Climate is a major determinant of regional seasons of allergenic pollen. In boreal and temperate vegetation zones cold period limits the pollination schedule of plants to certain months, while in warmer climate allergenic pollen may, in addition to periodical peaks, be present in the air all year round (Fang et al. 2001; Hurtado and Alson 1990; Murray et al. 2002; Savitsky and Kobzar 1996). Arid climate may also confine the pollen season to a few months (Halwgay 1994). More specificities arise from edaphic factors, land-use and cultivation, which all affect the distributions and productivity of allergy plants. Extensive geological formations, such as mountains, have their own unique vegetation and pollen flora. Also urban settlements with their characteristic temperature, land-use, high CO2 concentration and light environment are specific habitats for allergy plants (Ziska et al. 2004).
In boreal and temperate zones (after Allaby 2002) the local pollen season is created by three groups of wind-pollinated plants with different seasonality of pollination: (1) early-flowering trees and shrubs, (2) annual and perennial herbaceous plants pollinating during late summer/early fall near the end of the growing season, (3) herbaceous plants emitting pollen during several months or throughout the growing season (Edmonds 1979).
The first group - trees and shrubs flowering at the beginning of growing season or even in winter - start the season of allergenic pollen. Many tree genera with circumpolar distribution, such as birch (Betula), alder (Alnus) hazel (Corylus), poplar (Populus), oaks (Quercus) and mulberry (Morus), belong to this group. Hickory and Walnut are important spring-time allergenic plants in North America. Taxa belonging to Cupressaceae release pollen already during winter months in the Mediterranean region and in the Southern Great Plains, USA (Juniperus), as does the most important allergy plant in Japan, sugi (Cryptomeria). Olive (Olea) is an important allergy plants in springtime in the Mediterranean basin.
The duration and magnitude of pollen liberation of different species vary, but at a local scale a tree population may, under favourable conditions, release pollen during a relatively short period of time, resulting in sharp peak of high concentrations of airborne pollen. Because many species in this group represent the common regional forest, ornamental or cultivated trees, pollen concentrations may be very high during these peaks (Fig. 5.4).
The second group - annual and perennial herbaceous plants pollinating during late summer and autumn - contain two main allergenic taxa: ragweed (Ambrosia) and mugwort (Artemisia). Ragweed is the etiological agent in half of pollinosis cases in the US and its significance is increasing in many areas. Most ragweed species originate from North America. During last 80 years they have distributed over Australia and Japan, western parts of South America and Europe, limiting to south Scandinavia and Asian parts of Russia (Makra et al. 2005). Outside their natural habitats (Hall 1994), both ragweed and mugwort are weeds, and readily invade habitats recently disturbed by man, for example wastelands, abandoned fields and road sides, which brings them near the human population.
At a local scale ragweed and mugwort release pollen during several weeks of time per year (Fig. 5.4). Counts of airborne pollen are usually lower than those of common forest trees, but, for example, in Hungary where ragweed is abundant, daily average of 1,000 grains m-3 has been exceeded several times (Makra et al. 2005). It must be also noted that the concentrations of mugwort pollen may
March April May June July Au
Fig. 5.4 The seasonal development of airborne pollen counts (95% probability) in South Finland, boreal zone of Europe; yellow = alder, green = birch, red = grasses and blue = mugwort
Fig. 5.4 The seasonal development of airborne pollen counts (95% probability) in South Finland, boreal zone of Europe; yellow = alder, green = birch, red = grasses and blue = mugwort be underestimated at ground level, since the pollen grains of this species are heavy and do not readily rise to the position of the elevated pollen traps.
The third group is somewhat artificial and includes taxonomic groups, such as grasses (Poaceae), goosefoot (Chenopodium) and pigweed (Amaranthus), with many cross reacting species flowering in different times during the growing season. Human activities greatly affect the distributions and abundance of many of these species: important allergic genera as fescue (Festuca), rye grass (Lolium), blue grass (Poa), Bermuda grass (Cynodon), bent (Agrostis) are pasture grasses and/or cultivated as forage or lawn. Many of them, like orchard grass (Dactylis glomerata) and timothy (Phleum pratense) have been introduced from Europe and naturalised in North America and other continents.
Other examples of allergenic taxa releasing pollen during the whole growing season are pellitory (Parietaria) predominantly growing in the Mediterranean climate but found up to the UK, and cosmopolitans goosefoot (Chenopodium) and pigweed (Amaranthus). The latter are noxious weeds and invade recently disturbed soils. Both genera contain several cross-allergenic species flowering during different times of the growing season (Edmonds 1979; Fang et al. 2001; Hall 1994; Hurtado and Alson 1990). Pellitory-of-the-wall (Parietaria judaica) predominantly grows close to human habitats, in wall fissures and disturbed soils. Individual plants consist of shoots arising from common root-stock. Although most shoots flower during spring, some individuals have flowering shoots nearly a year round, which makes the pollen season practically continuous in some regions (Guardia and Belmonte 2004).
Many grass species are cross reacting, so even if individual species flower during a certain time period (specific for the particular taxa), the overall pollen season of grasses lasts several months (Zanotti and Puppi 2000). In East Australia and other southern hemisphere regions with similar climate such season lasts over 200 days (Green et al. 2004), while in temperate climate the period of high pollen concentrations is normally confined to a couple of months (WHO 2003). During the main season, counts over 1,000 grains m-3 of airborne grass pollen can be exceeded in some areas, UK for example, in peak season (Emberlin et al. 1999).
Pollen allergens are present not only in grains themselves but practically in all parts of the plant, especially when the pollination time approaches. The allergens are found, for instance, in small-sized (1-4 |im) particles called orbicules (El-Ghazaly et al. 1995). The orbicules (or Ubisch bodies) are sporopollenin granules stored together with pollen in anther lobes and often released before the liberation of the pollen itself. These small-size particles contain the same allergens as pollen and may penetrate deep into human airways, down to the alveoli triggering the allergic reactions (El-Ghazaly et al. 1995). Concentration of such small-size allergenic particles in the air can increase up to 2 weeks before the main pollen period, which may explain the early allergic reactions occurring a few days prior to flowering (Matikainen and Rantio-Lehtimaki 1998). Allergens are also present e.g. in birch leaves and in the sticky exudates surrounding bursting leaf buds and thus may cause problems to allergic people when branches are used as decoration indoors. The presence of allergens in grass leaves may also be a cause of allergy symptoms that may occur in connection with lawn mowing, although this does not seem to be thoroughly investigated.
Many countries have national networks of pollen monitoring stations. Currently available forecasts of pollinating season are mainly based on local observations together with information about the weather, growing seasons and typical flowering times of the local plants. Many regional or site-specific forecasting models based on regression of empirical pollen data sets against weather variables have been published in recent years (WHO 2003, numerous local regression models for trees, grasses, etc.). In particular, in Austria the local predictions for the onset of pollen seasons of alder, hazel and birch are based on thermal phenological models (http:// www.polleninfo.org). The National Pollen Research Unit, UK, provides regional pollen forecasts for several timescales and characteristics of the pollen season. The information on season start, expected severity and duration is released a month in advance. Medium range (5 days ahead) and short-range forecasts are used to predict day-to-day variation of concentrations once the season started. The techniques are based on regression of empirical pollen data against weather variables, as well as on local vegetation information (Emberlin et al. 1999; Adams-Groom et al. 2002). EU project A.S.T.H.M.A was an attempt to create regional service of short-term forecasts of pollen concentrations and health risks (http://www.enviport.com). In Denmark, a statistical model based on empirical pollen data and weather parameters is used to estimate the pollen amount of birch (Betula), grasses (Poaceae) and mugwort (Artemisia) for up to 48 h ahead (Janne Sommer, Danish Asthma and Allergy Association, personal communication). In Tulsa (Oklahoma, USA) a trajectory model is being used to predict the regional scale transport of Juniperus ashei pollen (http://pollen.utulsa.edu).
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