Modelled percentage of population sensitized to ragweed pollen at the baseline (left) and in the future assuming moderate greenhouse gases emissions scenario (RCP 4.5; right)

Source: Lake et al., 2017

Health issues

Thousands of plant species release their pollen into the air every year. The impact on human health is primarily evident in allergic diseases since exposure to allergens from airborne pollen or their inhalation can trigger allergic responses of nose (allergic rhinitis, commonly known as hay fever), eyes (rhino conjunctivitis), and bronchi (bronchial asthma). The prevalence of pollen allergy in the European population is estimated at 40%, making it one of the most common allergens in Europe (D’Amato et al., 2007). Even low pollen concentrations in the air can already induce allergy symptoms in highly sensitive persons. The allergic reactions to pollen are an important cause of sleep disturbance, impaired mental well-being and decreased quality of life, productivity loss or lower school performance for children, and associated healthcare costs. The great majority of allergy patients (90%) is thought to be un- or maltreated, despite the fact that appropriate therapy for allergic diseases is available at rather low costs (Zuberbier et al., 2014).

The role of pollen in the development and severity of allergic diseases depends on numerous factors, including the duration of exposure (related to the length of the pollen season and the time spent in allergenic environment), intensity of exposure (related to the pollen concentration in the air) as well as the allergenicity of the pollen. These factors have a large geographical and temporal variability, which results in differences in the prevalence of pollen-associated allergic rhinitis between locations and periods (Bousquet, 2020).

In Europe, grasses (Poaceae family) are the major cause of allergic reactions due to pollen (García-Mozo, 2017) given their wide geographical range. Among trees, the most allergenic pollen is produced by birch in north, central and eastern Europe, and by olive tree and cypress in the Mediterranean regions. Allergenic pollen is also produced by several herbaceous plants. Ragweed (Ambrosia artemisiifolia) requires special attention as a potential, extremely allergy-inducing invasive species in Europe.

Pollen allergies are typically highly seasonal. In most European countries, the main pollen season, covering pollen releases of various plant species, spans about six months, from spring to autumn, with geographical differences depending on the climate and vegetation (Bousquet, 2020). The European Academy of Allergy and Clinical Immunology (EAACI) defines the start of the pollen season for various species based on pollen concentrations in the air that affect human health. The start of the grass pollen season, for example, is defined when 5 out of 7 consecutive days carry more than 10 grass pollen grains/m³ air, and the sum of pollen in these 5 days is more than 100 pollen grains/m³ air (Pfaar et al., 2017). Emergency department visits and hospitalizations increase when grass pollen concentrations exceed 10 and 12 grains/m³ air, respectively (Becker et al., 2021). Similar criteria exist for birch, cypress, olive and ragweed (Pfaar et al., 2020).

The risk of allergy depends on the concentration of pollen in the air. However, the number of allergens released by a pollen grain (reflected in the so-called pollen allergen potency) can vary depending on the region, season, atmospheric pollutants, humidity and storm periods (Tegart et al., 2021). Pollen grains release, besides allergens, a wide variety of bioactive substances including sugars and lipids. When these substances are inhaled, they can also stimulate allergic reactions and determine the severity of the allergic reaction to pollen (the so-called pollen allergenicity) (Gilles et al., 2018). Additionally, the allergenicity of certain pollen species can be enhanced by environmental factors such as air pollutants. Long-term high NO2 levels in urban environments are associated with increased allergenicity of pollen of a number of species including birch (Gilles et al., 2018; Plaza et al., 2020). Also ozone could enhance allergenicity (Sénéchal et al., 2015). Hence, the combined exposure to air pollutants and allergens can have a synergistic effect on both asthma and allergy (Rouadi et al., 2020).

Pollen exposure may also cause inflammation of mucosal membranes, hence increasing the likelihood of respiratory infections, even in non-allergic persons (Becker et al., 2021). A study by Damialis et al. (2021) tested the correlation between Covid-19 infection rates and pollen concentrations during the first pandemic wave in spring 2020, while accounting for confounding factors like humidity, temperature, population density and lockdown measures. Pollen concentrations were found to explain on average 44% of the infection rate variability with higher rates at higher pollen concentrations (Damialis et al., 2021).

Observed effects

In the past decades, the prevalence of pollen-induced allergies has increased in Europe. This increase cannot solely be explained by changes in the genetics or health conditions of the population (D’Amato et al., 2007, 2020; Becker et al., 2021). The increase in prevalence of these diseases may be related to improved hygiene, increased antibiotic use and vaccination, and changes in lifestyle, dietary habits and air pollution (de Weger et al., 2021). In addition, climate change affects exposure to pollen and allergic sensitisation in several ways, including shift and prolongation of the pollen season, changes in pollen concentration and allergenicity, as well as shifts in geographical distribution of pollen.

Pollen: seasonal shifts and prolongation of season

Both the onset and the duration of pollen seasons are driven by meteorological variables, mainly temperature. In response to global warming, plants shift the timing of their developmental stages, including flowering and pollen release. A comprehensive study of global pollen datasets highlighted increases in pollen season duration (on average by 0.9 day per year) and pollen load over the last 20 years (Ziska et al., 2019). In urban areas, where most of the Europeans live, the higher temperatures exacerbated by the urban heat island effect, lead to earlier pollen season starts (D’Amato et al., 2014). Based on air temperature data, the Copernicus Climate Change Service visualizes the birch pollen season onset from 2010 to 2019, showing regional differences in the advancement of the start of the pollen season. Nevertheless, also radiation, precipitation and humidity affect pollen release and transport in the air, albeit less than temperature.

Pollen: concentration and allergenicity

Warmer conditions and elevated atmospheric CO2 concentrations stimulate plant growth. This can increase the pollen and allergen concentrations in the air, as well as the pollen allergenicity, which increases the risk for allergic reactions (Beggs, 2015; Ziska et al., 2019). Also altered humidity conditions, weather extremes and thunderstorms during the pollen season cause higher pollen and allergen concentrations in the air, which lead to more severe allergic reactions and asthma attacks (Shea et al., 2008; Wolf et al., 2015; D’Amato et al., 2020).

Pollen: geographical shifts

Global warming and the associated lengthening of the growing season facilitate a northward migration of invasive plant species in Europe, also those releasing allergenic pollen. The introduction of new allergens can increase local sensitisation, i.e., the process of people becoming sensitive or allergic due to exposure to allergens (Confalonieri et al., 2007). A particular example is Ragweed (Ambrosia), introduced in Europe several decades ago from the American continent with transport. Ragweed pollen is highly allergenic and released relatively late in the season (early September), potentially causing an additional wave of allergy and a lengthening of the allergic season (Vogl et al., 2008; Chen et al., 2018). Significant health and economic impacts in areas invaded by ragweed in Central and East Europe, France and Italy have already been reported (Makra et al., 2005). While the spread of ragweed in Europe is mainly driven by transport and agricultural activities, climatic changes facilitate the colonization of new areas. In addition, ragweed pollen grains can be easily transported hundreds to thousands of kilometres by air, hence causing peak pollen counts and associated allergy symptoms in areas where ragweed is not yet widespread (Chen et al., 2018).

Projected effects

The impacts of climate change on pollen seasons, concentrations and allergenicity are expected to lead to increased exposure of the European population to pollen and aeroallergens in the future. This will increase the likelihood of new allergic sensitisations, also for originally weak allergens (de Weger et al., 2021). Under the medium greenhouse gas emissions scenario (RCP 4.5) ragweed sensitisation is expected to spread across Europe and increase in some countries up to 200% by 2050 (Lake et al., 2017).

In already sensitized individuals, the duration and severeness of allergic symptoms is expected to increase under climate change due to longer pollen seasons and higher pollen allergenicity. If the period during which people are exposed to pollen prolongs, allergen avoidance as a coping strategy will become more complicated, affecting mental wellbeing.

The climate-driven changes in aeroallergens and associated triggered allergic reactions are projected to have implications for asthma prevalence and the associated medical costs (medication, emergency hospital visits) (Anderegg et al., 2021). Moreover, high temperatures and heatwaves, expected to increase in frequency and duration under the changing climate, aggravate respiratory problems and increase mortality for those suffering from asthma and other respiratory problems that result from allergies (D’Amato et al., 2020). Also, people’s susceptibility to viral infections may increase through exacerbating respiratory inflammation and weakening immune responses caused by allergens and pollen (Gilles et al., 2020).

Green infrastructure in cities, installed as climate change adaptation measures, may also increase the pollen loads and allergic reactions in the future (Cheng and Berry, 2013). A case-study in 18 green spaces in Brussels demonstrated that the allergenic potential of urban parks is expected to double as a result of combined changes in the duration of the pollen seasons, the allergenicity of pollen, and the sensitisation rates of the population (Aerts et al., 2021). Consideration of suitable tree species for urban environments is crucial when designing climate adaptation measures and engaging in spatial planning in order to avoid exacerbation of allergy risks.

Policy responses

Pollen concentrations of various trees and grasses are routinely monitored in all European countries. The measurements are used to determine the start and duration, as well as the intensity, of the pollen season. The measurements, in combination with chemical transport models, are also being used to set up allergy risk systems used in pollen information or early warning systems. The polleninfo portal, originating from a partnership between the European Aeroallergen Network and the Copernicus Atmosphere Monitoring Service (CAMS), provides daily updated pollen concentration forecasts and allergy risk assessments for all European countries.

In contrast to the pollen level, no routine measurements exist at the allergen level, neither for the number of allergens in a pollen grain, nor for allergen concentration in the air. Having access to this type of indicator would nevertheless help to explain the occurrence of pre-season allergy symptoms, especially in conditions in which high air pollution levels coincide with low pollen concentrations (Cabrera et al., 2021).

Setting general thresholds of pollen concentrations relevant across populations is difficult, as health effects depend also on a person’s sensitivity (Becker et al., 2021). Still, pollen information services can support individual patients to avoid negative health outcomes, especially when combing pollen monitoring and documentation of precise individual symptoms. For example, smartphone applications that combine individual symptom data and pollen concentrations could be used to determine personal pollen thresholds and reduce health impacts more efficiently (Becker et al., 2021).

Diagnosis, management, and coping

Pollen allergy is underdiagnosed and often un- or maltreated. Hence, awareness raising about the impact of allergies is needed to help people recognize, prevent and manage allergy symptoms. It is necessary to diagnose the type of pollen causing the allergy and start allergy medication before the start of the pollen season. During the pollen season, symptom prevention and coping is mainly based on avoiding exposure to allergens. Recommendations range from avoiding being outdoors, wearing sunglasses, avoiding drying clothes outside, keeping windows closed, and others. The EAACI has a dedicated website for patients with recommendations, and several countries also have national patient organizations that can advise allergy patients.

Spatial planning considerations

Establishment of hypoallergenic green spaces in and near cities, through careful tree species selection (Aerts et al., 2021) can reduce the prevalence of pollen allergies. Which tree species is suitable, depends on the locality, and the choice should consider the projected climatic changes. Removal of allergenic trees from existing green spaces is not recommended, to preserve biodiversity and ecosystem services, among others supporting adaptation to high temperatures under climate change (Aerts et al., 2021).

Control measures

The recent invasion by common highly allergenic ragweed (Ambrosia) prompted several European countries to develop and implement chemical and mechanical control methods. Also, the EU Directive 2002/32/EC on undesirable substances in animal feed establishes a legal standard for the concentration of Ambrosia seeds in feed to prevent further spread of the plant. Similarly, seed mixtures for birds must not contain more than 50 milligrams of Ambrosia seeds per kilogram.

Deploying a biological control agent against Ambrosia, such as the North American leaf beetle, could reduce the ragweed occurrence in Europe and lower the number of patients by approximately 2.3 million and the health costs by Euro 1.1 billion per year (Schaffner et al., 2020). However, the introduction of biological control agents can have negative effects on biodiversity by damaging non-target crops and native plant species and should be approached with caution.


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