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Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12393-6
10.1016/j.heliyon.2024.e36362
e36362
Research Article
Air mycobiome in the National Library of Greece following relocation to novel premises
Iliopoulou Stavroula a
Kourteli Maria a
Damialis Athanasios dthanos@bio.auth.gr
b⁎
Kapsanaki-Gotsi Evangelia a
Pyrri Ioanna a
a Section of Ecology & Systematics, Department of Biology, School of Science, National and Kapodistrian University of Athens, Panepistimioupoli, Athens, GR-157 84, Greece
b Terrestrial Ecology and Climate Change, Department of Ecology, School of Biology, Faculty of Sciences, Aristotle University of Thessaloniki, GR-54124, Thessaloniki, Greece
⁎ Corresponding author. dthanos@bio.auth.gr
20 8 2024
30 8 2024
20 8 2024
10 16 e3636221 4 2024
30 6 2024
14 8 2024
© 2024 The Authors. Published by Elsevier Ltd.
2024

https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
The aim of this work was to study the diversity and spatiotemporal fluctuations of airborne fungi in the National Library of Greece after its relocation from the Vallianeio historic building in the center of Athens to entirely new premises at the Stavros Niarchos Foundation Cultural Center, and also to compare the fungal aerosol in between the two sites. The air mycobiota were studied by a volumetric culture-based method, during the year 2019 in order to assess their diversity and abundance and to compare with those previously reported in the historic building. Twenty-eight genera of filamentous fungi were recovered indoors and 17 outdoors, in addition to yeasts registered as a group. The number of fungal genera recovered was almost similar in both premises, whereas seventeen genera indoors were identical, dominated by Penicillium, Cladosporium and Aspergillus. The mean daily fungal concentration was found to be 66 CFU m−3 indoors and 927 CFU m−3 outdoors in the new location vs 293 and 428 CFU m−3 indoors and 707 and 648 CFU m−3 outdoors in the previous one. The mean daily concentration indoors was consistently and significantly lower (P < 0.05) in the new building than in the historic one, although it was higher outdoors. The indoor/outdoor ratio for the total fungi was 0.07 in the new vs 0.41 and 0.66 in the previous one and reveals a superior indoor air quality in the new site. Air temperature and occupancy had a statistically significant impact on the concentration of indoor fungi. The remarkably reduced concentration of the mycobiota in the new premises indicated a considerable decline in fungal burden, mainly due to technological excellency of the facility and continuous preventive measures to ensure an enhanced indoor air quality in the National Library of Greece. This case study provides a paradigm about upgrading of indoor air after re-establishment of a facility in another setting.

Highlights

• Fungal indoor air quality improved in contemporary building with technical specifications.

• Indoor fungal levels were minimized in the rooms with entirely controlled conditions.

• Indicator species with deterioration or toxigenic potential were decreased or not detected.

• Outdoor air and occupants remain key factors in shaping indoor fungal community.

Keywords

Airborne fungi
Indoor air
Microbial ecology
Cultural heritage
Facility relocation
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pmc1 Introduction

The significance of preservation of culture heritage is commonly recognized. It has been demonstrated that fungal contamination is undoubtedly an important biodeterioration agent of heritage archives. Many studies consistently report a major burden of indoor air quality in libraries owing to fungal spore accumulation [1,2]. The production of a great variety of exo-enzymes by fungi, such as cellulases and keratinases, poses a serious threat for the preservation of documents of great value, which are kept in libraries [3,4]. In addition, genera of airborne fungi that are often detected in library bioaerosols are capable of producing toxic secondary metabolites, thus acting as allergens or causing health issues to operators and visitors [2,5,6].

Since air microbial contamination may have serious consequences, it is useful to obtain a better understanding of the factors that favor the fungal growth inside libraries. Temperature, relative humidity, ventilation and the number of people in the libraries affect the microbiological load of the air [3]. Variations of environmental conditions in libraries can create many problems to book conservation [7]. The appropriate conditions for effective preservation of documents in libraries are 18–20 oC temperature and 50–60 % relative humidity, according to the International Federation of Library Associations [8]. However, there are several airborne fungi, which can grow at low water activity (aw), such as the xerophilic species of genus Aspergillus [9]. Furthermore, fungal spores are often carried in the indoor environment from the outside, through ventilation or air-conditioning systems [3]. The more isolated a room is from the outdoor environment, the air is less contaminated [10]. The well-established and efficient ventilation and air-conditioning systems, which are often combined with the use of special filters, are proven to result in less microbiologically contaminated indoor environment [10,11]. Also, the intensive movement of people and books in the library premises is linked to higher concentration of fungal propagules in the air, which means that in new acquisitions the air could be more contaminated due to the book migration and visitors [12]. A continuous aerobiological monitoring has to be used to avoid deterioration of documents [13,14] and health risks for personnel and visitors [1,15], although that is not an easy task [3]. Environmental monitoring is a basic requirement for the implementation of preventive conservation measures [14,16,17].

The National Library of Greece was relocated in the year 2017, from the historic neoclassical Vallianeio building in the centre of Athens where it was housed for over a century, to the newly built and environmentally sustainable Stavros Niarchos Foundation Cultural Center (SNFCC), situated near Faliron Bay. The new building has technical specifications for controlled ventilation, air temperature and relative humidity and a green roof system (GRS). Prior to the relocation of the Library, a great effort has been made to clean up the books from settled dust in order to reduce the concentration of particulate matter and microbial load. Mechanical cleaning was the preferred method or specific applications for disinfection when required. The cleaning methods reduce the microbial populations on artifacts and in the air [11,18].

The main objective of the present study was to investigate how the indoor air mycobiome is shaped after the relocation of the artifacts in a contemporary building located near the seacoast, away from Athens center. The impact of environmental factors, human activities and/or interventions on fungal diversity and abundance in the library environment were analyzed and evaluated. In addition, the results of the present investigation were compared with the results of a previous study which has been conducted by the same method in the historic building prior to the relocation, in two subsequent years. The fungal indoor air content in the old building was analyzed from November 2014 to July 2015 and from October 2015 to July 2016 and the findings have been reported [19].

2 Materials and methods

2.1 Sampling site

The National Library of Greece (NLG) was relocated in the year 2017, from the Vallianeio building in the Athens center to the Stavros Niarchos Foundation Cultural Center (SNFCC) (website:https://www.snfcc.org/en/national-library-greece) and reopened for the public in 2018. A total of 720.760 items were transferred to the new building. The SNFCC is located at Kallithea Municipality, 4.5 km south of central Athens (37°56′22.9″N 23°41′30.1″E). It is surrounded by a park and at the eastern side has a canal with seawater from the nearby Faliron bay. The NLG building is half glass-half concrete with a green roof system (Fig. 1). Air temperature and relative humidity are regulated by an automatic, modern climate control system (Hanwell wireless environmental monitoring system with ML4106 wireless temperature and humidity transmitters) in specified areas of the building, which are the Book Retention (closed bookstacks), the Rare and Manuscript Transition Storage, the Vaults 1–4, the Bookcastle Reading room 2 (2nd floor) and 4 (4th floor), the Manuscripts, Archives and Special Collections Reading room and the Bookcastle Balcony (1st floor). The storage rooms and preservation laboratories have no windows in order to prevent airflow from the ambient environment and the access is limited to authorized personnel, especially in the vaults. The SNFCC is a cultural and recreational center and a tourist destination, increasing the number of people entering the NLG building.Fig. 1 A, B. The National Library of Greece at SNFCC. C. Reading room. D, E. Vaults for manuscripts and rare artifacts F. Closed bookstacks for the main library collection.

Fig. 1

2.2 Sampling campaign

Air was sampled at twenty sites inside the National Library (Fig. 1) and one in the outdoor environment for comparison. The indoor sampling sites included six reading rooms (Manuscripts Reading Room, Bookcastle Reading Room 2 and 4, Lending Collection Reading Room-Children's Section, Journals Reading Room and “Stavros Niarchos” Reading Room), six main book storage rooms (Book Retention, Rare and Manuscript Transition Storage and Vaults 1–4), six working areas including three laboratories (Bookbinding Lab, Preservation and Conservation Lab and Wet Lab), the Book Fumigation Room and two office rooms (National Library Administration and Cataloguing Department), and also two sites near the main entrance of the library building (NLG Lobby and Bookcastle Balcony). The Rare and Manuscript Transition Storage, the Manuscripts Reading Room, the Bookcastle Reading Room 4, the National Library Administration and the three laboratories are located on the fourth floοr. The “Stavros Niarchos” Reading Room occupies the third floor, whereas the Bookcastle Reading Room 2 and the Journals Reading Room are located on the second. In addition, the Cataloguing Department and the Book Retention are located at the back wing of the second floor, behind the Bookcastle. The Bookcastle Balcony is on the first floor and the Book Fumigation Room, the Lending Collection Reading Room-Children's section and the NLG Lobby are on the ground floor. The valuable books and manuscripts that are safeguarded in the vaults are in fabric and plastic cases, for additional protection. The access to storage rooms is restricted to authorized personnel to reduce effects from human presence to minimum.

Αir sampling was performed from January 2019 to July 2019 once per fortnight, between 9 and 11 in the morning, in all 21 sites consecutively. The sampling strategy involved a culture-dependent volumetric method based on impaction. Potato Dextrose Agar (PDA) was used as nutrient medium. A single-stage portable impactor for agar plates (Burkard for Agar plates–BAP; Burkard Manufacturing, Hertfordshire Co. Ltd, UK) with a laminar flow of 20 L/min was used to expose petri dishes for 3 min at 1 m above ground in the same specified spot for each sampling site every time. The petri plates were incubated at 25 °C in the dark for 5–7 days in order for fungal colonies to develop, be counted, studied and identified. The identification of each fungus to genus level was based on colony macromorphology and on micromorphology studied with a Karl Zeiss optical microscope at 400× magnification. Non-sporulating colonies were examined after two weeks of extended incubation in case of late sporulation. The equation of multiple-impaction transformation [20] Gregory, 1948 was applied to correct the colony counts and colonies were expressed as colony forming units per cubic meter of air sampled (CFU m−3).

2.3 Statistical analysis

The fungal diversity of each NLG sampling site was calculated using Simpson's Diversity Index (1-D). Subsequently, the total airborne fungal diversity of the library interior was compared to the air mycobiota outside the library. For the analysis, formula 1-D = Σn (n-1)/N (N-1) was used.

In order to assess the differences of air mycobiota between the two buildings, comparisons were conducted among sampling sites in the old (Vallianeio) and the new building (SNFCC), categorized by room type i.e. storage rooms, reading rooms, conservation labs, administration offices and outdoors. In order the results to be comparable, the derived data of a seven-month sampling period, January to July, were selected for evaluation for the two buildings. The indoor-outdoor concentration ratio (I/O) for the total fungi was calculated for the old and new building for the selected seven-month period.

All data were statistically analyzed at significant level 0.05 by using SPSS version 26 and Statistica 13.0. To compare the airborne fungal spore concentrations indoors vs outdoors, but also among different sites in the two library buildings, we performed full factorial ANOVA and ANCOVA. To check if the data distribution satisfies the normality prerequisite, we ran the Shapiro-Wilks test. As the data were not normally distributed, we used natural logarithms. Significant groups were identified after posthoc analysis, using the Bonferroni correction.

In addition to the fungal monitoring, meteorological parameters were additionally analyzed in order to investigate the relationship between the indoor airborne fungal load and air temperature and relative humidity, as well as the number of occupants at the NLG in the old and in the new building during the sampling process. We analyzed these data using General Linear Models and factorial regressions.

3 Results

3.1 Aeromycological analysis at the National Library of Greece (SNFCC)

In total, 294 samples were collected from 20 sampling sites indoors and one outdoors throughout a time-period of 7 months. Twenty-eight genera of filamentous fungi were identified indoors and 17 outdoors, whereas other genera belonging in Sphaeropsidales, Basidiomycota, Non-Sporulating Fungi (NSF) and yeasts were registered as groups or were unidentified. The Simpson's Diversity Index was 0.81 for the indoor and 0.73 for the outdoor environment. Eleven of the genera were recovered only inside the library.

The genera Penicillium and Cladosporium dominated the indoor airspora, as each accounted for 34.3 % and 28.8 % of the total airborne fungi recorded (Fig. 2). The group of yeasts followed with 8.3 % and Aspergillus with 3.2 %. In outdoor air, Cladosporium was dominant constituting 63.5 % of the total count, while the genera Alternaria, Penicillium and Aspergillus and non-sporulating fungi were also found in high rate (Fig. 2).Fig. 2 Contribution of the main taxa recovered to the total air mycobiota indoors and outdoors.

Fig. 2

Penicillium and Cladosporium were the prevalent genera indoors in all seasons (Fig. 3), with Penicillium surpassing Cladosporium during the cold months of the year. For the outdoor environment, Cladosporium was by far the dominant genus during the whole study period followed by Penicillium. Alternaria was recovered more frequently outdoors than indoors. Penicillium spores peaked during winter and early spring both indoors (214, 291 CFU m−3) and outdoors (506, 275 CFU m−3). Aspergillus spore concentrations were more stable in the interior of the library than in the outdoor air. Yeasts were regularly recovered in the interior of the library, in contrast to their occasional presence and low concentrations outdoors.Fig. 3 Temporal variation of the dominant genera and total fungi Α) indoors and Β) outdoors. Total fungi concentrations correspond to right axis whereas all the rest to the left axis.

Fig. 3

Furthermore, total fungal concentrations declined during winter (Fig. 3), whereas higher levels of airspora were observed during spring. Peaks of fungal concentration were observed throughout May to June, in most reading rooms, vaults and working areas (Fig. 3). In most indoor sampling sites, the daily fungal load never exceeded 500 CFU m−3, except for the National Library Administration office and the book fumigation room. An unusual peak of 937 CFU m−3 was recorded on January 10, 2019 at the National Library Administration office attributed mostly to Penicillium spores, whereas the concentrations remained low throughout the rest of the year. Αt the book fumigation room, the fungal concentrations were higher throughout the sampling period compared to the other indoor sampling sites. Τhe outdoor fungal spore concentrations exceeded 1000 CFU m−3 in almost 30 % of the samplings, especially during late spring and summer. The differences observed between the total fungal concentration in indoor and outdoor environment were statistically significant (P < 0.001). The total fungal concentration in the sampling sites indoors ranged from 0 to 354 (937) CFU m−3, whereas the range of total concentration outdoors was calculated at 104–3679 CFU m−3. The mean daily concentration was 66 CFU m−3 indoors and 927 CFU m−3 outdoors (Table 1). Data from the sampling sites indoors show that the highest mean daily concentration (303 CFU m−3) and number of taxa (15 genera) appeared in the book fumigation room, whereas the lowest mean daily concentration (34.7 CFU m−3) and number of taxa (4 genera) were found in the bookcastle reading room 4 (4th floor) and in the archives vault. The mean daily concentration (926.5 CFU m−3) and number of taxa (17 genera) outdoors were the highest reported from all sampling sites investigated during this study. When the sampling sites were grouped into 3 groups according to their use (storage rooms, reading rooms and working areas), it is clear that the lowest range was observed in the storage rooms, and especially the vaults, followed by reading rooms (Fig. 4). Simpson's Diversity Index ranged between 0.54 and 0.83 indoors, where the lowest value appeared in the National Library's Administration office and the highest in the bookcastle reading room 4 (4th floor), the NLG lobby and the bookcastle balcony.Table 1 Comparison of the mean daily concentration (CFU m−3) of total airborne fungi per sampling site grouped according to its usage in the National Library of Greece, between the historic Vallianeio and the SNFCC buildings.

Table 1Βuilding/Year
Room type	Vallianeio
2015	Vallianeio
2016	SNFCC
2019	
Reading rooms	359	273	44	
Book retention rooms	137	276	60	
Vaults for permanent storage	353	505	56	
Restoration laboratories	x	658	96	
Indoor (mean value of all sampling sites)	293	428	66	
Outdoor	707	648	927	
Indoor/Outdoor ratio	0.41	0.66	0.07	

Fig. 4 ANCOVA of total fungi concentration among room types in SNFCC, weighted for number of individuals per room.

Fig. 4

3.2 Fungal aerosol in relation to environmental factors

The air temperature and relative humidity are being controlled automatically and monitored in 10 areas of the NLG new building mainly where the books, documents and manuscripts are safeguarded. During the sampling procedure, the number of occupants present was also recorded. The lowest number of occupants was recorded in the vaults and the highest in the reading rooms and in the working areas.

The indoor air fungal load was analyzed in relation to air temperature, relative humidity and number of occupants present. An increased fungal spore concentration in the air was related to the air temperature and the number of occupants.

Analysis of variance of the air fungal propagules inside SNFCC building revealed statistically significant differences among the diverse library halls depending on the number of occupants present per room (Fig. 4). It is evident that working areas exhibit statistically significantly more fungi than all other areas when the number of occupants per site is evaluated as a dependent variable.

The mean concentration of the total fungi in the library atmosphere varies depending on temperature. The temperature is kept constant at a small range of 19–25 °C in the new premises, while the range was more extensive in the old one. The differentiation in the new building is not as significant as in the old one (Fig. 5).Fig. 5 Mean daily concentration in relation to daily average temperature in the SNFCC (N) building at the left and in the Vallianeio (V) building at the right. The Y axis is in natural logarithms.

Fig. 5

3.3 Comparison of NLG fungal community at the SNFCC and Vallianeio buildings

The fungal community recovered within Vallianeio building and the new premises at SNFCC was compared in order to elucidate how the indoor fungal aerosol is shaped after the relocation to a new contemporary building which meets the international standards for libraries. The comparison of the indoor airborne fungal diversity, as well as of the outdoor ambient air in the old and new building is shown in a Venn Diagram (Fig. 6). The diagram indicates that 17 genera were dwellers of both buildings' interior. Although an almost similar number of genera were registered inside both buildings, the composition of airspora differed significantly. In the outdoor environment, 11 genera have been identified at both locations. Ten genera have been reported exclusively in the outdoor ambient air at the Vallianeio building, whereas only 6 genera have been found exclusively at SNFCC. In total, 10 genera (Acremonium, Alternaria, Arthrinium, Aspergillus, Aureobasidium, Botrytis, Cladosporium, Fusarium, Geotrichum and Penicillium) appeared both indoors and outdoors at the two library buildings. The genera Acrodontium and Sporotrichum have been found only in the interior of both buildings. The number of fungal genera is higher indoors in both buildings, which is also supported by the Simpson's Diversity Index.Fig. 6 Venn Diagrams showing the diversity of fungal genera indoors and outdoors the National Library of Greece at the Vallianeio and SNFCC buildings. The genera detected indoors and outdoors in both buildings are marked with fonts in green color. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6

The fungal concentration comparison demonstrated clearly that in the new Niarchos building the spore concentrations were consistently and significantly lower (P < 0.05) than in Vallianeio (Fig. 7). The total fungal load ranged from 0 to 1002 CFU m−3 in the Vallianeio building with several peaks exceeding 2000 CFU m−3 and from 0 to 354 CFU m−3 in the SNFCC building with a single peak of 937 CFU m−3 in the administration office. The dominant genus Penicillium presented a similar abundance pattern with a concentration range from 0 to 1156 CFUm−3 and maximum values of 1800 CFU m−3 in the Vallianeio building and from 0 to 291 CFU m−3 and a maximum value of 900 CFU m−3 in the SNFCC building. The air mycobiota in the ambient environment were statistically significantly higher in the area were the SNFCC building is located. The concentration ranged from 153 to 1613 CFU m−3 with several peaks of more than 2000 CFU m−3 whereas in the Vallianeio building area ranged from 52 to 1058 CFU m−3 peaking at 1908 CFU m−3. It is interesting that concentrations remained lower inside the new premises throughout the sampling period, even though outdoor concentrations were higher in the area where the new building is situated as compared to the region of the Vallianeio building.Fig. 7 Factorial ANOVA of the average number of CFUs of total fungi per room type and outdoors and between the two libraries. The significance level is given at p < 0.05. The Y axis is given in logarithmic scale.

Fig. 7

4 Discussion

The National Library of Greece has a history of over two centuries and contains over a million books and magazines including a treasure of handwritten codices and historical archives of Greece. It has been housed at Vallianeio historical building in the center of Athens for over a century. The majority of the books were kept in a five-storey iron construction that air circulated among the levels as the floor of each storey was not compact. A part of the collection was stored in shelves that lined the walls of the main reading room. The manuscripts and the rare collections were safeguarded in a separate room. The building was naturally ventilated with no controlled conditions regarding temperature and relative humidity. The Library was transferred in the year 2017 to a new building in the premises of Stavros Niarchos Foundation Cultural Center (Fig. 1A–B). It is a contemporary building with controlled environmental parameters and a Green Roof System. The building has four vaults (Fig. 1D–E) where manuscripts, papyruses and all unique, rare collections are safeguarded under controlled conditions for T and RH and limited entrance only to authorized personnel. The bulk of the library's book collection is kept in suitable closed bookcases (Fig. 1F). There are four reading rooms (Fig. 1C) in three different floors and three Laboratories where all book maintenance takes place.

After the reopening of the library in the new premises, the indoor fungal aerosol was studied in order to reveal its diversity, abundance and distribution and to assess how the indoor mycobiome was shaped in comparison to Vallianeio building. The diversity and abundance of the air fungal load at Vallianeio building have been studied prior to relocation with the same volumetric method for two consecutive years [19].

Nieto-Caballero et al. [21] studied the indoor fungal aerosol behavior with fluorescence and molecular tools inside several schools in USA before and after building and/or ventilation system renovations, and they detected significant reductions in airborne fungal loads immediately following building rehabilitations. In the case of renovation in healthcare facilities, the construction activity dispersed large amounts of fungal spores causing infection outbreaks [22]. The present investigation seems to be the first one to study the fungal air load in an institution prior to its relocation in another place and following it.

In total, 28 genera were identified indoors and 17 outdoors whereas 27 were found indoors and 21 outdoors in Vallianeio building. Both buildings presented a higher fungal diversity indoors than outdoors. Penicillium and Cladosporium dominated the indoor airspora in the SNFCC building. The yeasts were also recorded in high concentrations inside the library. Outdoors Cladosporium prevailed during the whole sampling period, followed by Penicillium and Alternaria whereas yeasts were registered in very low numbers. A similar abundance pattern was also observed in Vallianeio building. It is evident that a core mycobiome consisted of the dominant, abundant and frequently encountered genera was the same in both buildings (Fig. 6). The diversity differences observed were mainly in genera rare in number and frequency. The diversity findings imply that the outdoor ambient air has little influence on indoor fungi. However, Penicillium and yeasts dominated indoors instead of Cladosporium and 11 genera were recorded only indoors, which indicate the existence of indoor sources that enrich the interior with fungal propagules, even in small amounts.

The differences observed between the total fungal concentration in indoor and outdoor environment were statistically significant (P < 0.001). The total fungal load indoors ranged from 0 to 937 CFU m−3 with a mean daily concentration of 66 CFU m−3, whereas the range of total concentration outdoors was calculated at 104–3679 CFU m−3 with mean daily of 927 CFU m−3. The mean daily concentration in Vallianeio building was 293 and 428 CFU m−3 indoors and 707 and 648 CFU m−3 outdoors, respectively for the two study periods. The comparison of fungal concentration between the two buildings demonstrated the statistically significantly lower concentrations (P < 0.05) registered inside the new library premises (Fig. 6, Table 1) despite the fact that outdoor fungal aerosol was more abundant in SNFCC area (Fig. 7, Fig. 8, Table 1). The indoor/outdoor ratio for total fungi was 0.07 in the new premises vs 0.41 and 0.66 in the previous one (Table 1) and indicates a high quality of indoor air compared to the outdoor in both locations, whereas it is superior in the new one. The I/O ratio<1 is an indicator of satisfactory indoor air quality although in some repositories may be much higher [23].Fig. 8 The statistically significant difference in the outdoor airspora of the two buildings locations.

Fig. 8

There are no regulations about the exposure limits for the concentration of airborne fungi. A commonly used baseline is that of 500 CFU m−3 recommended by the World Health Organization [24]. The mean daily concentration of 66 CFU m−3 for total fungi in the interior of the Library at SNFCC, which represents the mean of all sampling sites for the whole study period, seems to be the lowest mean value reported inside a library worldwide. The concentration level in SNFCC is much lower than that reported from libraries in other regions, i,e, from Poland [25,26], Cuba [23], Canada [6] and China [27].

Furthermore, the diverse rooms in SNFCC building were compared in pairs to the ones in Vallianeio according to their usage. It is evident that, indoor fungal aerosol abundance in the SNFCC is statistically significantly much lower in all room categories when compared to the respective ones in the Vallianeio building, despite the fact that outdoor air mycobiome abundance is statistically significantly higher in the area where SNFCC building is situated (Fig. 6, Fig. 7).

The mean daily concentrations varied significantly among the diverse areas of the SNFCC library building (Fig. 4). The four vaults where all the rare and valuable artifacts are safeguarded were the areas that fungal load was extremely low to almost non-detectable. This is attributed to the limited entrance only to authorized personnel. In the old building, the manuscripts room, an area with limited personnel present, also exhibited low air fungal values although temperature and relative humidity were not regulated. The highest fungal aerosol values in the working areas (Fig. 4, Fig. 7) were related statistically to the number of persons present, indicating that humans are a source of supply for fungal propagules to the library air. Occupancy is a determining factor of indoor airspora as also found by Grabek-Lejko et al. [25], Pasquarella et al. [28], Wu et al. [27], and Zielińska-Jankiewicz et al. [29] who observed a close link between the number of visitors and fungal propagule concentrations in museums and libraries studied.

The mean daily concentration of fungi in the reading rooms of the SNFCC is variable, although the rooms have similar size and number of visitors present at sampling time. The concentration was 21 CFU m−3 in the reading room located on the fourth floor directly underneath the green roof system (GRS). This value is much lower than those recorded (33–69 CFU m−3) in five reading rooms located in lower levels. An exception is the reading room for manuscripts with the highest mean daily concentration (69 CFU m−3), although it is smaller with less visitors. The impact of the GRS on the abundance of fungal aerosol is an intriguing topic for further study. An extensive systematic investigation in a primary school with GRS in Athens, revealed reduced fungal concentrations in the classrooms underneath a GRS [30].

The influence of temperature on the air fungal load is evident in the comparison of the SNFCC building to Vallianeio (Fig. 5). There was a statistically significant shift to significantly higher concentrations when temperature was increased (Fig. 5) demonstrating the positive influence of temperature on indoor fungal airspora. When fungal load was assessed in relation to temperature in SNFCC, no statistically significant relation was found, since there is little variation in temperature in different rooms. This was anticipated since temperature and relative humidity are regulated in order to meet suggested standards.

In addition, the measures applied for the cleaning and disinfection of books and other documentary material, prior to their transfer to the new settlement at SNFCC, has played a significant role in the reduction of particulate matter and fungal load in the air, as also found in libraries in Poland [11] and in Italy [18].

The assessment of the qualitative data in the indoor air of the National Library of Greece indicated that a core air mycobiome with the same dominant constituents actually exist in both locations, although considerably reduced in numbers The genera Penicillium, Cladosporium, Aspergillus predominate in descending order in the interior of the library, whereas Cladosporium precedes in the ambient air. The same genera were found as dominant in the National Archive of Cuba, with the genus Aspergillus to predominate [23].

Most of the fungi found in the air of libraries and archives are potential deteriogens for documentary resources and they exhibit cellulolytic or proteolytic activity, they produce acids and excrete pigments on the substrates they colonize [31]. A number of “indicator species” in certain genera, with a substantial deterioration, toxigenic or allergenic potential were decreased in numbers or were not detected in the new location. The dominant genera Cladosporium, Penicillium, Aspergillus and Alternaria, which comprise several species with cellulolytic, amylolytic or proteolytic potential [31], as well as the genus Trichoderma, which is a major threat for the books because of its high cellulolytic activity, were recovered in reduced numbers in the new location. The genus Stachybotrys besides its cellulolytic activity is of much concern because of its toxigenic potential since it may produce volatile organic compounds i.e. trichothecenes, which makes it unacceptable for indoor environments. It is interesting that it was not detected indoors at the SNFCC building, whereas it had been occasionally found in the old building. Similarly, the Myrothecium-like constituent was absent in the new location. The genus Beauveria, which is a hazard for libraries and museum collections since it is an entomogenous fungus associated to insects in several ways, was not detected in the SNFCC building.

The comparison of quantitative and qualitative data about the indoor air mycobiota in the National Library of Greece between the previous Vallianeio building and the present SNFCC building revealed that there is a considerable improvement of the fungal indoor air quality, following the relocation of the library in a new building with entirely controlled conditions. The monitoring of the indoor air quality in libraries is a reliable way to assess potential threats for documentary material and further implement preventive measures for human safety and conservation of cultural heritage.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data will be made available upon request.

CRediT authorship contribution statement

Stavroula Iliopoulou: Writing – original draft, Visualization, Investigation, Formal analysis. Maria Kourteli: Writing – original draft, Visualization, Investigation, Formal analysis. Athanasios Damialis: Writing – review & editing, Visualization, Validation, Formal analysis. Evangelia Kapsanaki-Gotsi: Writing – review & editing, Validation, Methodology, Conceptualization. Ioanna Pyrri: Writing – review & editing, Visualization, Supervision, Resources, Methodology, Funding acquisition, Formal analysis, Data curation, Conceptualization.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: The corresponding author, As. Prof. A. Damialis, is Section Editor in Heliyon Agriculture. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors are grateful to the late Dr. Filippos Tsimpoglou, former Director of the National Library of Greece, for the permission to conduct this research but most importantly for his enthusiasm and his continuous support. Also we thank Dr. Zoi Gkinni for her valuable help throughout the sampling process.
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