**2. Biological contamination of indoor air**

#### **2.1. Bacteria in the indoor environments**

The most common building-associated bacteria are saprophytic bacteria of the normal human skin, mouth, and nose that are emitted into the indoor air and bacteria originating from outdoors air [1-3]. Other common bacteria of indoor environments include heterotrophic bacteria that grow in the water reservoirs or moist sites of the building, such as bathroom sinks. Specifically, legionellae and nontuberculous environmental mycobacteria develop in biofilms of water pipelines or in water reservoirs of cooling systems. Actinobacteria (especially Streptomycetes), *Bacillus* species and various other bacteria grow in moist building materials together with fungi. Elements from bacterial structures released in air include bacterial cells, bacterial spores, peptidoglycans, microbial volatile organic compounds, exotoxins, and other bacteria growing metabolites [1].

Humans are an important source of indoor bacteria. The upmost layer of the normal human skin is continuously renewed, and skin scales containing bacteria are shed into the environ‐ ment [1]. Bacteria in the respiratory airways are eliminated through Pflügge droplets while talking, coughing, or sneezing. The level of air contamination is dependent on the number of persons inside a room and the efficiency of the ventilation system (natural or artificial ventilation). Bacteria that can be identified in indoor air are micrococci, staphylococci, streptococci, and corynebacteria [1].

The presence of pets in indoor spaces can represent an important source of air contamination and can be linked with the level of indoor endotoxins. The presence of dogs and cats can be the main predictors of endotoxin levels in house dust [1, 4-7]. Other predictors are the presence of vermin, such as mice, and infrequent cleaning, which indicates poor hygienic conditions in the home [1]. Storage of organic household waste indoors also increases bacterial contamina‐ tion in the indoor environment [1].

Bacteria actively growing or accumulating in the indoor environment may cause health effects and may require specific actions to control growth and prevent the exposures.

Bacterial growth can be found wherever standing water is present and Gram-negative bacteria can be identified in this environment. The presence of bacteria, either as viable bacteria or bacterial spores, mycotoxins, chemical markers like β-glucans and volatile organic com‐ pounds, and endotoxins, represents an indication of high humidity in the indoor environ‐ ment [1].

#### **2.2. Fungi in indoor environments**

Fungi are present in both outdoor and indoor air. The levels found in the outdoor air can range from less than 100 spores/m3 up to100 000 spores/m3 . Normally, the fungi levels are lower in indoor spaces compared with outdoor spaces. If the conditions that permit the fungal growth are present, for example, in damp houses, the levels of fungi in the indoor air can reach very high concentrations. Fungi level measurements are expressed in colony-forming units (CFUs) per cubic meter of air.

The levels of fungi and the type of fungi identified in indoor air depend on the season, construction features, age, and use of the building as well as the ventilation rate.

The most common fungal genera occurring in indoor environments are *Penicillium*, *Aspergil‐ lus*, *Cladosporium*, *Alternaria*, and yeasts [1-3]. These genera are also the most frequently occurring fungi in outdoor air.

Spores of fungi are present everywhere, and they are able to germinate wherever there is water available and the ranges between 0.80 and 0.98. Fungi need carbohydrates, proteins, and lipid to develop. They can find all these elements in house dust, construction materials like wall‐ paper or textiles, paint, glue, wood, paper and books, stored food, or deposit of cooking oil. Fungi can grow on inert materials like ceramic tiles [2].

### **2.3. Allergens in indoor environment**

Allergens are a group of agents that may cause a specific IgE-mediated reaction in humans. Fungal allergens can trigger type I allergic reactions and IgE sensitization to fungal species like *Alternaria*, *Penicillium*, *Aspergillus*, and *Cladosporium* spp. All these types of fungi species can induce allergic respiratory diseases, especially acute asthma attacks. Fungal allergens can also trigger type III-IgG-mediated allergic reactions (*Penicillium* and *Aspergillus)* and even type IV allergic reactions. This is the premise of the onset of hypersensitivity pneumonitis [2].

Allergens may also be present in indoor air due to house dust mites. The mite species that produce allergens are *Dermatophagoides pteronyssinus* (major allergens*—*Der p I and Der p II) and *Dermatophagoides farina* (major allergen*—*Der f I).

### **2.4. Endotoxins**

**2. Biological contamination of indoor air**

The most common building-associated bacteria are saprophytic bacteria of the normal human skin, mouth, and nose that are emitted into the indoor air and bacteria originating from outdoors air [1-3]. Other common bacteria of indoor environments include heterotrophic bacteria that grow in the water reservoirs or moist sites of the building, such as bathroom sinks. Specifically, legionellae and nontuberculous environmental mycobacteria develop in biofilms of water pipelines or in water reservoirs of cooling systems. Actinobacteria (especially Streptomycetes), *Bacillus* species and various other bacteria grow in moist building materials together with fungi. Elements from bacterial structures released in air include bacterial cells, bacterial spores, peptidoglycans, microbial volatile organic compounds, exotoxins, and other

Humans are an important source of indoor bacteria. The upmost layer of the normal human skin is continuously renewed, and skin scales containing bacteria are shed into the environ‐ ment [1]. Bacteria in the respiratory airways are eliminated through Pflügge droplets while talking, coughing, or sneezing. The level of air contamination is dependent on the number of persons inside a room and the efficiency of the ventilation system (natural or artificial ventilation). Bacteria that can be identified in indoor air are micrococci, staphylococci,

The presence of pets in indoor spaces can represent an important source of air contamination and can be linked with the level of indoor endotoxins. The presence of dogs and cats can be the main predictors of endotoxin levels in house dust [1, 4-7]. Other predictors are the presence of vermin, such as mice, and infrequent cleaning, which indicates poor hygienic conditions in the home [1]. Storage of organic household waste indoors also increases bacterial contamina‐

Bacteria actively growing or accumulating in the indoor environment may cause health effects

Bacterial growth can be found wherever standing water is present and Gram-negative bacteria can be identified in this environment. The presence of bacteria, either as viable bacteria or bacterial spores, mycotoxins, chemical markers like β-glucans and volatile organic com‐ pounds, and endotoxins, represents an indication of high humidity in the indoor environ‐

Fungi are present in both outdoor and indoor air. The levels found in the outdoor air can range

indoor spaces compared with outdoor spaces. If the conditions that permit the fungal growth are present, for example, in damp houses, the levels of fungi in the indoor air can reach very

. Normally, the fungi levels are lower in

and may require specific actions to control growth and prevent the exposures.

**2.1. Bacteria in the indoor environments**

490 Current Air Quality Issues

bacteria growing metabolites [1].

streptococci, and corynebacteria [1].

tion in the indoor environment [1].

**2.2. Fungi in indoor environments**

from less than 100 spores/m3 up to100 000 spores/m3

ment [1].

These are toxins integrated in the outer membrane of Gram-negative bacteria, which are composed of proteins, lipids, and lipopolysaccharides [2, 4-8]. The bacterial cell lysis deter‐ mines the release of endotoxins. In the air, endotoxins are bound to coarse particles. Exposure to different levels in air has different results: if the exposure is to high levels, respiratory symptoms or nonallergic asthma can develop, while if the exposure is at low and moderate levels, it is possible to have a protective action against allergies and asthma [2, 4-8].

### **2.5. (1 → 3)-** *β* **-D-glucans**

It is a structural element of the cell wall of fungi and some bacteria, and it can be found in house dust. (1 → 3)-β-D-glucans do not have allergic properties. However, they do have immune-modulating properties. Exposure to (1 → 3)-β-D-glucans in home environment can induce respiratory diseases [2, 9, 10] and may increase peak flow variability in asthmatic children.

### **2.6. Mycotoxins**

These are toxins (secondary metabolites) produced by fungi that interfere with RNA synthesis and can cause DNA damage. Some mycotoxins like aflatoxins produced by *Aspergillus flavus* and *Aspergillus parasiticus* induce carcinogenic effects [2, 11]. Indoor exposure to mycotoxins produced by *Stachybotrys chartarum* can be associated with acute pulmonary hemorrhage.

### **2.7. Fungi and Volatile Organic Compounds (VOC) in indoor air**

Fungi can produce around 200 volatile organic compounds. The identification of the VOCs in indoor air shows that there is a fungal growth present, even if the quantitative measurements are negative [11]. One of the most important substances that can be identified in indoor air is formaldehyde that can induce cancer.

#### **2.8. Viruses**

Indoor air humidity can induce a longer survival time of respiratory viruses and increased risk of respiratory infection and allergic diseases.
