Hygiene Inspections of Ventilation and Air Conditioning Systems in Accordance with VDI 6022
Hygiene of Ventilation and Air Conditioning Systems
Ventilation and air conditioning systems (VAC systems, referred to in German as RLT systems) are used for air exchange and air circulation and can additionally be employed for heating and humidifying indoor air. Broadly speaking, a distinction is made between ventilation systems with 100% fresh air supply and pure recirculation or mixed-air systems (where the supply air consists of a mixture of exhaust air and outdoor air). In all three cases, VAC systems make a significant contribution to indoor climate quality.
Due to the increasing shift away from fossil fuels toward electric heat pumps, the proportion of VAC systems and split units actively integrated into building heating concepts is expected to continue to rise in the future.
Especially for VAC systems installed in commercially used buildings, the German Ordinance on Industrial Safety and Health (Betriebssicherheitsverordnung) clearly regulates routine hygiene inspections (depending on system design, every two to three years). Operators are legally obliged to carry out these inspections in order to avoid exposing occupants to unnecessary health risks. Hygiene inspections must be conducted by qualified experts with sufficient training in accordance with VDI 6022, and the resulting samples must be analyzed by an independent and accredited laboratory.
VDI 6022 Sheet 1
The requirements for the hygienic condition of VAC systems have been defined in VDI 6022 Sheet 1 since 1998 and have been further specified in the revised Sheet 3 of VDI 6022, published in December 2025. The central guiding principle states that the supply air delivered by a ventilation system must not be of poorer quality than the air drawn in.
This deterioration may involve inorganic particles (e.g. asbestos or man-made mineral fibers in older systems) but primarily concerns contamination by organic particles, i.e. airborne microorganisms such as bacteria, yeasts, and molds. Prolonged exposure to elevated microbial contamination of indoor air can lead to lung diseases, allergies, and other health complaints. Microorganisms therefore represent a serious hygienic and health-related concern.
Microbiological Hygiene Inspection
During hygiene inspections focusing on the microbiological contamination of ventilation systems, so-called RODAC plates (replicate organism detecting and counting), also known as contact plates, are used for spot sampling of smooth surfaces. Two different culture media are applied to the surface: DG18 medium for detecting molds and TSA medium for detecting bacteria and yeasts.
After specific incubation periods of two days (TSA) and seven days (DG18), the colony-forming units (CFU) are quantitatively recorded. Due to the small size of the contact plates, it is essential to sample representative components of the VAC system. These include, for example, reference samples taken upstream and downstream of filters, as well as components such as heating and cooling coils, condensate trays, humidifier and fan chambers, droplet separators, supply air ducts, and air outlets.
Uneven or complex surfaces, as well as sensitive components showing visible contamination, must be sampled using sterile, moistened swabs in accordance with VDI 6022.
The 2018 revision of VDI 6022 Sheet 1 already supplemented surface sampling using contact plates with the impaction method for investigating indoor air. In this method, airborne microorganisms are collected from the air onto culture media (e.g. DG18 and MEA) using air samplers.
Unlike surface-based methods, it is not sufficient to simply count colony-forming units per area. Detected molds must be identified by an accredited laboratory down to genus and species level based on their morphological characteristics. Only this differentiated analysis allows an assessment of whether the detected molds originate from the VAC system or from the intake air.
To illustrate this, consider a simple example: if 100 CFU/m³ of mold species A are detected in outdoor or reference air, but 100 CFU/m³ of mold species B are detected in indoor air, it can be concluded that an unacceptable deterioration of the intake air has occurred, manifested by a shift in species composition.
With the revision of VDI 6022 Sheet 3, the analytical scope of surface sampling and impaction measurements has been expanded. Particle measurements (also known as total spore counts) are now additionally permitted for indoor air assessment. This method captures all airborne particles and analyzes them with respect to their microbiological components.
Impaction vs. Particle Measurement – Key Differences
Impaction measurements are particularly advantageous when mold species identification at species level is required. Ideally, the physiological characteristics of individual species (e.g. temperature and humidity requirements) allow conclusions to be drawn regarding affected components or contamination sources. However, not all spores present in the air are viable, meaning they do not germinate on culture media and are therefore not detectable. Consequently, impaction measurements never reflect the true total spore load in indoor air.
For example, if a bacteriolytic disinfectant is used during the cleaning of a VAC system, microbial cells may be inactivated but not destroyed. Spores may therefore continue to be released into the indoor air. Allergic or sensitizing effects in humans can occur regardless of spore viability.
In contrast, particle measurements allow all airborne spores to be detected and counted, regardless of viability. The limitation of this method is that spores can only be assigned to so-called spore types, and species or genus identification is not possible. The choice of method therefore depends on the specific objectives of the project and the limitations one is willing to accept.
Practical Differences
Beyond analytical aspects, impaction and particle measurements also differ significantly in sample logistics. The impaction method requires two different culture media with relatively short shelf lives (note: under no circumstances may culture media be used after their expiration date). After sampling, the media must reach the laboratory within two days.
Particle measurements, by contrast, require only a small glass microscope slide. Transport time is less critical, as fixed spores generally do not change. In addition, particle measurements can be evaluated immediately upon arrival at the laboratory and are not subject to the seven-day incubation period required for impaction samples. This results in a significant acceleration of project timelines.
Our Service for You
The GBA Group is a competent, independent, and reliable laboratory service provider for all aspects of hygiene inspections of ventilation systems. With our accredited asbestos and mold analytics, we examine your samples in accordance with the latest state of the art for any abnormal contamination. Our integrated management system guarantees tested and certified laboratory analytics in compliance with DIN EN ISO 17025 (quality management), DIN EN ISO 14001 (environmental management), and DIN ISO 45001 (occupational health and safety management).
In addition, we provide expert consultation during the planning and execution of hygiene inspections, if required. Once all preparations are complete, we supply all necessary consumables, sampling equipment (air samplers), and tested culture media for your project. Upon request, we also organize sample collection and transport to the laboratory, ensuring compliance with normative requirements regarding transport time and conditions.
After completion of all laboratory analyses, you will promptly receive a comprehensive laboratory report. We are also happy to support you with the professional interpretation of results, if needed.
Do you have any questions regarding hygiene inspections of ventilation systems or upcoming projects? Please feel free to contact us at any time, we look forward to hearing from you.
