The determination of pollen types and their proportions in the atmosphere of relevant urban areas have increasingly been evaluated in different regions. The final goal has been to elaborate a pollen calendar, providing data about the occurrence of pollen grains in the air, thus permitting medical treatment and prophylaxis.
MethodsAn aerobiological study was carried out in the atmosphere of the city of Salamanca, Spain, during eight years (2000–2007) by means of a Hirst type volumetric spore trap. A pollen calendar was elaborated following exponential classes obtained from 10-day average concentrations of the main pollen types.
ResultsMean annual pollen index was 16,916, coming from 72 different types of pollen. During the studied period, an increase of annual pollen levels was registered by means of regression analysis index. The most important types of pollen were Quercus, Poaceae, Cupressaceae, Olea, and Plantago. Arboreal pollen grains (62.7%) were more represented in airborne pollen spectrum than non-arboreal pollen (37.3%). Airborne pollen levels were particularly high between April and July, showing the highest values in May and June.
ConclusionsIn aerobiological terms Poaceae pollen seems to be a major risk for potential sensitised individuals due to its known allergenicity and its high atmospheric concentrations between late spring and early summer, followed by Cupressaceae, Olea and Platanus pollen grains, and taking into account the possible role of Fraxinus and Quercus in earl spring allergenic courses and in processes of cross-sensitivity, respectively.
Pollen is one of the most important sensitising aeroallergens. Individuals sensitised to pollen have increased in the last decades, especially in large cities and industrial areas.1,2 The presence of these airborne particles mainly depends on the kind of vegetation growing in surrounding places, plant phenology, season of the year and meteorological conditions.3 One of the most important preventive measures for sensitised patients is to provide information on the occurrence of different pollen types in order to diminish exposure to pollen allergens when levels are above clinical thresholds. For this reason, pollen calendars have been prepared in many towns around the world.4–9
The aim of this study was to establish the pollen calendar of the city of Salamanca for eight-year pollen records (2000–2007) and to present the annual distribution of the main pollen types in order to describe the atmospheric pollen profile in the Middle West of Spain. In addition, the present study adds information to previous studies on the aerobiology of some taxa in this area.10,11
Material and methodsThe city of Salamanca is located in the Middle West of the Iberian peninsula (40°58′N; 5°40′W), 800m above sea level, with a population of 180,000 inhabitants. The climate is Mediterranean continental12 characterised by cool winters, warm summers and a low annual rainfall level, which determines a drought season during the summer period. The city is surrounded by wide expanses of Mediterranean pastures to the South and West and by arable farming lands to the North-East, being London plane (Platanus hispanica Miller ex Münchh.) a tree-shape urban element most used as ornamental.
The aerobiological monitoring was performed using a Hirst type volumetric spore trap (Burkard 7-day spore trap), situated on the roof of a historical city centre building (Urban Board, Salamanca City Council) at a height of 20 metres above ground level. The sampler operated continuously from January 1st 2000 to December 31st 2007 with a flow of 10 litres per minute. Methods were standardised according to the Spanish Aerobiology Network recommendations for sampling, slide preparation and pollen counting13. Annual pollen index is an annual sum of mean daily pollen concentrations14 (expressed as pollen grains per cubic metre) and is used as a synonym of annual pollen counts.15 Pollens were identified using published keys16,17 and by comparison with a local pollen collection.
The pollen calendar was constructed following the technique adopted by Spieksma18: daily pollen levels of 10 days were summed and averaged over the eight years considered. These average sums were placed in exponential classes and depicted by columns of increasing height in the calendar. In addition, a five-day running mean was calculated during the studied period and plotted to assess the seasonal trend for total pollen content. Statistical analysis was carried out by SPSS (v.12) software package, applying regression analysis to assess the variation of annual pollen indexes. Lastly, in order to evaluate the airborne permanence of main pollen types, we used the classical term of Main Pollen Season (MPS) that delimits the period during which most pollen is recorded and in that case using 95% of the total annual pollen count.19
ResultsDuring the period 2000–2007 a total of 72 different types of pollen were identified, with the most abundant types being included in Table 1. The mean annual pollen index obtained in Salamanca during the studied period was 16,916. An increase in airborne pollen levels was observed by the index of regression analysis (R2=0.81) (Table 2). The highest seasonal values were recorded in late spring between May and June (35% and 27% average percentage of mean total pollen during the eight years analysed, respectively) and the lowest in autumn months (1.5% during October, November and December). The seasonal course of airborne total pollen grains is presented in Figure 2, by means of the average of the five-days running mean, showing an increase from early February until mid March, where levels decreased softly. In the first weeks of April, pollen counts rose until early May, when airborne pollen concentrations increased abruptly until early June. From mid June to late September, pollen levels decreased gradually, showing low concentrations during autumn weeks. Total arboreal pollen count was higher than non-arboreal (62.7% vs. 37.3%).
Aerobiological data for main recorded types of pollen in Salamanca during years 2000–2007
| Main pollen types | Annual Pollen Index1 | % over total pollen grains | Date of Peak day (value2) | MPS Length in days |
| Quercus | 4459 | 26.4 | 27/5/05 (904) | 61 |
| Poaceae | 3626 | 21.4 | 30/6/07 (355) | 151 |
| Cupressaceae | 1823 | 10.8 | 4/3/07 (550) | 154 |
| Olea | 792 | 4.7 | 17/5/06 (429) | 50 |
| Plantago | 661 | 3.9 | 13/7/07 (159) | 103 |
| Pinus | 648 | 3.8 | 14/4/05 (132) | 94 |
| Populus | 606 | 3.6 | 26/3/06 (271) | 39 |
| Platanus | 602 | 3.6 | 16/4/07 (297) | 34 |
| Rumex | 588 | 3.5 | 15/5/06 (75) | 90 |
| Urticaceae | 529 | 3.1 | 31/3/06 (59) | 217 |
| Castanea | 282 | 1.7 | 24/6/05 (78) | 40 |
| Chenopodiaceae | 197 | 1.2 | 9/8/06 (22) | 123 |
MPS: Main Pollen Season.
Statistical analysis by means of regression analysis for the total pollen counts, and arboreal and non-arboreal totals during the studied years
| Statistical analysis | Regression analysis [y=b1x+c] | |||||
| R2 | F | d.f. | Sig. | Const | Coef.b1 | |
| Total pollen | 0.81 | 24.94 | 7 | 0.002 | 1.5675 | 0.0013 |
| Arboreal pollen | 0.81 | 25.686 | 7 | 0.002 | −185.86 | 2079.7 |
| Non-arboreal pollen | 0.625 | 9.999 | 7 | 0.02 | 931.32 | 1037.2 |
R2: Determination index; F: Mean Squares statistic; d.f.: Degrees of freedom; Sig.: Significance; Const: Constant; Coef.b1: Coefficient b1.
Arboreal pollen peak days were also higher than non-arboreal (Table 1), with 904pollen/m3 on 27 May, 2005 for Quercus; 550p/m3 on 4 March, 2007 for Cupressaceae; and 429p/m3 on 17 May, 2006 for Olea. Non-arboreal taxa did not exceed 100p/m3 in the studied period, except for Poaceae, with the highest value (355p/m3) on 30 June, 2007; and Plantago (159p/m3) on 13 July, 2007.
A total of 32 pollen types were individually represented in the pollen calendar (Figure 3), reporting only pollen types reaching a 10-day mean pollen sum equal to or higher than 1p/m3. Three principal pollen seasons could be identified in the city of Salamanca on the basis of the predominance of different pollen types: winter season (Alnus, Cupressaceae, Fraxinus, Ulmus and Populus); spring season (Urticaceae, Pinus, Platanus, Quercus, Plantago, Betula, Rumex, Acer, Ericaceae, Olea, Ailanthus, Echium and Poaceae); and summer season (Castanea, Chenopodiaceae and Artemisia). There were no specific pollen types in the last months of the year, having no autumn season in the pollen calendar.
By means of the length of MPS, we evaluated the permanence of different pollen types in the atmosphere as shown in Table 1, having Urticaceae the longest MPS, followed by Cupressaceae, Poaceae, Chenopodiaceae and Plantago. The remaining pollen types presented a duration of 50 days or less, except Pinus, Rumex and Quercus.
DiscussionThe mean annual pollen index (16,916) recorded in Salamanca between 2000 and 2007 was lower than the mean annual total found in other European sites, such as Zagreb20 and other Spanish cities.21,22 The pattern was similar to that of other Spanish cities located around 200 kilometres South or South-East from Salamanca, such as Madrid23 and Cáceres24; but different to cities situated at the same distance to the North, such as León25 or even further afield, like Santiago de Compostela26 whose mean annual total was lower than that registered in Salamanca. In addition, the airborne pollen spectrum of Salamanca and percentages of each pollen type over total registered were different to those of other Iberian cities, probably due to the kind of vegetation present in different regions of the Iberian Peninsula, in addition to the composition and management of ornamental trees in urban environments.27
The increase of airborne pollen counts in the atmosphere of Salamanca during the 2000–2007 period, showed by the index of regression analysis (Figure 1; Table 2), was more accentuated in the case of arboreal pollen grains than in the case of non-arboreal pollen. In other European cities28,29 an increase in pollen levels coming from main arboreal species, such as Betula or Platanus, has also been observed. A similar fact has been described in the Iberian Peninsula30,31 for the case of Quercus and Olea. The increment in pollen concentrations could be related to the known effect of climate change on higher pollen levels and subsequently a great number of allergic processes.32,33
Regarding the main allergenic pollen types in the Mediterranean area and in the Iberian Peninsula34,35 there was a potential risk for allergic patients in the case of Poaceae, because of its high levels between May and July and its long permanence in the atmosphere; Platanus, whose pollen grains were located during few weeks but with important daily concentrations in April, and Olea with also high pollen counts in May. The presence of Fra e 1, homologous of Ole e 1 from Fraxinus36, could explain some of the early allergic courses that appear during February. Other possible symptom eliciting allergens could come from Cupressaceae pollens, whose levels are also high in late winter. Despite its quantitative first position among identified pollen types in Salamanca, Quercus could only be responsible for allergic symptoms when airborne pollen grains are abundant37, taking into account processes of cross-sensitivity with other pollen types38. This fact coupled with recent studies reporting monosensitisation to Quercus pollen39, pointed out its possible role in sensitised individuals living in the Salamanca area.
This study was partially supported by Junta de Castilla y León, Project Code SA091A07, Public Health Council of Junta de Castilla y León agreement code H42, and by Salamanca City Council. The first author would like to thank the University of Salamanca for his research grant. Authors thank anonymous reviewers for their useful comments which have improved the final manuscript.
