Bioaerosols

Bioaerosols are defined as airborne particles with compounds of biological origin, for example, pathogenic or non-pathogenic and living or dead fungi and bacteria, their secondary metabolites, bacterial endotoxins, mycotoxins, viruses and pollen grains (Douwes et al. 2004, Ghosh et al. 2015). Due to their ubiquitous nature, bioaerosols are detected in most enclosed environments (Nevalainen et al. 2015). Their distribution is highly dependent on seasons, and their concentrations are higher in summer and fall and lowest in winter (Salonen et al. 2017, Salonen et al. 2015). In indoor environments, the presence of bioaerosols is controlled through cleaning, maintenance and ventilation systems (Salonen et al. 2015, Ghosh et al. 2015).

Endotoxins

Endotoxins are biologically active liposaccharides and components of the outer membrane of gram-negative bacteria (Duchaine et al. 2001, Rennie et al. 2008). They are ubiquitous contaminants in indoor environments and are found in dusts and aerosols. Geographical region, season, cultural differences and habits affect the endotoxin levels in schools (Jacobs et al. 2014a). According to a review by Salonen et al. (2016), some of the factors which affect the endotoxin levels of indoor floor dust are, for example, age of the building, cleaning, farm or rural living, flooring materials (carpets, in particular), number of occupants, the presence of dogs or cats indoors, and relative humidity. However, it was concluded in their review that the research data are inconsistent and additional studies are needed. Studies on endotoxins and other particles in house dust have traditionally been based on vacuumed dust samples collected from floors or mattresses, as it is cheap and highly feasible (Fahlbusch et al. 2003, Schram et al. 2005, Schram‐Bijkerk et al. 2006, Noss et al. 2008, Samadi et al. 2010, Frankel et al. 2012). However, the majority of the samples may consist of large or heavy particles, such as sand, that would not become airborne, and the power of the vacuum, sampling area and time have a major impact on the results (Noss et al. 2008, Mazique et al. 2011). Several air sampling methods have been used as alternatives (Park et al. 2000, Dales et al. 2006, Wheeler et al. 2011, Morgenstern et al. 2005), but they may also be biased in that they may not represent the actual concentrations or measures of long-term inhaled exposure (Duchaine et al. 2001, Mazique et al. 2011). To compensate for the shortcomings of all these methods, an electrostatic dust fall collector was developed and is nowadays widely used instead of the vacuuming method (Jacobs et al. 2013, Noss et al. 2008).

Indoor fungi

Indoor environments in buildings are evolutionary new ecosystems. The number of known fungal taxa is estimated at around 80,000, but only 150 to 250 of these taxa are found in buildings (Samson 2011). Thus, only a limited number of fungal species dominate the indoor mycobiota, even though buildings provide diverse ecological niches (Nielsen et al. 2004). The moisture requirement of different fungal genera or species varies. Usually, a water activity (aw, which is an indicator of the availability of water) of 0.95–0.99 is favourable for fungal growth, while aw values of 0.65–0.90 and 0.88–0.99 are favourable for the growth of xerophilic fungi and yeasts respectively (Su-lin et al. 2011). The temperature in buildings is typically 20– 25°C, and the pH range in building materials is typically 5–6.5. These conditions are optimal for mesophilic fungal genera, such as Aspergillus, Trichoderma and Penicillium (Vacher et al. 2010). Sufficient light and oxygen are also critical for the growth of fungi in indoor environments (Voisey 2010, Airaksinen et al. 2004b). Moisture migration through the structures may result in microbial growth, and fungal spores might move indoors under the influence of negative pressure (Airaksinen et al. 2004a, Seppänen and Fisk 2004, Airaksinen et al. 2004b).

Modified wood products, wood polyethylene composites and plywood are susceptible to infestation by fungal genera such as Aspergillus, Trichoderma and Penicillium (Thacker 2004, Doherty et al. 2011). Some of the substrates for indoor fungi are inner wall materials used in buildings, such as prefabricated gypsum boards, cork liners and mineral wool; polyurethane used in composites, painted surfaces, fibre glass insulation and ceiling tiles; and paper and glue used in indoor surfaces. Additionally, nutrients in house dust and water favour fungal growth on all building materials. Thus, it is very likely that any hygroscopic or moist natural or synthetic material may serve as a substrate for saprophytic, biodeteriogenic or cellulolytic fungi and enable them to grow indoors (Samson 2011, Li et al. 2015). The mould growth on building materials causes changes in the structure and porosity of plywood and concrete and penetrates the building material in search of nutrients. Over time, the building material will become more fragile as the structure weakens (Andersen et al. 2011b, Viitanen et al. 2010).

In schools, Trichoderma species are typically found on wet manufactured wood and gypsum boards (Lübeck et al. 2000, McMullin et al. 2017), and Trichoderma spp. and Aspergillus versicolor have been associated with moisture damage (Salonen et al. 2015). Moreover, species of the indoor fungal genera Trichoderma and Aspergillus are known to be capable of plastic degradation, for example, in a structure made of concrete that contains plasticizers (Danso et al. 2019, Gregory 2009). In school environments in continental and moderate climates, the most common indoor fungal genera are Cladosporium spp., Penicillium spp., and Aspergillus spp. (Salonen et al. 2015).

In mould-damaged buildings, the indoor mycobiota might be extensive and form a significant indoor source of fungi (Gutarowska and Piotrowska 2007). Surfaces covered with fungal biomass release conidia into indoor air, and indoor settled dust may be enriched with these conidia and may preserve them. Viable conidia in settled indoor dust, thus, serves as a reservoir for recolonization of favourable ecological niches in the building (Kildesø et al. 2003). Air filters, such as exhaust air filters in the air handling unit, and ventilation ducts may also be colonized by fungi. Indoor fungi can be useful indicators of IAQ; therefore, a deeper understanding of their biology is important (Cabral 2010).

House dust contains mainly textile fibres and human-based materials, as well as fungi and bacteria (Rintala et al. 2012) and material from plant and animal sources. Fungi and their residues are sampled from air, surfaces, dust or building material. Particle measurement techniques especially developed for biological particles are needed for the sampling of airborne fungi. In culturing methods, air samples are collected directly on an agar surface (impactors) or a liquid medium (impingers) (Nevalainen et al. 2015). These samples are quantitatively assessed as the concentration per square metre of air (Reponen et al. 2011, Pasanen 2001). For a largescale study, air sampling using air samplers is often too costly and laborious. Therefore, settled dust sampling methods for measurement of long-term exposure have been developed for indoor sampling (Gehring et al. 2008). Floor sampling is typically done by vacuuming a specific area for a pre-determined time (Karvonen et al. 2014). Then, the dust is weighed, and the results are expressed as weight per gram of dust or per square metre. Sampling of mattress dust is often used in allergy-related studies (Hyvärinen et al. 2006). However, dust samples collected by these methods do not represent airborne fungi, and therefore, passive collectors for gathering settled dust from surfaces have been developed (Noss et al. 2010). Passive sampling provides a longer sampling time and, therefore, reflects the long-term airborne exposure. Fungal species useful as bioindicators for fungal infestations in buildings.

Fungal identification in indoor mould samples is important in order to recognize genera or species that can be used as bioindicators of fungal infestation and water damage in buildings. The traditional methods use morphological characterization of fungal contamination to provide simple and fast genus-level identification, but species-level identification usually requires DNA-based methods (Samson 2011). Toxicity profile bioassays, mycoparasitism analysis and fluorescence emission methods could be useful for separating indoor isolates into morphotypes, and this could enable screening for certain indicator species and speed up the identification procedure (Castagnoli et al. 2018). The species which have been proposed to be indicative of moisture in buildings are tertiary colonizers that need a high aw >90 and produce conidia in slimy masses which are not easily aerosolized, for example, species of the genera Trichoderma and Stachybotrys (Nielsen et al. 2004, Li et al. 2015).

Mycoparasitic fungi prey on other fungal species and kill their fungal prey by invasion and secretion of certain enzymes, and feeding on the released nutrients (Karlsson et al. 2017). The necrotrophic species T. atroviride may be the bestknown species that exhibits strong necrotrophic mycoparasitism. Necrotrophic mycoparasites are destructive, have a wide host range and nonselectively prey on live and dead fungal biomass (Karlsson et al. 2017). The strong necrotrophic mycoparasitism exhibited by T. atroviride may indicate availability of fungal prey on wet building materials. Li et al. (2015) suggest that the presence of viable conidia of Trichoderma species, such as T. atroviride, in airborne or settled dust, extracted from exhaust filters, should be considered as a significant indicator, based on which further investigation should be conducted to confirm the presence of this species (Li et al. 2015, Castagnoli et al. 2018).

Source: Camilla Vornanen-Winqvist. 2020. "Indoor air contaminants, symptoms and effects of mechanical ventilation in school buildings"