Price-performance comparison of HEPA air purifiers and lower-cost MERV 13/14 filters with box fans for filtering out SARS-Cov-2 and other particulate aerosols in indoor community settings

BackgroundSARS-Cov-2 aerosols have potential to accumulate in airspaces of poorly ventilated, indoor spaces such as classrooms, offices, homes, restaurants, and bars. Separately, toxic aerosol pollution (e.g. wildfires, wood burning) is frequently encountered in these locations with ventilation relying on outside air. Portable high efficiency particulate air (HEPA) air purifiers are useful to remove both types of aerosols without relying on outside ventilation. To meet or exceed recommended 4 to 6 air exchanges per hour (ACH), high price-points for select HEPA air purifier models without incurring excessive noise generation make them unaffordable for many households and communities. Do-it-yourself (DIY) alternatives with box-fans and HVAC filters can be much lower cost but their clean air delivery rate (CADR) and noise generation varies greatly depending on choices of filters, number of filters, and fan speeds.ObjectiveTo compare cost-effectiveness and noise-generation of aerosol filtration by commercially available, portable HEPA air purifiers and do-it-yourself (DIY) alternatives built from box fans and HVAC filters rated at MERV 13 or 14 using single and quad filter designs.MethodsThe comparison is based on three metrics: clean air delivery rate (CADR), the noise generated (dBA), and affordability ($$) using an ISO-certified aerosol measurement device to measure input/output particle filtration of particles sizes ranging from 0.3 microns to 10 microns, airspeed meter to measure airflow, and NIOSH sound meter app to measure noise.ResultsAccounting for reduced filtration efficiency of MERV 13/14 filters compared to HEPA, estimated clean air delivery rate (CADR) of a do-it-yourself (DIY) setup using 2” and 4” filters with a box fan running at fan speed 1 for tolerable noise was 278 cfm ($38) to 371 cfm ($58), comparable or better than a commercial HEPA air purifiers running at maximum speed with low noise generation at 282 cfm ($375). However the quad filter designs, popularly known as the Corsi-Rosenthal box, achieved gains in CADR of only 60% over a single filter design but in contrast to 200-250% gains reported by UC-Davis.ConclusionsDIY alternatives using single 1”, 2” and 4” MERV 13/14 filters compare favorably to commercially available systems in terms of estimated CADR and noise but at five to ten times lower cost. Compared to use of one filter, an improvement in CADR (air flow) was observed with a popular DIY configuration involving quad filter design, popularly known as the Corsi-Rosenthal box, not as high as reported by a recent study by UC-Davis

Evaluation of Antimicrobial Effect of Zinc Pyrithione against Airborne Fungi and Bacteria Growth Collected onto New and Loaded HVAC Fibrous Filters

Microbial growth onto HVAC filters was observed in real conditions with possible degradation of the indoor air quality. The filtration performance of marketed antimicrobial filters containing zinc pyrithione was tested under laboratory conditions and compared to that of similar filters with the same classification, F7 (EN779:2002). The filtration performance of the two tested filters during loading with PM10 particles was quantified in an experimental setup with filter pressure drop measurement and particle counting upstream and downstream of the filters. The microbial growth on the new and loaded filters, both contaminated with a microbial airborne consortium composed of two bacteria (Gram-positive and -negative) and fungi, was quantified by colony-forming units after conditioning the filters for a few days under controlled temperature (25 °C) and humidity (50% or 90% relative humidity). The results reveal that there was no degradation of the filtration performance of the filters treated with the antimicrobial agent. The efficiency of the antimicrobial treatment, i.e., the ability to inhibit the growth of microorganisms during the incubation period, was significant with the new filters regarding the fungal growth, but the results demonstrate that the antimicrobial treatment became inefficient with the loaded filters.

Gray Box Time Variant Clogging behaviour and Pressure Drop Prediction of the Air Filter in the HVAC System

Identification and prediction of clogging behavior in heating, ventilation, and air conditioning (HVAC) filters is crucial to avoid issues such as system overheating, energy waste, lower indoor air quality, etc. Researchers are focusing more on the particle loading characteristics of a filter medium in a laboratory environment under steady-state conditions, fixed particle concentrations, area of porosity, dust feed and volumetric flow rate. However, recent research still shows uncertainties in modeling as well as the implementation problems of constructing the HVAC laboratory test bench and equipment. In addition, subjects such as non-uniform particle deposition depreciation of the condition and various type of mechanical filters such as fibrous, fabric, granular, and membrane filter or electrostatic filters which typically used in HVAC systems perform under some assumptions and still need more research. The studies become even more difficult acquiring a large number of time-varying and noisy signals. Another approach among studies is data-driven knowing that Building Automation System (BAS) is not equipped with appropriate sensor measuring the clogging, it is needed to drive the clogging mathematical model from the pressure drop signal. This paper bridges the gap between particle-size study and black box modeling of HVAC filter which has not received much attention from authors. The proposed method assumes that the pressure drop is the result of two time-varying functions; f(t), which represents the dynamics of clogging and, g(t), which refers to dynamics of remained terms. The exponential and polynomial of second order functions are proposed to express the clogging behavior. The software package based on Particle Swarm Optimization Artificial Bee Colony (PSOABC) algorithm, is developed and implemented to estimate the coefficients of the clogging functions based on smallest RMSE, high coefficient of correlation and acceptable tracking. Five Air Handling Unit (AHUs) are selected for practical verification of the model and the results show that the applied method can successfully predict clogging and pressure drop behaviour of HVAC filters.

Distribution of SARS-CoV-2 RNA Signal in a Home with COVID-19 Positive Occupants

Although many COVID-19 patients quarantine and recover at home, the dispersal of SARS-CoV-2 onto surfaces and dust within the home environment remains poorly understood. To investigate the distribution and persistence of SARS-CoV-2 in a quarantine home, samples were collected from a household with two confirmed COVID-19 cases (one adult and one child). Home surface swab and dust samples were collected two months after symptom onset (and one month after symptom resolution) in the household. The strength of the SARS-CoV-2 molecular signal in fomites varied as a function of sample location, surface material and cleaning practices. Notably, the SARS-CoV-2 RNA signal was detected at several locations throughout the household although cleaning appears to have attenuated the signal on many surfaces. Of the 24 surfaces sampled, 46% were SARS-CoV-2 positive at the time of sampling. The SARS-CoV-2 concentrations in dust recovered from floor and HVAC filter samples ranged from 104-105 N2 gene copies/g dust. While detection of viral RNA does not imply infectivity, this study confirms that the SARS-CoV-2 RNA signal can be detected at several locations within a COVID-19 quarantine home and can persist after symptoms have resolved. In addition, the concentration of SARS-CoV-2 (normalized per unit mass of dust) recovered in home HVAC filters may prove useful for estimating SARS-CoV-2 airborne levels in homes.

Filtration Efficiency, Breathability, and Reusability of Improvised Materials for Face Masks

There is paucity of data on the performance of different improvised materials to cope with the COVID-19 pandemic. The goal of this study is to evaluate the filtration efficiency and breathability of improvised filtration and commonly available mask materials, as well as to assess their reusability. Materials readily available to the general public such as cotton, fragrance and additive-free dry baby cleaning wipes, and those abundantly available in the hospital setting, such as sterilization wraps, were chosen for testing, amongst others. In the COVID-important 2–5 m particle range, two-layers of cotton provided filtration efficiency between 34%–66%. Amongst potential filter materials, 300-weight sterilization wraps provided approximately 80% filtration efficiency and are readily available in the healthcare setting. The addition of sterilization wrap to cotton fabrics brought the filtration efficiency to above that of the sterilization wrap (80%-90%) at the expense of added pressure drop. Four-layers of dry baby wipes performed very well with a filtration efficiency of 85% and a reasonable pressure drop (1/3 of procedure mask). Since the material is advertised as pure spunlace polypropylene and designed to contact the skin during cleaning, it would appear generally safe as a filter insert. Of improvised filters, polypropylene electrostatic HVAC filters performed the best with filtration efficiencies of >99%, but are not recommended due to the risk of confusion with glass-based HVAC filters and uncertainty regarding trace materials used in the filter. The filtration efficiency of two-layers of cotton fabrics with one-layer of sterilization wrap slightly improved over 10 laundry cycles, while the performance of other non-wovens, like dry baby wipes, degraded more rapidly and should be considered disposable. In summary, we found that a two-layer cotton fabric can provide a comfortable, breathable and reusable option. The addition of a sterilization wrap or four-layers of pure spunlace fragrance free dry baby wipes can significantly improve filtration and block expiratory aerosols at the expense of an added pressure drop.

Emerging investigator series: air conditioning filters as a sampler for semi-volatile organic compounds in indoor and near-building air

SVOCs were measured in HVAC filters to compare seasonal differences, incoming vs. outgoing air, and semi-quantitatively determine indoor air concentrations.