Moulds are microscopic, filamentous fungi that grow as a microscopic network of filamentous cells, known as the mycelium. Under the right conditions, the mycelium will form cellular structures producing spores that are dispersed by wind, rain or small insects. When the spores fall on a suitable substrate, new fungal colonies can develop.

Moulds and health

Some moulds are potentially harmful to health. They can be pathogenic or can produce toxins that can contaminate food for example. However, there are many species that are useful for the traditional preparation of food, the decomposition of organic matter or for industrial applications.

Compared to all the micro-organisms present in the air, mould spores are among the most numerous and diverse living particles that we breathe in. The small size of the spores facilitates their penetration into the respiratory tract. Exposure to mould, especially prolonged exposure, is not without health risks.

Fungal spores are regarded as important airborne allergens that can cause allergic diseases such as allergic rhinitis and conjunctivitis, allergic asthma, hypersensitivity pneumonitis, etc. 

In some cases, the allergenic aspect can add to the toxicity. The ingestion of mycotoxins, secondary metabolites of moulds, can cause serious health problems. In addition, the inhalation of large quantities of toxins can cause damage to the respiratory system.

The irritant power of some microbial volatile organic compounds (MVOC) emitted in the air by some moulds could also be a serious source of problems.

Moulds in indoor environments

Many biological contaminants can be present in indoor environments: bacteria, moulds, viruses, dust mites, animal allergens, etc. 

The majority of micro-organisms present in indoor air are of natural origin and can come from the outdoor environment. This is particularly the case with fungal spores. Their concentration and nature vary to a greater or lesser degree according to the season. These fungal spores present in the outdoor environment can be introduced into the indoor environment. They can then be found in the indoor air and accumulate in the dust deposited on the floor and furniture. Moulds, like bacteria, can also have an origin resulting from various anthropogenic outdoor activities such as construction / demolition of buildings, composting centers or waste sorting, etc.

Moulds can sometimes originate from an indoor source. If this is the case, the presence of moisture is often the cause of the problem. In the housing of our regions, moisture is frequently present in excess. In addition to natural flooding, water infiltration or water leaks, human activity such as breathing, showering or cooking, when coupled with insufficient ventilation, is a potential source of moisture.

In addition, in the case of a visible fungal problem, dust mites are also often present as they feed on mould. Both of these microorganisms are a major source of allergens and should be kept out of homes. However, moulds are not always visible and can, like dust mites, be found in bedding, carpets, chairs, etc.

Buildings with centralised air conditioning systems, especially those with humidifiers, are sometimes the site of specific microbial growth. Various types of moulds can be isolated in humidification tanks, sometimes in the presence of very high concentrations of bacteria.

The communities, and particularly those hosting vulnerable populations (nurseries, schools, homes for the elderly), need special attention. Inside hospitals, nosocomial infections due to multi-resistant bacteria or moulds are a growing concern.

Moulds in other environments

  • Agricultural environment

Farmers and livestock breeders are regularly exposed to thermophilic microorganisms that proliferate, for example, in wet feeds where the temperature rises during fermentations. Thermophilic bacteria or moulds are often involved in cases of extrinsic allergic alveolitis (EAA), also known as ‘hypersensitivity pneumonitis’. These pathologies are also encountered in the storage (e.g. grain silos), transport and processing of foodstuffs into finished products. Examples of EAA of occupational origin are ‘farmer’s lung’, ‘bird keeper’s lung’, ‘mushroom worker’s lung’, etc.


  • Industrial environment

The industrial sector encompasses a wide range of activities, which means that workers can be confronted with many health problems related to microbiological contamination.

Sanitary facilities (showers), which are essential for certain activities carried out in the industrial sector, are often sites of fungal and bacterial growth (e.g. the bacteria responsible for legionellosis).

Microbes can also develop through very specific processes. This is the case with so-called ‘white water’ or cutting fluids used for cooling during milling or metal cutting. 


  • Composting site

Thermophilic moulds and various types of thermophilic bacteria are frequently found in composting facilities. Other contaminants, such as volatile organic compounds, can also be detected. A health impact may be perceived by people working in or living near these facilities. This is also the case in the waste industry, whether it is collection, storage, sorting or processing.

At Sciensano, the BCCM/IHEM fungal collection preserves more than 15,000 different strains of molds and yeasts of bio-medical interest. Sciensano also focuses on molds that have an impact on human and animal health. For example, our hospital environment surveys help prevent nosocomial mycosis.

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