It is well known that fungi can secrete a lot of volatile organic compounds (VOCs), which are one of many products of their primary or secondary metabolism [34,35]. The secretion of these chemical compounds is also directly related with a sick unit which is called sick building syndrome (SBS). Secretion of volatile substances as well as production of a huge amounts of extracellular polysaccharides (EPS) occurs especially when the molds grows in closed rooms, poorly ventilated with a high relative humidity [36]. It is well known that fungi exhibit greater metabolic activity under higher relative humidity [36,35]. The symptoms associated with secreted by fungi VOC the most often are manifested by lack of comfort, headaches, eye pain, pharyngitis, dry cough, dizziness, blurred vision, difficulty in concentration, sensitivity to smell, fatigue, apathy, or even a greater tendency to colds. These mainly neurotoxic symptoms are enhanced with a higher concentration of bioaerosol in indoor air [37]. It is characteristic that after leaving such a building or a room, these symptoms usually disappear quickly [38,39].

Such harmful conditions can exist only in one room, in the whole apartment or even in the whole building, especially if the latter has been built with inadequate materials to the typical climate in this region. The growth of fungi inside these spaces is always accompanied by the release of carbon dioxide and volatile organic compounds. Compounds secreted by fungi belong to such groups as aromatic and chlorinated hydrocarbons, alcohols, ketones, aldehydes or esters [40]. Among alcohols the most often are found 1-octanol, 2-octanol, methylbutanol or ethylhexanol. Apart from alcohols, terpenes and ketones are also often produced, such as 2-pentanone, 2-hexanone, 2-heptanone, 3-octanone, α-pinene, β-pinene, camphene, limonene, or methylfuranes. Among the aromatic hydrocarbons produced by fungi the most often are found cyclohexane and benzene [40].

The type of MVOC produced depends not only on the species of fungus but also on the nature of the substrate on which it grows. Moreover, differences exist also between strains from the same species. Often isolated and the most typical for indoor air fungi from the genus Aspergillus, Penicillium, Cladosporium and Fusarium are capable to produce a wide range of volatile compounds. It was showed that these species are able to secrete: formaldehyde, acrolein, nonanal, 2-pentanone, 2-hexanone, 2-heptanone, limonene, methylfuranes, camphene, or α-pinene [40]. It has also been shown that produced by many species of fungi formaldehyde and β-glucans shows strong irritation effect to mucosal membranes [41]. Not only fungi have the ability to produce MVOC. Such compounds can also be produced by bacteria and actinobacteria. Some compounds, such as monoterpenes, can also come from certain building materials, furniture, carpets and other sources [42].

Showing the negative effects of fungi present in the direct human environment, it is impossible to ignore their toxic effects on the human body. With regard to human and animal health, the commonly known problem pose mycotoxins [43]. It is disturbing that among the fungal population there are observed virulent strains that have the potential for increased production of mycotoxins. This phenomenon may be related with the increasing use of plant protection products (pesticides, fungicides), of which the mutagenic effect is well known [44].

Mycotoxins are poisonous products of secondary metabolism of molds. It is believed that these compounds are needed by fungi in their natural environment, as most of these substances are produced against other microorganisms (including other species of fungi) as a manifestation of competition for ecological niches [45-47]. So far, more than 400 different mycotoxins have been described [48]. Mycotoxins can cause both acute and chronic disorders in humans [49]. To induce acute intoxication, which often ends with death, it is necessary to get a milligram dose per kilogram of body weight [48,49]. To poison by mycotoxins most often comes through ingestion of food contaminated with these compounds. However, taking into account the fact that mycotoxins can accumulate in different structures of mycelium and can diffuse into the culture medium the primary mycotoxicosis can occur even by inhalation of spores, conidia or fragments of mycelium. For example, Stachybotrys, chartarum produces mycotoxins which concentrated in conidia, phialides and conidiophores. A certain amount of these toxic compounds are also secreted into the medium, especially during the sporulation period. Another species, Aspergillus flavus and Aspergillus parasiticus produces mycotoxins which accumulate in conidia and sclerotia [50].

The significantly increased incidence of cancer in people inhabit moldy rooms in most cases is undoubtedly related with long-term exposure to mycotoxins [51,52]. Studies performed in Poland show that even a quarter of the dwellings are damp. From further calculations it seems that as many as 24% of the residents are at risk from mycotoxins and allergens secreted by fungi [15].

It is believed that tumor changes, especially cancer of liver or kidney, may be delayed effects of long -term exposition to low doses of mycotoxins of the human body [51,52]. Among the most important and the most commonly encountered in the environment mycotoxins are included aflatoxins, ochratoxin A, zearalenone, trichothecenes and fumonisins [48]. The first of the mentioned appears particularly dangerous to humans and animals because of their carcinogenic properties due to the benzofuran ring [53]. The most widely distributed in the environment producers of dangerous mycotoxins are numerous species of the genus Aspergillus. Many species from this genus, and in particular A. fumigatus, A. flavus, A. terreus, A. niger, A. clavatus and A. chevalierii were isolated from cellulose or collagen contained materials [54,55]. The mycotoxin production itself and the intensity of this process depend on the fungal strain, and its genotype. It also depends from its developmental stage, from the relative humidity and temperature, and from the chemical composition of the substrate, in which conditions that fungus grows. In the regard to the latter, the nature of the nitrogen source plays a particularly important role for the toxic compound production [48,53]. Mycotoxins are produced intensively at higher temperatures and under the higher relative humidity, and particularly inside the dwellings with poor ventilation [56]. For example, the optimal temperature for the formation of aflatoxin is 33oC. It has also been found that under the some levels of inorganic nitrogen, phosphates and under the conditions of oxygen depletion and elevated carbon dioxide the production of this toxin is inhibited [57].

It is especially important that most mycotoxins have affinity for certain organs and tissues. Kidney and liver-specific affinity for ochratoxin A seems particularly dangerous due to the prevalence of Aspergillus ochraceus in the environment. Equally dangerous are trichothecenes which appears to have gastrointestinal affinity and zearalenone which has a significant negative effect on the reproductive system [52,58-60].

In addition to the harmful effects of mycotoxins, allergens and volatile organic compounds produced by fungi, these organisms as pathogens can cause dangerous infections both superficial as subcutaneous or systemic. Only the genus of Aspergillus, which in the air of the dwellings occurs very often [61-64] contains dangerous pathogens. These include A fumigatus, A flavus, A terreus, A nidulans, A niger and A clavatus. The mentioned species as opportunistic pathogens especially in the case of predisposed individuals may be the cause of lung aspergillosis, keratitis or otitis [64]. As it was previously mentioned house dust may be an important reservoir of fungal spores or other propagules not only of mold but also of dermatophytes especially in homes with humidity problems. The presence of the latter is more likely in the flats where inhabitants have different domestic animals that leave the parts of the animal corneous epidermis or dander. This situation is conducive to the development of dermatophyte infections, whose etiological factors are zoophilic dermatophytes such as Microsporum canis (Figure 4) or Trichophyton mentagrophytes [12,65].

Summarizing, with the presence of molds in dwellings always are connected some health problems, depending on individual sensitivity. The problem of the presence of fungi in flats and public buildings described in this article is becoming more real when we take into the considerations the fact that every day approximately every fourth Pole is exposed on fungal spores [15]. Spores of fungi and bacteria are ubiquitous in buildings and easily adapt to the niche available for them, as long as there is an adequate relative humidity and also a minimum amount of water available in the substrate. It should be underlined that the first ones are able to develop at significantly lower relative humidity levels as well as under the lower substrate humidity.