Removing indoor particles using portable air cleaners: Implications for residential infection transmission

Reducing indoor exposure to influenza particles can be an important strategy to manage residential infections. Many portable air cleaning (PAC) technologies are currently employed in residential environments but very little research has been performed to evaluate and compare their performance in terms of particle removal associated with influenza. This study evaluates the effectiveness of portable air cleaners at removing airborne NaCl particles as an analogue to the influenza virus and applies the results to an IAQ mass balance model to evaluate the performance in controlling residential exposures and mitigating infection risks. Various devices representing different PAC technologies were tested using a pull down particle challenge in a full scale stainless steel chamber. Particle generation and measurement were conducted using a 6-jet atomizer and a paired aerodynamic particle sizer (APS)-scanning mobility particle sizer (SMPS), respectively. PAC incorporating HEPA filtration, electrostatic precipitation, ion generation and electret filtration were tested. We found that particle exposures released during a cough or sneeze event in a typical Quebec City residential room in Canada can significantly be reduced using HEPA, electrostatic precipitation and electret filtration PACs when compared with a situation where no PAC is being used. Modelling analysis demonstrates that the use of these PACs can mitigate the risks of influenza infection via airborne route for a caregiver or a spouse sharing the same room. The implications of this study are significant considering low ventilation rates of Quebec City residences.     

Particle loading rates for HVAC filters, heat exchangers, and ducts

The rate at which airborne particulate matter deposits onto heating, ventilation, and air-conditioning (HVAC) components is important from both indoor air quality (IAQ) and energy perspectives. This modeling study predicts size-resolved particle mass loading rates for residential and commercial filters, heat exchangers (i.e. coils), and supply and return ducts. A parametric analysis evaluated the impact of different outdoor particle distributions, indoor emission sources, HVAC airflows, filtration efficiencies, coils, and duct system complexities. The median predicted residential and commercial loading rates were 2.97 and 130 g/m(2) month for the filter loading rates, 0.756 and 4.35 g/m(2) month for the coil loading rates, 0.0051 and 1.00 g/month for the supply duct loading rates, and 0.262 g/month for the commercial return duct loading rates. Loading rates are more dependent on outdoor particle distributions, indoor sources, HVAC operation strategy, and filtration than other considered parameters. The results presented herein, once validated, can be used to estimate filter changing and coil cleaning schedules, energy implications of filter and coil loading, and IAQ impacts associated with deposited particles. PRACTICAL IMPLICATIONS: The results in this paper suggest important factors that lead to particle deposition on HVAC components in residential and commercial buildings. This knowledge informs the development and comparison of control strategies to limit particle deposition. The predicted mass loading rates allow for the assessment of pressure drop and indoor air quality consequences that result from particle mass loading onto HVAC system components.     

Ultrafine particle removal by residential heating,ventilating, and air‐conditioning filters

This work uses an in situ filter test method to measure the size‐resolved removal efficiency of indoor‐generated ultrafine particles (approximately 7–100 nm) for six new commercially available filters installed in a recirculating heating, ventilating, and air‐conditioning (HVAC) system in an unoccupied test house. The fibrous HVAC filters were previously rated by the manufacturers according to ASHRAE Standard 52.2 and ranged from shallow (2.5 cm) fiberglass panel filters (MERV 4) to deep‐bed (12.7 cm) electrostatically charged synthetic media filters (MERV 16). Measured removal efficiency ranged from 0 to 10% for most ultrafine particles (UFP) sizes with the lowest rated filters (MERV 4 and 6) to 60–80% for most UFP sizes with the highest rated filter (MERV 16). The deeper bed filters generally achieved higher removal efficiencies than the panel filters, while maintaining a low pressure drop and higher airflow rate in the operating HVAC system. Assuming constant efficiency, a modeling effort using these measured values for new filters and other inputs from real buildings shows that MERV 13–16 filters could reduce the indoor proportion of outdoor UFPs (in the absence of indoor sources) by as much as a factor of 2–3 in a typical single‐family residence relative to the lowest efficiency filters, depending in part on particle size.     

Characterizing gaseous air cleaner performance in the field

As part of an ongoing effort to better understand the performance of indoor air cleaners in buildings, the National Institute of Standards and Technology (NIST) has completed a series of gaseous air cleaner field tests and model simulations. This paper focuses on experiments to measure the removal of decane with a sorption-based in-duct gaseous air cleaner and a sorption-based portable air cleaner in a single-zone test house. Due to the lack of standardized gaseous air cleaner field testing protocols, a field test method was developed using semi-real-time concentration measurements and mass balance analysis. A total of 24 experiments were completed with directly measured single-pass removal efficiencies ranging from 24% to 56% and removal efficiencies based on a transient whole building mass balance ranging from 30% to 44%. Experimental results revealed important factors affecting field performance such as air cleaner contaminant loading for the in-duct air cleaner and room air mixing for the portable air cleaner. An additional six tests were conducted to evaluate the predictive capability of the indoor air quality model CONTAM.     

Impact of placement of portable air cleaning devices in multizone residential environments

An advantage of portable air cleaners is that they can be positioned in different parts of a building and used where air cleaning is needed. This makes them a popular choice for use in residential buildings. In typical indoor particle modeling efforts, perfect air mixing and uniform contaminant concentration distribution are assumed. However, nonuniform spatial concentrations of particles are more reflective of most environments. Using experiments to validate computational fluid dynamic and particle tracking models and applying these models in numerical based parametric analysis, this paper analyzes the overall contaminant removal in a multi-room residential building. Simulations varied (1) particle size (0.74, 3.4 and 10 μm), (2) clean air delivery rate (CADR) of the air cleaner (50 m³/h and 500 m³/h), and (3) position of portable air cleaner in different rooms. The results show very large variation of the overall particle removal for different positions of portable cleaning device. In extreme cases, the effective positioning of cleaning device can result in a factor of 2.5 change in overall particle removal and, consequently, strongly affect occupant exposure to particles.     

Real-time measurements of PM2.5 and ozone to assess the effectiveness of residential indoor air filtration in Shanghai homes

Portable air cleaners are increasingly used in polluted areas in an attempt to reduce human exposure; however, there has been limited work characterizing their effectiveness at reducing exposure. With this in mind, we recruited forty-three children with asthma from suburban Shanghai and deployed air cleaners (with HEPA and activated carbon filters) in their bedrooms. During both 2-week filtration and non-filtration periods, low-cost PM_2.5 and O₃ air monitors were used to measure pollutants indoors, outdoors, and for personal exposure. Indoor PM_2.5 concentrations were reduced substantially with the use of air cleaners, from 34 ± 17 to 10 ± 8 µg/m³, with roughly 80% of indoor PM_2.5 estimated to come from outdoor sources. Personal exposure to PM_2.5 was reduced from 40 ± 17 to 25 ± 14 µg/m³. The more modest reductions in personal exposure and high contribution of outdoor PM_2.5 to indoor concentrations highlight the need to reduce outdoor PM_2.5 and/or to clean indoor air in multiple locations. Indoor O₃ concentrations were generally low (mean = 8±4 ppb), and no significant difference was seen by filtration status. The concentrations of pollutants and the air cleaner effectiveness were highly variable over time and across homes, highlighting the usefulness of real-time air monitors for understanding individual exposure reduction strategies.     

Returning characteristics of pollutants for a local domain in the presence of returning and recirculating airflow in indoor environments

Heating, ventilating, and air-conditioning (HVAC) systems usually supply air, which is a mixture of fresh air from the outdoor environment, and return air from rooms via the ventilation ductwork. This air reduces the heat load and cost impact of air conditioning using outdoor air. This recirculation of room air in air-conditioning systems is reasonable in terms of energy saving; however, the deterioration of air quality might be a concern because of the recirculation of contaminated room air. Here, we numerically investigate the effect of pollutant recirculation/return on the formation of concentration distributions of local pollutants in indoor environments when the mixing ratio of recirculated air in the HVAC system changes. We discuss the detailed structure of the formation mechanism of local pollutant concentration distributions using various indices for indoor ventilation efficiency in simplified room models. Among the indices, visitation frequency and net escape probability are the ones that directly assist in evaluating the recirculation/return characteristics of indoor pollutants. As a result, when the proportion of air that is recirculated becomes large, the number of pollutants returning to a target local domain, the visitation frequency, increases exponentially, and the net escape probability—which directly expresses the probability of pollutant discharged from the target domain—is close to zero.     

Particle removal efficiency of the portable HEPA air cleaner in a simulated hospital ward

Use of a HEPA (high efficiency particulate air) filter in a room is believed to assist in reducing the risk of transmission of infectious diseases through removing the particles or large droplets to which pathogens may be attached. Use of a portable HEPA filter(s) in hospital wards is hypothesized to increase the effective ventilation rate (for particles only). Use of a portable HEPA filter is also hypothesized to increase the effective airflow rate of the general ward to the standard of an isolation ward for emerging infection diseases. This may be a good solution for housing patients when the number of beds in an isolation ward is insufficient. An experiment was conducted in a full scale experimental ward with a dimension of 6.7 m × 6 m × 2.7 m and 6 beds to test these hypotheses for a portable HEPA filter. The removal efficiency for different size particles was measured at different locations. The influence of the portable HEPA air cleaner on the airflow pattern was also studied through smoke visualization and computational fluid dynamics (CFD) simulations. Results show that the HEPA filter can effectively decrease the particle concentration level. The effective air change rate achieved by the HEPA filter (for particle removal only) is from 2.7 to 5.6 ACH in the ward. The strong supply air jet from the portable HEPA filter interacted with the room airflow pattern and became dominate, introducing global airflow mixing in the room. Background noise levels were also measured and noise level in the room increased when the maximum airflow of the filter was used.     

Contaminant particles removal by negative air ionic cleaner in industrial minienvironment for IC manufacturing processes

This study was carried out in a closed test chamber under natural decay, negative ionic air cleaner application, as well as air mixing mechanism with negative ionic air cleaner. Among three operation modes, the air mixing mechanism with negative ionic air cleaner can reduce particles better under the flow field condition. In the air mixing, especially vigorous one (5 ACH (Air Changes per hour)), enhanced the air cleaning effect. The highest removal efficiency was measured at a height of 0.6 m from the floor and it was decreased substantially with an increase in height. The relative effectiveness of negative ionic air cleaners was predicted to decrease with an increasing particle size. We also found that there was a limited horizontal diffusion of ions. The empirical curves fit based for the concentration gradient of NAI (Negative air ionization) generated was developed for estimating the NAI concentration with different heights and distances from the source of negative ionic air cleaner discharge.     

Flow impact of an air conditioner to portable air cleaning

In order to reduce indoor pollutant exposure, people become increasingly interested in portable air cleaning devices, which can be positioned with flexibility. Such purification devices usually discharge cleaned air with strong momentum, which can interrupt indoor airflow created by air-conditioning units. If a well-organized air circulation to a portable air cleaner is not achieved, indoor air purification cannot be fully ensured. This study has used both measurement and computational fluid dynamics (CFD) modeling to investigate the flow interaction between an air conditioner and a portable air cleaner to purify indoor gaseous pollutants. A workshop environment conditioned by an air conditioner and cleaned by a portable air cleaner was mimicked in an environmental chamber to obtain data for validation of a CFD program. Then CFD was applied to evaluate factors that may affect air purification including: positioning of the air conditioner and air cleaner, air conditioner diffuser types, air-conditioning cooling or heating running mode, and location of pollutant sources. The study finds the simulation results are in good agreement with the corresponding experimental data. The positioning coordination of an air conditioner and an air cleaner, and selection of air conditioner diffuser types shall assure a good air circulation cycle to the air cleaner to improve air purification efficacy. In addition to the cleaner effectiveness, it is also recommended to evaluate an air cleaning device in terms of the absolute pollutant concentration, if the portable air cleaner is under the interaction of an air conditioner and the local performance data are interested.     

Effectiveness and cost of reducing particle‐related mortality with particle filtration

This study evaluates the mortality‐related benefits and costs of improvements in particle filtration in U.S. homes and commercial buildings based on models with empirical inputs. The models account for time spent in various environments as well as activity levels and associated breathing rates. The scenarios evaluated include improvements in filter efficiencies in both forced‐air heating and cooling systems of homes and heating, ventilating, and air conditioning systems of workplaces as well as use of portable air cleaners in homes. The predicted reductions in mortality range from approximately 0.25 to 2.4 per 10 000 population. The largest reductions in mortality were from interventions with continuously operating portable air cleaners in homes because, given our scenarios, these portable air cleaners with HEPA filters most reduced particle exposures. For some interventions, predicted annual mortality‐related economic benefits exceed $1000 per person. Economic benefits always exceed costs with benefit‐to‐cost ratios ranging from approximately 3.9 to 133. Restricting interventions to homes of the elderly further increases the mortality reductions per unit population and the benefit‐to‐cost ratios.     

Filtration effectiveness of HVAC systems at near‐roadway schools

Concern for the exposure of children attending schools located near busy roadways to toxic, traffic‐related air pollutants has raised questions regarding the environmental benefits of advanced heating, ventilation, and air‐conditioning (HVAC) filtration systems for near‐road pollution. Levels of black carbon and gaseous pollutants were measured at three indoor classroom sites and at seven outdoor monitoring sites at Las Vegas schools. Initial HVAC filtration systems effected a 31–66% reduction in black carbon particle concentrations inside three schools compared with ambient air concentrations. After improved filtration systems were installed, black carbon particle concentrations were reduced by 74–97% inside three classrooms relative to ambient air concentrations. Average black carbon particle concentrations inside the schools with improved filtration systems were lower than typical ambient Las Vegas concentrations by 49–96%. Gaseous pollutants were higher indoors than outdoors. The higher indoor concentrations most likely originated at least partially from indoor sources, which were not targeted as part of this intervention.     

An integrated modeling tool for simultaneous analysis of thermal performance and indoor air quality in buildings

To analyze the thermal performance and indoor air quality (IAQ) in building simultaneously and quickly, we have developed an integrated modeling tool to simulate the dynamic indoor multi-parameters distributions and concentrations. The tool can take the parameters including indoor temperature, indoor humidity, and pollutant concentrations (e.g., volatile organic compounds (VOC) CO₂, particulate matter (PM)), as well as the heating/cooling load of heating, ventilating, and air-conditioning (HVAC) system into account. It couples a new zonal approach based on room air age. This paper presents the basic concept and flow chart in developing the modeling tool, and demonstrates the tool's application in a hypothetical health care building. The tool could be used for design of HVAC system with IAQ control devices and for the simultaneous analysis of thermal performance and IAQ in buildings.     

Collection of biological and non-biological particles by new and used filters made from glass and electrostatically charged synthetic fibers

Synthetic filters made from fibers carrying electrostatic charges and fiberglass filters that do not carry electrostatic charges are both utilized commonly in heating, ventilating, and air-conditioning (HVAC) systems. The pressure drop and efficiency of a bank of fiberglass filters and a bank of electrostatically charged synthetic filters were measured repeatedly for 13 weeks in operating HVAC systems at a hospital. Additionally, the efficiency with which new and used fiberglass and synthetic filters collected culturable biological particles was measured in a test apparatus. Pressure drop measurements adjusted to equivalent flows indicated that the synthetic filters operated with a pressure drop less than half that of the fiberglass filters throughout the test. When measured using total ambient particles, synthetic filter efficiency decreased during the test period for all particle diameters. For particles 0.7-1.0 mum in diameter, efficiency decreased from 92% to 44%. It is hypothesized that this reduction in collection efficiency may be due to charge shielding. Efficiency did not change significantly for the fiberglass filters during the test period. However, when measured using culturable biological particles in the ambient air, efficiency was essentially the same for new filters and filters used for 13 weeks in the hospital for both the synthetic and fiberglass filters. It is hypothesized that the lack of efficiency reduction for culturable particles may be due to their having higher charge than non-biological particles, allowing them to overcome the effects of charge shielding. The type of particles requiring capture may be an important consideration when comparing the relative performance of electrostatically charged synthetic and fiberglass filters. PRACTICAL IMPLICATIONS: Electrostatically charged synthetic filters with high initial efficiency can frequently replace traditional fiberglass filters with lower efficiency in HVAC systems because properly designed synthetic filters offer less resistance to air flow. Although the efficiency of charged synthetic filters at collecting non-biological particles declined substantially with use, the efficiency of these filters at collecting biological particles remained steady. These findings suggest that the merits of electrostatically charged synthetic HVAC filters relative to fiberglass filters may be more pronounced if collection of biological particles is of primary concern.     

Combined use of an electrostatic precipitator and a high‐efficiency particulate air filter in building ventilation systems: Effects on cardiorespiratory health indicators in healthy adults

High‐efficiency particulate air (HEPA) filtration in combination with an electrostatic precipitator (ESP) can be a cost‐effective approach to reducing indoor particulate exposure, but ESPs produce ozone. The health effect of combined ESP‐HEPA filtration has not been examined. We conducted an intervention study in 89 volunteers. At baseline, the air‐handling units of offices and residences for all subjects were comprised of coarse, ESP, and HEPA filtration. During the 5‐week long intervention, the subjects were split into 2 groups, 1 with just the ESP removed and the other with both the ESP and HEPA removed. Each subject was measured for cardiopulmonary risk indicators once at baseline, twice during the intervention, and once 2 weeks after baseline conditions were restored. Measured indoor and outdoor PM_2.5 and ozone concentrations, coupled with time‐activity data, were used to calculate exposures. Removal of HEPA filters increased 24‐hour mean PM_2.5 exposure by 38 (95% CI: 31, 45) μg/m³. Removal of ESPs decreased 24‐hour mean ozone exposure by 2.2 (2.0, 2.5) ppb. No biomarkers were significantly associated with HEPA filter removal. In contrast, ESP removal was associated with a ?16.1% (?21.5%, ?10.4%) change in plasma‐soluble P‐selectin and a ?3.0% (?5.1%, ?0.8%) change in systolic blood pressure, suggesting reduced cardiovascular risks.     

空调系统新风供给形式对室内颗粒污染物的影响

着重探讨了风机盘管加新风的空调系统和两种变风量空调系统形式中,新风量,新风过滤效率和室外含尘粒子对室内颗粒污染物的影响,最后指出对一次,二次风的混合进行中效过滤的全空气系统,能够明显提高新风稀释和排除室内粉尘的能力,完全能够满足是用户对环境日益增加的健康和舒适方面的要求。     

Particle transport characteristics in indoor environment with an air cleaner: The effect of nonuniform particle distributions

Air cleaners are expected to improve the indoor air quality by removing the gaseous contaminants and fine particles. In our former work, the effects of the air cleaner on removing the uniformly distributed particles were numerically investigated. Based on those results, this work further explores the performances of the air cleaner in the reduction of two nonuniform particle distributions generated by smoking and coughing. The Lagrangian discrete trajectory model combined with the Eulerian fluid method is employed to simulate the airflow pattern and particle transport in a room. In general, the results show that the particle fates have been resulted from the interaction between the emitting source and the air cleaner. And the position of the air cleaner is a key parameter affecting the particle concentration, for which a shorter distance between the air cleaner and the human body corresponds to a lower concentration. Besides, the air velocity emitted from the human mouth and the orientation of the air cleaner can also influence the transport of particles.     

Achieving 'excellent' indoor air quality in commercial offices equipped with air-handling unit--respirable suspended particulate

A study was carried out to investigate the feasibility of achieving ultra low respirable suspended particulates (RSP) in commercial offices without major modification of existing ventilation systems by enhancing the particulates removal efficiency of existing central ventilation systems. Four types of filters which include pre-filters, cartridge filters, bag filters and high efficiency particulates air (HEPA) filters were tested in a commercial building in Causeway Bay. The results show that an RSP objective of <20 microg/m3 could be met by removing RSP from both the return air and outdoor air supply simultaneously. This level of performance is classed as 'excellent' by the Hong Kong Government, Environmental Protection Department. Filters with efficiency that exceed 80% placed both in the return air and outdoor air were sufficient to meet the objective. It is not necessary to install HEPA filters to achieve the 'excellent' class. The outdoor air filter has great influence on the steady state indoor RSP concentration while the effective cleaning rate is governed by the return air filter. Higher efficiency filters increased the static drop but the volume flow of the air fan was not affected significantly. The additional cost incurred was <5% of the existing operation cost. PRACTICAL IMPLICATIONS: This paper reports a field study of RSP control for an indoor office environment. The results are directly applicable to building service engineering in the design of ventilation systems using air-handling units. Field observations indicated that indoor RSP in an office environment could be suppressed below 20 microg/m3 within 1 h by the simultaneous filtration of outdoor air and return air. Outdoor air filtration has a great influence on the steady state indoor concentration and return air filtration governs the cleaning rate. It is believed that the results of this study could be extended to the cleaning of other indoor pollutants such as volatile organic compounds.     

Ventilation, temperature, and HVAC characteristics in small and medium commercial buildings in California

This field study of 37 small and medium commercial buildings throughout California obtained information on ventilation rate, temperature, and heating, ventilating, and air-conditioning (HVAC) system characteristics. The study included seven retail establishments; five restaurants; eight offices; two each of gas stations, hair salons, healthcare facilities, grocery stores, dental offices, and fitness centers; and five other buildings. Fourteen (38%) of the buildings either could not or did not provide outdoor air through the HVAC system. The air exchange rate averaged 1.6 (s.d. = 1.7) exchanges per hour and was similar between buildings with and without outdoor air supplied through the HVAC system, indicating that some buildings have significant leakage or ventilation through open windows and doors. Not all buildings had sufficient air exchange to meet ASHRAE 62.1 Standards, including buildings used for fitness centers, hair salons, offices, and retail establishments. The majority of the time, buildings were within the ASHRAE temperature comfort range. Offices were frequently overcooled in the summer. All of the buildings had filters, but over half the buildings had a filter with a minimum efficiency reporting value rating of 4 or lower, which are not very effective for removing fine particles. PRACTICAL IMPLICATIONS: Most U.S. commercial buildings (96%) are small- to medium-sized, using nearly 18% of the country's energy, and sheltering a large population daily. Little is known about the ventilation systems in these buildings. This study found a wide variety of ventilation conditions, with many buildings failing to meet relevant ventilation standards. Regulators may want to consider implementing more complete building inspections at commissioning and point of sale.     

Health benefits and costs of filtration interventions that reduce indoor exposure to PM2.5 during wildfires

Increases in hospital admissions and deaths are associated with increases in outdoor air particles during wildfires. This analysis estimates the health benefits expected if interventions had improved particle filtration in homes in Southern California during a 10‐day period of wildfire smoke exposure. Economic benefits and intervention costs are also estimated. The six interventions implemented in all affected houses are projected to prevent 11% to 63% of the hospital admissions and 7% to 39% of the deaths attributable to wildfire particles. The fraction of the population with an admission attributable to wildfire smoke is small, thus, the costs of interventions in all homes far exceeds the economic benefits of reduced hospital admissions. However, the estimated economic value of the prevented deaths exceed or far exceed intervention costs for interventions that do not use portable air cleaners. For the interventions with portable air cleaner use, mortality‐related economic benefits exceed intervention costs as long as the cost of the air cleaners, which have a multi‐year life, are not attributed to the short wildfire period. Cost effectiveness is improved by intervening only in the homes of the elderly who experience most of the health effects of particles from wildfires.