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Airborne-microbe filtration in indoor environments
Abstract
The critical aspects of filter sizing and a methodology for predicting a filter's performance against allergens, bacteria and viruses are presented. The use of 80- and 90-percent filters produces air-quality improvements that approache those with HEPA filters. The combination of filtration and UVGI offers an efficient way of controlling both the largest microbes and spores and the smallest bacteria and viruses.
... Generally, these systems employ several filtration steps, with filters of increasing efficiency. Most commonly used filters are produced from cheap glass or synthetic fibers like polyester, polypropylene, acrylic and polyamide fibers, and are easily disposable [14]. ...
... Thus, a filter is classified as coarse if is able to retain up to 50 % PM10, ePM2.5 if it can retain up to 50 % of PM2.5 and as ePM1 if it can retain at least 50 % of PM1 particulate matter from air [15]. This class of filters are effective for most indoor environments, such as offices, commercial spaces, or transports [14]. High efficiency filters are highly recommended as the last step of filtration for specific sterile environments including operating theatres, bio-laboratories or nanofabrication laboratories, as well as equipment such as biosafety cabinets. ...
... Filters can be classified as HEPA (High Efficiency Particulate Air Filters), if it has an efficiency between 85 up to 99.995 %, or ULPA (Ultra Low Particulate Air Filters), if it has an efficiency higher than 99.995 % [16]. This class of filters is also considered highly efficient to capture viruses [14]. ...
According to the World Health Organization, low-quality air results in 3.2 million premature annual deaths of
which 21 % are attributed to respiratory infections. The recent coronavirus (COVID-19) pandemic, responsible
for millions of deaths worldwide, has highlighted the issue of airborne pathogens on human health. Air filtration
is an established method to ensure good indoor air quality, being effective at capturing airborne particles and
dust, as well as pathogens, and preventing diseases such as asthma. Nevertheless, with usage, water from air
humidity, dust, and dirt, including fungi and bacteria spores, can accumulate on filters creating ideal conditions
for the propagation of microorganisms. Thus, the filter can become a secondary source for airborne pathogens. In
this work, antiviral and antibacterial filters were prepared by spray coating Ag2O, CuO, and ZnO particles into
commercially available air filters. The reliability of the filters was tested through particle release in air, through a
scanning mobility particle sizer, and filtration performance was measured against an aerosol of iron nanoparticles in nitrogen gas produced through a spark discharge generator. Additionally, the biosafety of the filters
was measured in vitro through cytotoxicity tests against L-929 mouse fibroblast cells. Antibacterial activity was
measured in vitro against two clinically relevant pathogens: Streptococcus pneumoniae and Pseudomonas aeruginosa
and the anti-viral performance was measured against human respiratory syncytial virus. In general, the results
have shown that spray coating is a reliable solution, with no detectable cytotoxicity effects or particle release in
the air. After functionalization, both Ag2O and CuO compounds have been shown to have total antiviral activity
(over 99 %), while only Ag2O presented a clear antibacterial action. In conclusion, spray coating as postmanufacturing functionalization of air filters can be an efficient and low-cost solution against airborne
pathogens.
... The Working Group was convened on June [13][14]2005, to discuss and critique the initial draft of the concepts and principles. Following this meeting, a report was drafted that incorporated the Working Group's oral and written suggestions. ...
... Therefore, it is often necessary to model filter performance based on MERV curves and extend the curve to the region of interest. The details of filter modeling have been addressed in detail by Bahnfleth (2002 &2002a), Kowalski et al (1999), and . It can be seen in Figure 8.5 that removal efficiencies actually increase below about 0.1 micron. ...
UVGI air disinfection systems that operate on airstreams have applications in hospitals, schools, commercial buildings, homes, and in many other facilities. There are two main types forced air UV disinfection systems: In-duct UV systems and stand-alone recirculation units (also called unitary UV systems). A third class of systems exist, barrier systems, which are UV fixtures placed in doorways to disinfect air passing from room-to-room, but they are relatively uncommon. The primary advantage of forced air UV disinfection systems is the ability to control and predict performance. Well designed UV air disinfection systems can produce extremely high levels of clean air.
... mm for most bacteria [40]. In other explorations of the impacts of particle filtration and other size-dependent processes on virus and bacteria disease transmission, some researchers have assumed that the individual virus or bacteria particles are aerosolized and exist suspended as individual organisms [11,41]; however, it is likely more appropriate to consider the particles as larger expelled droplets that contain aggregates of the smaller infectious particles within Refs. [8,35,42,43]. ...
... Removal of smaller particles generally results in higher air flow resistance (Kowalski and Bahnfleth 2002); however, this is complicated by geometry issues and selection of filtration method and/or medium. Filters come in common depths of 1, 2, 4, 5 and 6 inches with consequent increases in filter media surface area and decreases in airflow resistance for the same filter medium. ...
Occupant concern about indoor air quality (IAQ) issues has led to the increased use of more effective air filters in residential heating and cooling systems. A drawback of improved filtration is that better filters tend to have more flow resistance. This can lead to lower system airflows that reduce heat exchanger efficiency, increase duct pressure that leads to increased air leakage for ducts and, in some case s, increased blower power consumption. There is currently little knowledge on the magnitude of these effects. In this study, the performance of ten central forced air systems was monitored for a year. The systems used either a Permanent Split Capacitor (PSC) or a Brushless Permanent Magnet (BPM) blower. Each system was operated with a range of filter efficiencies ranging from MERV 6 (the lowest currently permitted in ASHRAE Standard 62.2) up to MERV 16. Measurements were recorded every ten seconds for blower power, filter pressure drop, supply and return plenum pressures together with plenum and indoor temperatures. These detailed continuous measurements allowed observation of filter loading effects as well as the initial change in system performance when filters were swapped. The results of the field measurements were used in simulations to examine more general system performance effects for a wider range of climates. The field tests showed that system static pressures were highly influenced by filter selection, filter loading rates varied more from house to house than by MERV rating and overall were quite low in many of the homes. PSC motors showed reduced power and airflow as the filters loaded, but BPM motors attempted to maintain a constant airflow and increased their power to do so. The combined field test and simulation results from this study indicate that for MERV 10-13 filters the effects on energy use are small (<1%) over a wide range of performance conditions and climates. However, using higher efficiency MERV 16 filters leads to problems in terms of potential for significantly increased energy use (>5%) and usability. In systems using low MERV filters that are already close to blower performance limits the addition of a MERV 16 filter pushed the blowers to their performance limits.
Ultraviolet Light Disinfection for animal facilities.
A real-time monitor of bioaerosols, Fluorescence Laser Aerodynamic Particle Sizer (FLAPS), was evaluated for its ability to detect bioaerosols and measure bioaerosol filtration efficiency. Biological spores (Bacillus subtilis and Cladosporium sphaerospermum) were aerosolized and injected into a wind tunnel based on ASHRAE Standard 52.2. The sensitivity of the FLAPS was adjusted by the photomultiplier tube (PMT) gain to distinguish between bioaerosols and non-biological aerosols such as polystyrene latex and potassium chloride particles. Suspensions of biological spores were carefully prepared to remove interference with culture media. Aerosol concentrations upstream and downstream of a clean synthetic media filter (MERV-14) with electrostatic charge were monitored by FLAPS and by an Andersen impactor. The calculated filtration efficiency from the measurement of FLAPS was 96% for B. Subtilis and 100% for C. sphaerospermum. These results agree with those obtained with the Andersen impactor (91% for B. subtilis and 99.5% for C. sphaerospermum).
Ventilation performance in two units equipped with a single, hanging air conditioner having no filter, and another two units equipped with a central air conditioner with HEPA filter were compared for the air change rate, air velocity profile, and microbe concentration in a 814-bed public hospital from February to May 2006. It was found that microbe concentrations in units using a single air conditioner were about two times higher than those in central air conditioner units. Within the same type of air conditioner, increasing ventilation rate resulted in decreased microbe concentrations.
Increased attention is now being given to particles found indoors that have human health implications. The objective is to reduce the indoor particle concentrations of such pollutants as radon progeny, tobacco smoke, fibers and microbiological matter. Radon progeny (radioactive isotopes of polonium, bismuth and lead) are chemically active (unlike their chemically inert parents), and can adhere to indoor surfaces including airborne particles, room surfaces or lung tissue. Modeling of the expected lung dose (based on their alpha radiation) indicates that radon progeny not attached to particles have greater health risks than those attached to particles. Environmental tobacco smoke particles are liquid droplets of condensed tobacco combustion vapors that contain numerous organic compounds including known and suspected carcinogens and tumor promoters. It is the particulate phase of tobacco smoke that is suspected of causing lung cancer. Several particle control techniques exist (including mechanical and electrostatic filters) that may be used as part of a residential forced air heating/cooling system to substantially reduce the indoor concentrations of respirable particles. Unducted or stand-alone control devices are also available, and previous reports discuss an evaluation of these devices. In this paper the authors discuss the effects of air cleaning devices on indoor particle concentrations.
Stricter particulate emission regulations and the desire to produce cement as economically as possible serve to focus attention on the fabric filtration technologies available for use in cement processes baghouses. There are two fabric filtration technologies employed in cement baghouses: conventional in-depth filtration and expanded polytetrafluoroethylene (ePTFE) membrane surface filtration. The decision of which filtration technology to use greatly affects the performance of the baghouse and the process. Membrane surface filtration provides the lowest particulate emissions of any fabric filtration media, while also generally providing additional benefits such as consistently higher airflow, longer filter life, and reduced associated maintenance costs. Membrane filtration provides these benefits in material preparation and finish grinding, pyroprocessing, and electrostatic precipitator retrofit baghouses.
An experimental and modeling study was undertaken to evaluate the utility of local air filters for infection control. The ventilation provided by the portable filter that was tested not only exceeded regulatory guidelines, but was remarkably uniform throughout the room in spite of large variations in the air velocity. Furthermore, even partially enclosed regions with single openings that are commonly found in furnished rooms were adequately cleaned. The uniform ventilation is attributed to the rapid mixing arising from the flow of filtered air discharged from the filter. Ventilation of semi-enclosed regions appears to occur by a pulsationtype flow driven by relatively small pressure variations in the room. It is concluded that portable air filters are highly effective in removing particles from typical examination rooms and, because they also reduce the amount of contaminants entrained into the HVAC system, might be suitable for infection control at low-risk health facilities.
Different experimental techniques were applied to determine the effects of different air filter media on the viability of bacteria. Rinse suspensions of unused filter media were employed in standard inhibition tests to determine the effects of filter ingredients on bacterial growth under ideal nutritional conditions. Furthermore, a new test procedure was proposed and validated to determine the survival of viable microorganisms in fibrous air filters as a function of different parameters. Samples of filter media were challenged with microbial aerosols in an experimental set-up designed for measuring the collection efficiencies of fibrous filters. The loaded filter samples were then challenged with clean air under controlled conditions for a definite time span and numbers of viable microorganisms in the filter media were determined as colony forming units. The filter samples were retrieved from unused filter media usually employed in common air conditioning and ventilation systems.Under ideal nutritional and moisture conditions, growth of investigated microorganisms in nutrient broth and on nutrient agar was not inhibited by the inclusion of filter samples or rinse solutions of different filters in the growth medium with one exception. M. luteus and E. coli collected in air filter media and exposed to low air humidity (RH = 30–60%) showed a decline in their viability as a function of time (within 1 h). The decline rate was dependent on the type of bacteria employed and also the filter material itself.