Health Effects of Climate Change (HECC) in the UK: 2023 report. Chapter 6: Outdoor airborne allergenic pollen & fungal spores.

Aeroallergens are airborne particles that can cause or exacerbate allergic disorders including pollen and fungal spores. Aeroallergens can trigger hay fever and exacerbate asthma which affects about 11% of the UK population. Chapter 6 considers the seasonality of these allergens and how they may be impacted by climate change. The chapter was led by academic experts from University of Leicester, with contributions from the University of Worcester, University of Exeter and UKHSA. Weather and climate are well-recognised drivers of aeroallergen production. Climate also impacts atmospheric transport of pollen grains, including atmospheric transport of allergenic pollen from the continent. There is thus significant potential for a changing climate to shift the start-date, duration, and severity of pollen seasons and associated health risks. The authors present new empirical analyses assessing the relationship between fungal spores and temperature using a 52-year data set (1970 to 2021) and review the published evidence and surveillance data for oak, alder, birch, and grass pollen at 6 sites across the UK (1995 to 2020). The link between aeroallergen production, seasonality and temperature differs widely between species. Current evidence outlined in this chapter suggests that there has been a significant trend towards higher concentrations of pollen (such as birch) or increased length of the pollen season (such as oak). Other species (such as alder and grass pollen) show a mixed picture, however for grass pollen, this is likely due to the high number of different grass species and interacting variables that affect these seasons. The occurrence of the first high day for grass pollen annually is getting earlier, however, and heatwaves are predicted to shorten the season duration. The authors note that trends over time are most pronounced in the Midlands; however, existing surveillance provides early indications only, and data is still relatively limited. The impact of climate change on pollens is likely to be mixed and vary considerably across the UK for different species and based on level of warming. In future decades, the first high pollen day is likely to occur earlier for alder, oak and grass pollen, while alder and birch pollen seasons are expected to continue to increase in severity in the Midlands and further north and west over the next 2 decades. In contrast, trees in the south and the east of the UK are likely to become stressed due to increased frequency and severity of heat and drought, which is expected to reduce pollen output and duration of the pollen season. Grass and nettle family pollen seasons are not expected to increase or decrease over time. It is likely that pollen potency will increase and this will enhance the season for hay fever sufferers in most years, although this may decline from the 2030s and with higher levels of warming. The authors’ assessment of trends in fungal spores found an earlier start of the season for all spores, partly associated with warmer temperatures in spring and summer coupled with higher precipitation. In a warmer and wetter future climate, there could be a further advance in the start of the season for many spores. Notably, the authors highlight the potential for interactions between pollutants and airborne fungal spores. Exposure to some urban air pollutants has been shown to increase the allergenicity of some fungal spores. The authors suggest that there are likely to be health benefits for allergy suffers under decarbonisation scenarios involving electrification of transport and associated air pollutants, though there is limited evidence to quantify possible health co-benefits. The results presented in this chapter highlight the relationship between aeroallergen species and climate, with several implications for public health. Firstly, earlier and prolonged pollen seasons may increase population exposure to airborne spores and extend the allergy season, meaning that hay fever and allergy sufferers may suffer symptoms earlier and for longer periods of the year. These trends will be highly variable by region and species; therefore, aeroallergen forecasting, preparedness, and response will need to be highly localised. For example, local health organisations should provide information in locally appropriate ways, outlining the risks, protective behaviours and support. Communication pathways should also exist to warn and inform residents, in addition to professionals. Secondly, it is possible that where temperatures reach levels high enough to cause pollen-producing species to wither or die, this will result in reduced aeroallergen exposure resulting in fewer hay fever and allergy symptoms. This is most likely in the south and west regions of England and at higher levels of warming.

The results in this chapter highlight several research gaps and priorities, including the need to: develop allergen-specific and highly localised public health forecasts, given that weather and climate impacts on aeroallergens range widely between species; continue advancement of taxa-specific forecasts and research to support these as the climate changes will remain a priority; build the evidence on how airborne fungal spores interact with air pollutants and how decarbonisation strategies could maximise health co-benefits associated with aeroallergens; develop the evidence-base to inform urban planning and green infrastructure development on the implications of alternate designs for aeroallergen production, including potential health co-benefits and trade-offs.