RTÉ Radio 1's Today with Claire Byrne, Dr David O'Connor from DCU and GP Dr Yvonne Williams on dealing with Europe's extreme pollen levels and how to relieve the symptoms
Different plants release their pollen at different times of the year. Hazel and alder can start the process very early in the year, followed by trees such as birch and oak during spring. Grasses then become the dominant allergenic pollen during late spring and summer, before weed pollens, including nettle, become more prominent later in the year. Met Éireann describes the Irish pollen calendar in the same broad tree, grass and weed phases, with weed pollen occurring from late summer into autumn.
Together, these overlapping seasons mean Ireland's pollen calendar can stretch from the opening weeks of the year through to September. Of course, this does not mean everybody with hay fever suffers for nine months. People are not allergic to "the pollen season". They become sensitised to particular pollen types.
Someone predominantly allergic to grass may experience symptoms during a relatively defined period in summer. Someone sensitised to several trees, grasses and weeds can potentially experience symptoms over a much longer period. This helps explain a common question: why am I still sneezing when the grass pollen season is supposed to be finishing? The answer may simply be that what is in the air has changed.
From RTÉ Brainstorm, why hay fever is going to get worse because of climate change
Climate change is redrawing the pollen calendar
Climate change gives us another reason to rethink a fixed pollen season. Flowering and pollen release are strongly influenced by environmental conditions, particularly temperature and rainfall. A systematic review of 93 studies found clear relationships between meteorological conditions and the timing and concentration of airborne pollen, although the effects vary between plant species and locations.
More recently, the 2026 Europe report of the Lancet Countdown on health and climate change concluded that climate change has prolonged the pollen season in Europe by around one to two weeks. For alder, birch and olive, pollen seasons during 2015–2024 began around one to two weeks earlier than during 1991–2000.
Climate change does not simply mean "more pollen". It can mean earlier pollen, longer periods of exposure, changes in the amount of pollen produced and changes in the geographical distribution of allergenic plants. Recent reviews show that rising temperatures and wider climatic changes are altering pollen seasons and human exposure to airborne allergens.
When grass pollen begins to disappear, the allergy season has not necessarily ended. The composition of the air has simply changed.
In other words, climate change is redrawing the pollen calendar. Attention often focuses on spring arriving earlier, but the other end of the calendar matters too. For an allergy sufferer, what matters is not simply when one plant begins flowering, but the total period during which allergenic pollen is circulating in the air.
Long-term European monitoring has already shown substantial changes in the timing and intensity of pollen seasons, although these changes are not identical for every pollen type or location.
What is actually in the air?
This is why pollen monitoring is increasingly important. A pollen count is much more useful when we know which pollen is being counted. A high concentration of one species does not necessarily carry the same allergy risk as the same concentration of another. Nettle provides a useful example: pollen concentration and allergenic potency are two different things, and nettle is generally regarded as less allergenic than major pollen allergens such as grass and birch.
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From RTÉ Radio 1's Drivetime, tips to help those suffering from hayfever
Pollen is not the only biological material present. As summer progresses into autumn, airborne fungal spores such as Alternaria and Cladosporium can also become important allergens. Met Éireann notes that fungal spores occur throughout the year but tend to peak during late summer and autumn.
When grass pollen begins to disappear, the allergy season has not necessarily ended as the composition of the air has simply changed. The humble nettle therefore offers more than an irritating encounter on a country walk. Its late-summer pollen provides a useful reminder that Ireland's allergy season is longer and more complex than many of us realise. As our climate continues to change, that final sting in the tail may become an increasingly important part of understanding when and why we sneeze.
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The views expressed here are those of the author and do not represent or reflect the views of RTÉ
