Guidance

Outdoor airborne allergens

Outdoor airborne allergens are tiny particles in the air that can cause allergic reactions when people who suffer from hay fever or asthma breathe them in. The primary outdoor allergens include pollen and fungal spores.

• From: UK Health Security Agency and Centre for Climate and Health Security
• Published: 7 August 2025

Applies to England

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Topic: environmental exposures
Air-related exposure: airborne allergens
Part of: learning about airborne allergens

Key resources for outdoor airborne allergens:

• Outdoor airborne allergens
  

• Indoor airborne allergens
  

More content

• Bioaerosols and health - video from Leicester University
  

• HECC chapter 6: Outdoor airborne allergic pollen and fungal spores

Pollen

Pollen is produced by plants, and is a significant cause of seasonal allergies such as hay fever. Pollen as a category includes grass pollen, tree pollen (such as birch, alder, hazel, and oak), and weed pollen (like mugwort and ragweed). The pollen season generally has 3 main phases (1):

1. Tree pollen: late March to mid-May

2. Grass pollen: mid-May to July

3. Weed pollen: end of June to September

Climate change is expected to impact the seasonal timings, concentrations, species composition, and allergenic potency of pollen in the UK. Pollen seasons are expected to shift in start date, duration, and severity. Because of heatwaves and drought it is expected that tree pollen will occur in higher concentrations whilst grass pollen seasons become shorter, particularly in southern England.

Warmer temperatures and altered precipitation patterns can at first lead to earlier and longer pollen seasons, potentially increasing the severity of symptoms for allergy sufferers. Over time however, tree pollen output may decrease as trees become stressed from drought, which are predicted to become more frequent. While this might benefit people with pollen allergies, it poses a broader threat to public health in the UK. It is possible that the spatial distribution of plants that cause allergies may change as invasive species become more suited to the UK’s climate. Common ragweed for example, is an invasive species from which a single plant can produce a billion grains of pollen per season that can cause strong allergic reactions. A study which used models to predict ragweed pollen concentration suggests that there will be a four-fold increase in pollen by 2050, which in turn will lead to higher rates of pollen sensitisation (2).

Temperature effects on plant growth depends on the species of plant in question. A warming climate makes it more likely that plants will have their optimal temperatures exceeded (3). Studies have also found that there has been a clear increase in atmospheric pollen, which could not be attributed to temperature but could be influenced by the human-caused increase of carbon dioxide concentrations (4). Another study suggests that the increasing frequency of temperature extremes could contribute to the extension of pollen seasons and explain why pollen is becoming more potent, across the northern hemisphere (5).

Climate change impacts biodiversity and speeds up plant growth rates, which impacts the composition and concentration of bioaerosols. An example of this is ragweed, which has been found to flower more quickly in urban areas that have high carbon dioxide levels, causing them to produce more pollen (2). Different populations are also exposed to different levels of allergens depending on their location, for example there are different composition and concentrations between rural and urban areas.

Fungal Spores

Fungi are an essential part of the ecosystem and are key to breaking down and recycling nutrients from dead material. They are a natural part of our environment. Spores from fungi, including moulds, are prevalent in the outdoor air and can cause allergic reactions. In many places outdoor airborne fungal spores exceed pollen concentrations 100 to 1000 times.

Fungal spores are present in the air throughout the year but their concentrations can vary depending on the season and environmental conditions. The most studied outdoor fungal aeroallergens in the UK are Alternaria and Cladosporium (the only fungal spores for which there are published allergenic thresholds) and Aspergillus. In some countries up to 78% of patients with allergic respiratory symptoms are sensitised to Alternaria. The threshold values for Alternaria and Cladosporium are often exceeded during the UK fungal spore season. Aspergillus is another important fungal allergen in severe asthma and other respiratory conditions. It not only causes allergies but it can also colonise the human airways and can cause infection (6, 7).

For fungal spores, warmer temperatures earlier in the year as well as increased rainfall may result in an earlier start to the season for many spore-producing species, with some expanding their geographic range. Meteorological factors such as temperature, precipitation, and relative humidity significantly influence fungal seasonality. More widespread flooding can also mean an increased prevalence of mould and damp in housing (6, 7).

For a useful overview of how to minimise the health impacts of outdoor allergens see these posters from Allergy & Anaphylaxis Australia.

References

1. Met Office, ‘What’s the Pollen Outlook This Year?’ 2024 (viewed on 29 March 2025)

2. Hamaoui-Laguel L and others, ‘Effects of climate change and seed dispersal on airborne ragweed pollen loads in Europe’ Nature Climate Change 2015: Volume 5 pages 766-771 (viewed on 20 March 2025)

3. Hatfield JL, Prueger JH, ‘Temperature extremes: Effect on plant growth and development’ Weather and Climate Extremes 2015: Volume 10(A) pages 4 to 10 (Viewed on 21 March 2025)

4. Ziello C and others, ‘Changes to Airborne Pollen Counts across Europe’ PLoS ONE 2012: Volume 7(4) (Viewed on 21 March 2025)

5. Ziska LH and others, ‘Temperature-related changes in airborne allergenic pollen abundance and seasonality across the northern hemisphere: a retrospective data analysis’ The Lancet Planetary Health 2019: Volume 3(3) pages e124-e131 (Viewed on 21 March 2025)

6. Anees-Hill S, Douglas P, Pashley CH, Hansell A, Marczylo EL, ‘A systematic review of outdoor airborne fungal spore seasonality across Europe and the implications for health’ Science of the Total Environment 2022: Volume 818 (Viewed on 21 March 2025)

7. UK Health Security Agency, ‘Health Effects of Climate Change Chapter 6: Outdoor airborne allergic pollen and fungal spores’ 2012 (viewed on 18 March 2025)

8. Allergy & Anaphylaxis Australia, ‘How to reduce exposure to environmental allergens’ (Viewed on 8 May 2025)

Published 7 August 2025