Non-Thermal Catalysis for healthier air
1 Are air handling systems contaminated by micro-organisms?
Numerous studies have found that air handling systems are contaminated by bacteria and fungi and have documented their role in the transport of these contaminants [1,2,3,4].
This research work very often focuses on hospitals as these establishments receive people who are particularly vulnerable to infection. It has thus been proven [5,6,7] that the most contaminated elements inside buildings are firstly filters, followed by fire prevention equipment, air vents, air conditioners and lastly dust in the spaces above suspended ceilings, on walls, wallpaper and in carpets.
Fungi have been the subject of many publications due to their extremely high pathogenic risk. An inspection of 820 air conditioning units revealed that on average they were contaminated by 1252 CFU/m3, with measured values ranging from 17 to 9100 CFU/m3. The species identified included Alternaria, Aspergillus flavus, Aspergillus fumigatus, Aspergillus niger, Aspergillus ochraceus, Aspergillus versicolor, Botrytis cinerea, Cladosporium herbarum, Epiccocum purpurascens-sterilia, and Penicillium spp. Bacteria (Propionibacterineae, Staphylococcus, Streptococcus and Corynebacterineae) were detected in the ventilation ducts and on the filters.
2 Do these contaminants survive longer on filters?
In a study conducted by Mittal et al., Staphylococcus epidermidis, Escherichia coli, Brevundimonas diminuta, Bacillus atrophaeus, coliphage MS-2, and Aspergillus brasiliensis were selected to represent the different classes of micro-organisms likely to be present on filters. The authors thus showed that under normal conditions of use of a filter (regulated air speed, temperature and humidity), the most fragile organisms, i.e. S. epidermidis, E. coli and B. diminuta, managed to survive for 2 to 6 days on the filter whilst the more resistant (B. atrophaeus, MS-2 coliphage, A. brasiliensis) survived longer than the duration of the tests (6 days). A test conducted over a period of 210 days even showed that B. atrophaeus survived with no loss of viability.
To address the high risk of contamination, antimicrobial filters were developed to avoid the growth of these contaminants and the risk of bioaerosols being transported through the filters. Various antimicrobials were used in the pretreatment of filters, including iodine and silver [9,10]. Although antimicrobial treatment slows down the carriage of bioaerosols in the indoor air, it does not completely prevent it because particles are deposited on the filters creating a barrier between the antimicrobial agent and the micro-organism. It has now been proven that micro-organisms can even develop on so-called “antimicrobial” filters.
The same conclusion was reached by comparison between a filter with a standard glass fibre medium and another using three layers of hygroscopic polymer. The micro-organisms tended to survive and develop for a shorter time on the three-layered filter for the first few months but this advantage subsequently disappeared due to the supply of nutrients provided by the build-up of particles on the filter after 6 weeks.
3 Is there a correlation between particulate cleanliness and control of airborne biocontamination?
The Société Française d’Hygiène Hospitalière (French Society for Hospital Hygiene) answered this question by analysing five clinical studies conducted in operating theatres, concluding that, contrary to what was thought, it was “difficult to conclude that there is a correlation between the concentration of bacteria in the air and the particle count”.
4 What is the explanation for the fact that there is no proven correlation?
Several factors may explain it. One of the reasons lies in the measurement itself. To understand this, the principles of filtration and the measurement of particles in the air must first be explained.
4.1 Filtration principle
Filters act as a sieve, preventing the largest particles passing through. However this is not the only way they operate. Other effects such as inertia, electrostatic interception or diffusion also participate in filtering smaller particles.
Taking all these effects together, it is proven that the retention capacity of High Efficiency (HEPA) filters is lower for particle sizes between 0.03 and 3 microns (see Figure 1, blue curve), which represents almost half of the total number of particles in the air according to the Whitby diagram (Figure 2), or 95% of the total surface of all the particles.
Figure 1: filtration efficiency of HEPA filters according to particle size (source: GVS)
Figure 2: Whitby diagram
This size range covers most viruses and many bacteria and spores, both fungal and bacterial. This is why high efficiency filters are classified according to standards (EN 1822: 2019, ISO 29463: 2017) allowing them to be compared on a test bench with regard to their particle retention capacity on sizes between 0.12 and 0.25 micros. The filters with the best classification are those with the highest efficiency in this size range. This standard is suitable to assess the integrity of filters after manufacture. The test of their integrity when they are installed in rooms is covered by other standards, in particular ISO 14644-3.
4.2 Particle counting principle
Airborne particle counters based on the scattered light principle are most frequently used for particle counting (recommendations of standard ISO 14644). Their range covers 0.1 to 10 microns. Measurement errors on the first channel measured can be as high as 70% for these devices! (see Figure 3 according to standard ISO 21501 ).
Figure 3: acceptable errors for counting according to standard ISO 21501. The values measured by the first particle size channel of optical counters are tainted by an error rate of up to 70%
In practice, particle counting for sizes smaller than 0.3 microns is rarely used for various reasons. Firstly, the smaller the particles to be counted, the more costly the equipment needed to count them. For example, the cost of a counter for which the smallest measurable particle size is 0.1 microns is three times higher than a counter starting at 0.3 microns. Moreover, the smaller the size, the more sensitive the counter is to contamination. Therefore, counters used to count particles of 0.1 to 0.2 microns are usually used in ultra-clean environments (ISO 6 or less according to ISO 14644-1). Only a few applications in microelectronics use this type of device, the most common being counters counting sizes larger than 0.3 microns.
Thus, on the one hand, microbiological contaminants between 0.03 and 3 microns in size are less well filtered by high efficiency filters than other particles and on the other hand, the sizes of particles counted by particle counters only cover part of the non-filtered contaminants (sizes larger than 0.3 microns) with a high level of uncertainty concerning the measurement.
Moreover, the survival of viruses, bacteria and spores and their propensity to grow on filters, the tightness of these filters when they are mounted and how well their integrity is maintained over time are all factors which may explain the lack of correlation between particulate contamination and airborne micro-organisms. It is therefore incorrect to think that standard or even high efficiency filtration alone is sufficiently effective to protect against microbiological air contamination. It is also incorrect to think that particle counting alone is sufficient to assess airborne biocontamination.
4.3 Particle counting versus microbiological air sampling
The ease of use and the speed at which results are obtained via particle counting mean that the concentration of particles in the air is measured rather than biocontamination. Several days or even weeks are usually needed to know the level of biocontamination in a room (as compared to a few minutes for particle counting). This means operations have to be stopped until the results of the samples are known. However the risk of surgical site infection is prevented and reduced by controlling airborne contamination not the concentration of particles in the air. Cristina et al. conclude in their study that “microbiological measurement remains the best method to assess the air quality in operating theatres”. The limits of high efficiency filtration are therefore better understood.
5 Despite this, the standards recommend that the most sensi-tive zones should be treated by large filtering surfaces such as unidirectional airflow filtering ceilings.
Numerous studies, summarised in the WHO report, ”suggest that unidirectional (laminar) airflow ventilation systems must not be used to reduce the risk of surgical site infection in patients undergoing total hip or knee replacement surgery”. Yet these are operations which carry a very high risk of infection requiring the greatest protection for the patient. So why is unidirectional airflow contraindicated when it offers such a large filtering surface and is so highly recommended for high-risk areas?
It is specified that “standard ventilation systems in operating theatres are of the mixed or turbulent flow type. The role of these systems is to homogenise the air, aerosols and particles in the room. Ventilation systems which use a unidirectional (or laminar) flow move the air in one direction at constant speed and in parallel flow lines in order to create a zone in which the air, aerosols and particles from the room are removed. Laminar airflows are routinely used in an environment where contamination by particles may have serious consequences, such as in orthopaedic implant surgery. However, unidirectional airflow systems are complex, costly and require careful maintenance. In many low-income countries, neither standard ventilation systems nor laminar airflow systems are affordable or maintained regularly. Natural ventilation is often the only option. A systematic review of the literature was made to determine if a unidirectional airflow ventilation system is more effective in reducing surgical site infection than a standard ventilation system. […] A systematic review and eight observation studies [19,20,21,22,23,24,25,26] comparing unidirectional airflow and turbulent airflow were identified. Most of the studies concerned total hip and knee replacements and only a few studies were available for other types of surgery [20, 21, 23]. The meta-analyses showed that unidirectional airflow did not provide any advantage in relation to standard ventilation (turbulent airflow) in reducing the occurrence of surgical site infection in total hip or knee replacements. […] Considering these results and the related costs, the group of experts decided to suggest that unidirectional (or laminar) airflow ventilation systems should not be used preventively to reduce the risk of surgical site infection in patients admitted for total hip or knee replacement surgery.” It should be noted that for an equal room volume to be treated, a unidirectional airflow system will have a much higher energy consumption since it will treat 10 to 15 times more airflow than a turbulent airflow system (hypotheses of a turbulent airflow system with an air change rate of 30 volumes/hour and a speed under flow of 0.25 to 0.35 m/s)
Zheng et ass. conducted a meta-analysis of 123,788 hip replacements and “did not find any convincing proof that unidirectional airflow is superior to turbulent airflow for the prevention of Surgical Site Infection’”. On the contrary, they even showed that the number of surgical site infections increased with a unidirectional airflow in relation to a turbulent airflow.
6 What are the risks of filter biocontamination?
Filter biocontamination may lead to the biocontamination of the environment because, as we have seen, micro-organisms are less well filtered due to their size or due to a filter integrity or installation defect or because of the way filters are handled when they are being changed for example[29, 30]. Potentially highly pathogenic contaminants can then be found in the direct environment, i.e. in the air and on surfaces (walls, ceilings, equipment, patients, etc.) where they will be able to develop. The health consequences can be very serious and may even lead to death in the most vulnerable.
Microbiological contamination is not the only risk. Gram-negative bacteria release endotoxins into the air as they multiply or during cell lysis. These endotoxins can be dangerous for humans and are not stopped by filters. They are composed of lipopolysaccharides (LPS). According to the INRS (the French National Research and Safety Institute),“their presence in working atmospheres, mentioned by B. Ramazzini in 1713, was confirmed in the mid-twentieth century. […] Contamination via the respiratory tract in a professional environment is the cause of coughs, dyspnea, and asthma, often in combination with a fever. […] Professions identified as being affected are increasingly varied: people working in […] the agrifood industry, those working in rooms without humidifiers or air conditioning, […]”.
In the same way, moulds release mycotoxins which are toxic for the liver, nervous system and immune system and which also have CMR (Carcinogenic, Mutagenic and Reprotoxic) properties. They also release Volatile Organic Compounds which cause filters to smell unpleasant and which act as a trigger or exacerbating factor for respiratory pathologies such as asthma. Moulds can also be carriers of allergens which may cause asthma, allergic rhinitis and respiratory pathologies.
7 What alternative is there to mechanical filtering?
From the point of view of inert particles, mechanical filtration currently remains the best treatment solution with a proven performance record, although not all filters offer the same quality.
From a microbiological point of view however, mechanical filtration is a “real breeding ground for germs” which, far from protecting staff and the environment, may become a significant source of contamination. It is therefore vital to combine it with destructive technologies. Non-thermal catalysis is the effective solution adapted to indoor environments.
Internal tests have been conducted with non-thermal catalysis on bacteria (L. pneumophilia), bacterial spores (B. subtilis) and viruses (bacteriophage T2). During tests, samples of vegetative and sporulated forms of bacteria were taken using two biocollectors: one produced by Bertin Technologies (Coriolis) and the other by AES Laboratoire (Sampl’air). Counting using the Coriolis was performed in two ways: 1- filtration on a membrane, then transfer of the filter to an agar medium and finally incubation for counting, 2- addition of two fluorophores (Invitrogen LIVE/DEAD BacLight kit) to a sample of the liquid recovery medium from the Coriolis. This test based on the integrity of the bacteria and spore cell membrane enables dead or live micro-organisms to be identified using a fluorescence microscope. The infectious or non-infectious nature of bacteriophages before and after treatment by catalysis was assessed using the Sampl’air and Petri dishes containing an agar medium covered in E. coli bacteria. Viruses which are still alive infect the bacteria, creating identifiable lysis plaques on the surface of the agar. The results demonstrated 2 to 3-log bacteria reduction in one pass.
Other tests conducted by an independent laboratory proved the effectiveness of the technology when set up in an air handling type unit. The results showed total removal of Aspergillus brasiliensis and Staphylococcus epidermidis.
Non-thermal catalysis destroys the microbiological contaminants and any contaminant made of organic matter. For example, the mechanism leading to the destruction of endotoxins by catalytic oxidative procedures has been demonstrated [33,34,35]. In particular, direct contact of the micro-organisms with the free radicals and hyperoxidant species formed during the photoactivated catalysis process initially leads to the oxidation of the lipopolysaccharide layer, i.e. the external layer of the cell membrane, especially polyunsaturated fatty acids. Thus, lysis of Gram-negative bacteria initially leads to the oxidation of the endotoxins in the membrane wall.
CALISTAIR has conducted extensive tests in the field and has had its technology validated in the laboratory by independent entities according to the standards currently in force.
8 What are the key takeaways?
Firstly, there is no miracle solution but using combinations of technologies compensates for the weaknesses of the individual techniques. Non-thermal catalysis was developed with this in mind. This technology is very effective in destroying airborne biocontaminants which are not destroyed by high efficiency filters. If destructive technologies are not used, pathogenic micro-organisms find a favourable environment for their growth and development on these filters and are then spread by air. They are subsequently found in the environment and are the source of infection in the medical sector in particular.
The combination of non-thermal catalysis and standard mechanical filtration thus guarantees air free from contaminants.
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