Indoor bioaerosols have been investigated worldwide, mainly because we humans now spend most of our time indoors, the fraction for industrialized countries ranging from 80% [1,2] to 95% [3]. These studies have mainly been concerned with airborne fungi, while the indoor airborne presence of pollen grains has received less attention. Furthermore, pollen allergens in indoor air could occur predominantly as particles of smaller diameter than pollen grains [4].

Although the term “sick building syndrome” has been used to denote the origin of indoor allergic and environmental illnesses, the main biological causes of building-related disorders are fungi, bacteria, viruses, protozoa, pollen, house-dust mites, insect pests, algae, pigeons, and rodents [5,6]. It is recognized that airborne allergies cause more problems worldwide than all other allergies combined. Furthermore, exposure to substances found in the indoor environment is known to trigger such respiratory illnesses as asthma. Studies of the association between human illness and environmental exposure focused initially on the outdoor environment in the form of pollen and mould counts. The emphasis has now shifted, however, to exposure in indoor environments [7,8], and although pollens as bioaerosols are often not considered in air pollution studies their concentration may measured in air quality standards [9].

Different proposals have been made to reduce or eliminate indoor bioaerosols, with efficiencies close to 100%, basically by filtering the air with electrostatically charged activated carbon [4,10] or using HEPA (high efficiency particulate air) filters [11,12]. Nevertheless, it has also been argued that the simple use of air-conditioning can reduce indoor bioaerosols [13].

Most indoor airborne pollen research has been on homes [13–19], including mobile homes [3]. Other contexts that have been investigated are workplaces [4,20] and schools [4]. Work on airborne pollen in hospital environments has been less frequent [21].

Most researchers agree that there is a close relationship between outdoor and indoor pollen, although there are some discrepancies [14,20]. There are various possible routes by which pollen may enter the indoor airborne environment, such as through the ventilation system [22], but there is a trend of thought that humans themselves are the vector, either transporting these bioaerosols [23] adhered to their clothing [16] or stirring them up from the floor where they are abundant, as people walk about inside a building [19,24–26].

Although there are techniques than permit sampling allergens in the air [27] quantification of indoor airborne pollen continues as accurate method of study. Various samplers have been used to detect indoor airborne pollen. Gravimetric samplers were commonly used initially, but have progressively declined in popularity [28–31]. Personal volumetric samplers are extensively used, examples being the Rotorod [3,15,19,32], the button inhalable aerosol sampler [33], Lanzoni [20,21], and the Burkard personal sampler [17,22]. In some cases, a fixed or continuous sampler has been used to compare indoor and outdoor sampling data, examples being the Burkard seven-day [18] and the Lanzoni [20] samplers.

The aim of the present work was to sample the indoor atmosphere of a hospital to determine the temporal and spatial variation in pollen concentrations, to compare these concentrations with outdoor measurements, and then to assess the possible factors that might influence the presence of pollen, such as the level of isolation of the room, the number of people transiting the room, the weather, and seasonality.

The average I/O ratio found in the present work (0.27) is higher than most values reported in the literature: 0.02 [19,25], 0.025 [33], 0.041 [24], 0.05 [15,20], 0.083 [29], and 0.24 [3]. But it is lower than one case: 0.33 [14]. These differences could be because the present study corresponded to a place in which there is a great movement of people, and the entrance doors are open much of the time. The summer I/O ratio was the highest (0.39), and the spring ratio the lowest (0.23). This is coherent with the findings of Sterling and Lewis [3], who suggested that an explanation for their results could be that air-conditioning, which is most often used in summer, is an entrance route for pollen, or that windows and doors were more likely to be open, thus permitting the entrance of pollen.

The indoor pollen concentration found (25.1 grains/m³) is also high compared to most literature values: 1.2 grains/m³ [15,19], 10 grains/m³ [32], 16 grains/m³ [18]. But it is lower than one case: 71 grains/m³ [33]. The vegetation surrounding the places sampled in those papers is not reported, but in our case there were abundant ornamental trees and shrubs, mainly Cupressaceae (Platycladus orientalis, Cupressus arizonica, C. sempervirens), which would be the origin of the main pollen types found. One must therefore accept that major mixing of indoor and outdoor air is occurring, which is one of the most serious problems of indoor air quality [36].

Like other workers [18,20], we found that the concentrations measured with the continuous sampler were higher than with the personal sampler. This could be because the outdoor site selected for sampling with the personal sampler was at the entrance to the hospital, and protected from the wind. The resulting relative lack of air circulation could therefore have reduced the presence of pollen. As was mentioned above, the exception was the ornamental Cupressaceae, for which the outdoor concentration measured with the personal sampler was higher than with the fixed sampler. A phenological study was conducted for this pollen type in the period 2008–2009 (unpublished data), finding that the highest indoor concentration found in the hospital coincided with the maximum peak for Platycladus orientalis, the most abundant ornamental tree around the hospital, in February 2008.

The correlation found between the number of people moving through the entrance and the outdoor pollen counts would seem to support the idea that the pollen is transported by people walking [19,24,25,26] or by a re-suspension process that depends directly on particle size [37]. However, the outdoor sampling was made about ten metres away from the entrance, so that only simultaneous sampling immediately outside the entrance could support or refute this idea. Furthermore, no correlation was found between the number of people present in the wards and the pollen count.

Although people could be the vectors responsible for a certain amount of indoor pollen carried from outdoors to indoors, we consider mainly the doors and windows, and then the air-conditioning system, to have been the principal routes of the entrance of pollen. The indoor pollen concentrations reached somewhat more than a quarter of the outdoor concentration, with peaks as high as some hundreds of pollen grains per cubic metre on some occasions. This pattern depended on the vegetation surrounding the building—mainly ornamental trees and shrubs. Indoor air circulation could have been the reason for there being no differences between indoor open wards and isolated rooms, and even between independent floors. The outdoor pollen seasonality was also observed indoors, as there was a close correlation in the presence of airborne pollen between the two sets of places. Air-conditioning can alter the I/O ratio either directly as a route for the entry of pollen, or because its use affects the likelihood of doors and windows being closed or open.