For a thoughtfully designed indoor environment
The assessment of mould contamination and infestation must be conducted from a wide range of perspectives. It requires a high level of expertise and a comprehensive understanding of ecological, hygienic-microbiological, analytical-diagnostic, and medical interrelationships.
As there is no universal method for assessing mould infestations, a wide range of techniques is employed to ensure an optimal and targeted approach. Furthermore, familiarity with relevant guidelines and standards, combined with state-of-the-art laboratory equipment, is essential for obtaining meaningful and well-founded results to determine the cause of the mould infestation.
There are numerous pathways by which microorganisms enter indoor air. Moulds, yeasts, and bacteria are abundant in outdoor air and enter indoor spaces through open windows and doors. Bacteria and yeasts primarily originate from the soil and are introduced into indoor air while adhering to soil or dust particles. Many moulds produce airborne spores that are optimally adapted for dispersal through the air. In addition to naturally occurring airborne microorganisms, one must also consider those introduced by humans, which are spread via droplets from speaking or coughing or through the shedding of colonised skin and hair particles. In some cases, particles containing bacteria are also stirred up from textiles or flooring. The majority of these microorganisms are classified as environmental microbes; these are generally harmless to healthy individuals. However, pathogens can also be transmitted via indoor air. In such instances, the concentration of the microorganisms is a critical factor. If a threshold concentration is exceeded, the risk of illness rises significantly. Individuals with impaired infection defences or a weakened immune system are at particular risk. Hygiene measures are intended to help minimise health risks.
The term “hygiene” encompasses not merely cleanliness but, in a scientific sense, also the effort to prevent disease and to maintain or enhance the well-being and performance of all individuals. This entails minimising human exposure to environmental stressors. Given that people spend approximately 80% of their time indoors, this applies particularly to living and working environments. The aim should be to achieve a physiologically optimal indoor climate, generally characterised by room temperatures of 20–22°C and a relative humidity of 50–60%. Pathogens should be absent from indoor air.
In workplaces and large building complexes, HVAC systems can help establish and maintain these conditions. However, bacteria and fungi are sometimes spread via the airflow, including through HVAC systems. Significant bacterial agents include actinomycetes, spore-forming bacteria (Bacillaceae), cocci, micrococci, and corynebacteria. Among moulds, representatives of the genera Cladosporium, Penicillium, Alternaria, Aspergillus, and Botrytis are most frequently detected. Yeasts such as Rhodotorula spp. and Saccharomyces spp. are also present. These microorganisms include pathogens that can be harmful to health, potentially causing infections—particularly in immunocompromised individuals—and triggering allergies in many people.
If pathogens enter the body through the lungs, this can rapidly lead to an illness affecting the entire system. The close proximity to the bloodstream within the lungs facilitates the spread of pathogens throughout the body or allows the transfer of their metabolic products into the blood. However, since the human immune system also operates within the bloodstream, the body is able to respond immediately to an infection.
A common cause of hygiene issues in HVAC systems is microbial contamination of filters, where the combination of condensation and accumulated dust creates favourable conditions for microbial growth. Microorganisms are inevitably drawn in from the outdoor air; under suitable conditions, they can grow through the filter media and be released into the indoor air. Regular maintenance and hygiene inspections of HVAC systems and their filter units can minimise associated health risks for employees and should therefore be carried out at regular intervals. Filter replacement schedules and maximum service lives are specified in the VDI 6022 standard, setting the limit at one year for coarse filters and two years for fine filters. Shorter intervals may be required depending on specific needs, such as the level of contamination or manufacturer recommendations.
Various moulds and bacteria can act as pathogens or trigger allergies. The numerous species differ in their potential or likelihood to cause disease. Many species are harmless to healthy individuals, whereas some can infect even healthy people; this applies particularly to certain bacterial pathogens. These differences stem from the specific characteristics of the individual species and their metabolic activities.
Under the current Biological Agents Ordinance, microorganisms are classified into risk groups based on various criteria. These criteria include, for example:
Natural occurrence and distribution of the species
Production of toxins (endotoxins, exotoxins, mycotoxins)
Routes of entry for the pathogen (mouth, lungs, skin, gastrointestinal tract)
Incubation period of any resulting illness
Severity and course of a potential illness (chronic, acute)
Possible secondary illnesses or long-term effects
Diagnostic options and therapies (antibiotics)
Transmissibility of an illness
Known resistance to antibiotics
The current risk classifications are highly accurate for bacteria, as bacterial pathogens have long been extensively studied. In contrast, further research is needed regarding moulds. Current risk classifications do not take into account recent research findings and are often based on the identification of individual fungal species from clinical isolates—that is, from human tissue samples. However, it is known that moulds exhibit different appearances and growth patterns during infections, which can lead to identification errors. Recent research and experience therefore suggest that certain species should be viewed more critically than their current risk group designation implies. A case in point is Aspergillus niger, which is known to cause infections of the ears, lungs, and other organs. Yet, under the current Biological Agents Ordinance, A. niger is classified as a relatively harmless environmental microbe in Risk Group 1.
Although the classification is based on a broad set of criteria, the resulting risk assessment cannot be applied across the board to everyone. It is important to note, in particular, that the classification is primarily determined by infectious potential rather than sensitising potential. Sensitisation can lead to an allergy, which may also result in severe physical effects and symptoms of illness. Many personal or individual factors—such as genetic predisposition—can significantly increase an individual’s risk of becoming sensitised. For instance, it is known that individuals whose parents suffer from allergies face a significantly higher risk of developing an allergy themselves. Similarly, regarding infections such as athlete’s foot, specific population groups—including elite athletes, diabetics, and smokers—are at greater risk than the general population. Therefore, the risk posed to an individual by a specific type of fungus must always be evaluated on a case-by-case basis.
Microorganisms that are “unlikely” to cause disease in humans (this includes species that may cause illness in severely immunocompromised individuals or species to which “only” allergenic potential is attributed). With few exceptions, this category includes moulds.
Microorganisms that can cause disease in humans and pose a risk to workers; spread of the pathogen among the general population is unlikely; effective prevention or treatment is usually possible. Moulds of the species Aspergillus fumigatus and Aspergillus flavus are classified in this category.
Pathogens that can cause severe human disease and pose a serious risk to workers; there may be a risk of spread within the population; prevention or treatment is usually possible.
Pathogens that can cause severe human disease and pose a serious risk to workers; there may be a high risk of spread within the population; effective prophylaxis or treatment is generally not available.
Contact samples are collected from surfaces that are relevant to hygiene or potentially contaminated, using contact plates with various culture media (DG18 and MEA). The media are subsequently incubated in the laboratory at 25°C for 5–7 days and then analysed (including differentiation of the genera and most common species of moulds).
An impactor sampler serves as a universal and versatile solution for capturing airborne biological constituents. The MBASS 30 system from Holbach, equipped with the LKS30 sampling head, is distinguished by its specially designed aluminium housing that accommodates standard commercially available agar plates (culture media). This offers great flexibility in the choice of culture medium, enabling a meaningful microbiological assessment tailored to the specific question at hand.
A calibrated airflow of 30 L/min is drawn over the agar plates using a vacuum pump. The sampler’s intake head contains a perforated plate featuring a specific number of holes of uniform diameter; this directs the airflow in a controlled manner over the agar plates and accelerates the air-entrained particles sufficiently to ensure their deposition based on the principle of inertial impaction. The arrangement of the holes (323 in total) ensures even distribution across the culture medium. Depending on the dust load, the following air sample volumes are used for collection: 50, 100, 200, or 300 litres. The decision regarding the air volume to be sampled is made on-site.
Analysis of the microbiological results (including the differentiation of mould genera and the most common species) and the professional evaluation are carried out in specialised microbiological laboratories.
Note: Certification confirms that a product met the requirements of the applicable testing programme at the time of testing. It does not provide a comprehensive assessment of a product’s suitability for health or of the health effects of an entire indoor environment.
Testing building products against published criteria since 1982.
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