Hay fever imposes a huge socio-economic burden through healthcare, costs of treatments and loss of productivity. It can also have a major impact on the individual in many ways including a decrease in the quality of life and underperformance in education and exams.

Prevalence increased notably between the early 1960s and the mid 1990s2 but the rate of increase slowed down during the last decade to about 1 per cent annually. It is not clear why the UK has such a high rate of hay fever but it is likely that several factors have contributed including clean living (hygiene hypothesis), lack of endoparasites, high use of antibiotics for children in previous decades, diet and air pollution.

The pollen seasons

In the UK some pollen can be in the air in all months of the year and fungal spores can be abundant in the summer and autumn, giving little respite for some sufferers. Depending on weather and location, pollen from early flowering trees can be in the air in December and January. Alder and hazel are the most important of these as they cross react with birch, our most important allergenic tree, and act as primers for this. A severe alder pollen season can lead to more pronounced reactions to birch later in the year. This season affects about 20 per cent of hay fever sufferers and tends to alternate in severity between high and low years due to inherent biological rhythms. It is followed by the oak pollen season which affects about 20 per cent of people with hay fever.

The worst time of the year for most hay fever sufferers is from late May through to early August. This is the grass pollen season which is by far the most important allergenic pollen in the UK, affecting about 95 per cent of people with pollen allergy. Pollen from summer flowering trees and weeds, and spores from some fungi will also be in the air during these months but fewer people are allergic to these. Spore counts can be very high in rural areas and various types are associated with different weather. The main weed pollen season is in late summer, affecting about 20 per cent of sufferers. This overlaps with the main fungal spore season which runs from late September to late November, when basidiospores from mushrooms and toadstools are released to the air in vast numbers.

Although pollen from most species of pine in the UK is not allergenic it can cause mechanical irritation of the eyes and mucous membranes due to its large size. Many people ask about oilseed rape although very few are actually allergic to it. The main problem is caused by volatile organic compounds (VOCs) released by this crop which give it its pungent smell. These irritate the mucous membranes producing sneezing and hay fever-like symptoms. A person could be allergic to one, several, or many types of pollen and spores. But whatever the trigger, the allergic reaction and the potential resulting symptoms are basically the same. 

Monitoring pollen counts

The UK Pollen Network monitors pollen and spores in the air using standard Hirst-type volumetric traps, which are also the standard method over most of Europe and North America. These traps are sited on exposed roof tops and operate continuously to suck in air at 10 l/min. Particles in the airflow adhere to a tape which rotates at 2mm/hour behind a critical orifice to give a time related sample. The tapes for each 24 hours are mounted on microscope slides and the pollen and spores on them are identified and counted. A calculation is made to give the daily average pollen count for each type as grains per cubic metre of air. The data is collated and sent to the National Pollen and Aerobiology Research Unit which uses it, together with other information, in forecasting for the Met Office and national media.

Pollen monitoring sites in the UK have the longest records worldwide, with some having daily pollen count data for over 40 years. The long term data is analysed against meteorological factors and land use for a number of reasons. These include producing pollen forecast models, and helping to interpret the biotic impacts of climate change and the epidemiology of allergic diseases.

The data produced each day through the season is used in the forecast models to inform the public about the conditions for the next few days and to provide information to healthcare bodies and to businesses, such as pharmaceuticals and retail outlets, about the pollen season outlook for the next few weeks. The forecasts are given in terms of risk (eg low to very high, or on a scale of 1-10). People differ in their sensitivity to pollen not only between individuals but also on an individual basis due to things like stress and hydration. Also the pollen exposure experienced in a region will vary with local vegetation and topography. The forecasts provide an overview based on the likely reaction of the majority of people to the general conditions in that area.

The impact of climate change

Pollen seasons in the UK have been altering in response to climate changes in several ways over the last few decades. The most dramatic is that warmer winters and springs in the 1990s and early 2000s have resulted in earlier starts to tree pollen seasons. For example, the mean start of birch pollen seasons has become earlier by about five days per decade over the last 30 years3. Similar trends are evident across northern Europe4.

The situation is different for grasses as flowering in most species is triggered by the longer days of late spring and early summer. For this reason the start of the grass pollen season has not got much earlier, but the seasons are lasting longer due to longer growing seasons. The combined result of earlier tree seasons and longer grass ones means that the period covered by the main pollen seasons has been extended by about four weeks. This is accentuated by the fact that some pollen seasons, including grass, have been getting more severe since the mid 1990s, probably because of changing weather patterns.

Extensions have also occurred in the distributions of some allergenic plants. One with a high impact is ragweed (Ambrosia species), for example, which is highly allergenic. The culprit is not native to Europe but was introduced from North America accidentally to Hungary and to France near Lyon. In 1990 it was limited to these and adjacent areas, but since 1995 it has spread over most of Europe reaching southern Scandinavia5 and becoming more prevalent in the UK, enabled by warm days in late summer which the plant needs to set seed. Sensitivity to Ambrosia pollen has increased in the areas which it has colonised. Climate change also has indirect effects through temperature and drought stresses on plants, which can result in the production of pollen grains with increased allergen load per unit weight or with modified expression of allergens.

Predictions of climate changes cannot be made with any certainty. However, current opinion is that changes will occur over the coming decades leading to a warmer climate for the UK. The ‘Synthesis Report’ of the Intergovernmental Panel on Climate Change (IPCC)6 published in November 2007 predicts an increase of at least two to three degrees in global temperatures based on its mid-range estimate. The Met office and other climate models generally predict that the UK will become wetter in winter and drier in summer, with a mean temperature rise of one to two degrees this century.

Average summer rainfall for 2100 is likely to decrease by 5-20 per cent from present. Average winter rainfall is very likely to increase, providing a soil moisture store for early growth of the grass in the spring and tending to increase the amount of pollen produced. Drier weather in spring and summer will give more days with good conditions for pollen dispersal, leading to more high count days and increased severity of seasons. Overall this will bring longer growing seasons, extending the main hay fever seasons by about four to six weeks and leading to higher incidence of hay fever. This will be compounded by increased prevalence due to sensitisation to new aeroallergens resulting from plant migrations and changes in agriculture. All of these aspects indicate that the demand for treatments and remedies for hay fever will continue to rise for the foreseeable future.


  • Björkstén et al (2008). Worldwide time trends for symptoms of rhinitis and conjunctivitis: Phase III International study of Asthma and Allergies in Childhood in Pediatr Allergy Immunol; 19: 110-124.
  • Devenny et al (2004) Atopy in Aberdeen school children over 35 years. BMJ; 329: 489-490.
  • Emberlin J. ‘Grass , tree and weed pollen’ in Allergy and Allergic Diseases AB Kay (Ed) 2nd edition Wiley-Blackwell (2008).
  • Emberlin J, Detandt M, Gehrig R, Jaeger S, Nolard N, Rantio-Lehtimaki A (2002) Responses in the start of Betula (birch) pollen seasons to recent changes in spring temperatures across Europe. International Journal of Biometeorology; 46: 159-170.
  • European Information Service (EPI): www.polleninfo.org
  • The AR4 Synthesis Report. Intergovernmental Panel on Climate Change:

Professor Jean Emberlin is Director of the National Pollen and Aerobiology Research Unit at the University of Worcester