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The COVID-19 pandemic revealed opportunities to improve prevention practices in healthcare settings, mainly related to the spread of airborne microbes (also known as bioaerosols). This scoping review aimed to map methodologies used to assess the implementation of portable air cleaners in healthcare settings, identify gaps, and propose recommendations for future research. The protocol was registered in the Open Science Framework and reported following the checklist provided by the Preferred Reporting Items for Systematic Reviews and Meta-Analysis – an extension for Scoping Reviews (PRISMA-ScR) statement. The search strategy was performed in five databases and one grey literature source. At the last selection phase, 24 articles that fulfilled our inclusion criteria were summarized and disseminated. Of these, 17 studies were conducted between 2020 and 2022; one of them was a protocol of a multicentre randomized controlled trial. The outcomes measured among the studies include airborne microbe counts, airborne particle concentrations, and rate of infections/interventions. The leading healthcare settings assessed were dental clinics (28%), patient's wards (16%), operating rooms (16%), and intensive care units (12%). Most of the devices demonstrated a significant potential to mitigate the impact of bioaerosols. Although some indoor air quality parameters can influence the mechanics of aerosols, only a few studies controlled these parameters in their analyses. Future clinical research should assess the rate of infections through randomized controlled trials with long-term follow-up and large sample sizes to determine the clinical importance of the findings.
The COVID-19 pandemic raised awareness of the high risk that represents respiratory pathogens spread by aerosols – particularly in enclosed spaces with poor ventilation [
]. Although it is not a new concern in health facilities since it has long been a top priority for the Centers for Disease Control and Prevention (CDC), the pandemic outbreaks did evidence the need for evolution and innovation to mitigate the impact of aerosols [
The Centers for Disease Control and Prevention STRIVE initiative: construction of a national program to reduce health care-associated infections at the local level.
Aerosols are liquid or solid particles suspended in the air by natural or artificial sources. Depending on their weight, these particles can remain suspended in the air for hours and travel long distances through the airborne route [
]. In cases when it is impossible to reduce the sources of aerosols or the dilution ventilation is insufficient, the implementation of portable air cleaners has been proposed as a coadjutant measure in residential, commercial buildings, and healthcare settings [
The portable air cleaners use different technologies such as fibrous media air filters, generally rated as high-efficiency particulate air filters (HEPA) or ultra-low particulate air filters (ULPA), ultraviolet air filtration, and electronic air cleaners, including electrostatic precipitators and ionizers, alone or in combination [
]. Fibrous media air filters remove particles by capturing them on fibrous filter materials. Electrostatic precipitators and ionizers remove particles by an active electrostatic charging process. Ultraviolet air filtration reduces viable airborne micro-organisms by killing or deactivating them [
]. Gas-phase air-cleaning technologies include adsorbent air filters such as activated carbon, chemisorbed media air filters, photocatalytic oxidation, plasma, and intentional ozone generators, designed to remove gaseous air pollutants or convert them to harmless [
The effectiveness of different portable air cleaners was reported to range from 12 to 99% depending on the technology used, setting, and outcome assessments across the studies [
]. In theory, one can assume that lowering the airborne particle concentrations and airborne microbial counts in the indoor air would result in lower rates of infection. This scoping review aimed to map and summarize overall research (published and grey literature) assessing the implementation of portable air-cleaning technologies in healthcare settings; additionally, to report the outcomes measured across the studies, the characteristics and range of the used methodologies, challenges, and limitations, and to propose recommendations for future research.
Methods
Protocol and registration
This scoping review was registered in the OSF database (doi: https://osf.io/8g9ap), conducted following the guidelines for conducting systematic scoping reviews of the JBI Briggs Reviewers Manual, and reported following the checklist provided by the PRISMA-ScR statement (Supplementary Table S1) [
The inclusion criteria were guided by the review question: What outcomes have been measured in existing research to assess the implementation of portable air cleaners in healthcare settings? Studies that aimed to assess the implementation of portable air-cleaning devices in healthcare settings (medical and dental clinics and offices, urgent care centres, large hospitals) in real or quasi-real-life scenarios compared to no implementation were considered. No limitations of language or publication date were established.
An initial limited search was performed in PubMed to analyse text words contained in the title and abstracts across the articles that fulfilled our eligibility criteria. The following MeSH terms and keywords were combined: Hospital∗ OR ‘Health Facilities’ OR ‘Dental Clinic∗’ AND ‘Air Filters∗’ OR ‘Air Purifier∗’ OR ‘Portable Air Cleaner∗’ OR ‘Air Circulation’ OR ‘Air Filtration’ OR ‘High-Efficiency Particulate Air Filter∗’ OR ‘Ultraviolet Air Filtration’ OR ‘Plasma Air Filtration’. After selecting keywords and index terms, a second search was performed by two independent reviewers. Five databases (PubMed, Embase, Scopus, Cochrane Library, and Web of Science) were used to identify all the published articles on the topic, and one grey literature source (Grey Matters) was used to identify unpublished articles (Supplementary Table S2).
The reference list of included articles was also assessed to search for additional studies, and search alerts were activated in each database. Sources were last accessed in June 2022. All citations found were imported into a reference manager (EndNote, version 20.3, Thomson Reuters), and duplicates were removed automatically and manually.
Extraction of data and charting
Two independent authors (M.A. and J.D.) extracted and charted data from the included studies. The following information was tabulated: author, country, year of publication, aims, healthcare setting, description of the device used (commercial name, airflow settled, noise, and type of technology), and outcomes measured. The studies were also summarized and charted according to the outcome assessed, describing how these outcomes were measured (methodology and measurement tool) and reported results.
Results
Study selection
In total, 2023 citations were identified, and 425 duplicates were removed. After title and abstract analyses, 31 articles were selected for full-text reading, of which one report was not retrieved. Six articles were excluded: one extended abstract [
The effectiveness of an air cleaner in controlling droplet/aerosol particle dispersion emitted from a patient’s mouth in the indoor environment of dental clinics.
Protocol: EPOS trial: the effect of air filtration through a plasma chamber on the incidence of surgical site infection in orthopaedic surgery: a study protocol of a randomised, double-blind, placebo-controlled trial.
Indoor air quality evaluation using mechanical ventilation and portable air purifiers in an academic dentistry clinic during the COVID-19 pandemic in Greece.
A quasi-experimental study analyzing the effectiveness of portable high-efficiency particulate absorption filters in preventing infections in hematology patients during construction.
], 17 studies were conducted and published between 2020 and 2022 during the COVID-19 outbreaks. However, only one study described the detection of SARS-CoV-2 RNA in air samples collected in addition to other airborne microbes (a range of other bacterial, viral, and fungal pathogens) [
]. The outcomes measured – alone or combined – among the included studies were airborne microbial counts, airborne particle concentrations, and rate of infections or interventions (Figure 2). The characteristics of the included studies are described in Table I, including the commercial names and technology of the devices tested, as well as the main technical descriptions when the authors or the manufacturers' websites supplied the information.
Figure 2Outcome measurements across the included studies to assess the implementation of portable air cleaners in healthcare settings over time.
Protocol: EPOS trial: the effect of air filtration through a plasma chamber on the incidence of surgical site infection in orthopaedic surgery: a study protocol of a randomised, double-blind, placebo-controlled trial.
Indoor air quality evaluation using mechanical ventilation and portable air purifiers in an academic dentistry clinic during the COVID-19 pandemic in Greece.
A quasi-experimental study analyzing the effectiveness of portable high-efficiency particulate absorption filters in preventing infections in hematology patients during construction.
The healthcare settings assessed were dental clinics (28%), patients' wards (16%), operating rooms (16%), intensive care units (12%), single-bed patient rooms (8%), emergency units (8%), renal units (4%), and haematology units (4%), and rooms for high-resolution oesophageal manometry (4%), including teaching hospitals and clinics in different countries (Australia 9%, Belgium 4%, Brazil 4%, Canada 9%, Germany 4%, France 4%, UK 9%, Greece 4%, Italy 8%, Sweden 4%, Singapore, Turkey 8%, USA 25%).
Assessment of airborne microbial counts using portable air cleaners
Airborne microbial counts were assessed in 11 studies (three of them were assessed in addition to airborne particles), one randomized clinical trial, and 10 in-situ experiments in real-life scenarios by different sampling methods and measurement tools [
] as shown in Table II. Arikan et al. and Pouvaret et al. assessed surface microbial counts in addition to airborne microbial counts by a surface swab test [
In all the studies, the sources of aerosols were patients and procedures performed during the sampling period. Table II also shows the calibration of the measurement tools (when reported), airborne microbes assessed, analysis method, and outcomes reported using portable air cleaners in each study assessing microbiological contamination. Regarding indoor air parameters that can influence microbiological results, only four of these studies assessed temperature or relative humidity [
Assessment of airborne particle concentrations using portable air cleaners
The concentration of airborne particle or particle matter of different aerodynamic diameters was measured in 12 studies through in-situ experiments in real [
Indoor air quality evaluation using mechanical ventilation and portable air purifiers in an academic dentistry clinic during the COVID-19 pandemic in Greece.
] scenarios, alone or combined. Table III shows details of the measuring tools used (when reported), airborne particle sizes measured, and outcomes reported using portable air cleaners in each study assessing airborne particulate concentration.
Table IIISummary of the methodologies, measurement tools, and outcomes assessing airborne particles
Indoor air quality evaluation using mechanical ventilation and portable air purifiers in an academic dentistry clinic during the COVID-19 pandemic in Greece.
0.5–10.0/particle counter (BioTrak; TSI, Minneapolis, MN, USA)
–
–
–
Significant reduction in total airborne particle values
T, temperature; RH, relative humidity; CO2, carbon dioxide; PAC, portable air cleaner; AGPs, aerosol generator dental procedure; NI, not informed; ACH, air changes per hour; HVAC, heating, ventilation, and air conditioning; HRM, high-resolution manometry; ISO, International Organization for Standardization.
a ISO 1 indicates the cleanest and ISO 9 the dirtiest air.
In one study, in addition to particle concentration levels, they also assessed the aerosol size distribution with an aerodynamic particle size spectrometer [
]. Seven studies calculated the pre-existing ACH in the rooms (Table III). Capparé et al. mention that the pre-existing ACH was calculated, but the data is missing in the results [
Indoor air quality evaluation using mechanical ventilation and portable air purifiers in an academic dentistry clinic during the covid-19 pandemic in Greece.
Rate of infections or interventions using portable air cleaners
Five studies measured the correlation or association between the implementation of portable air cleaners and decreased rates of infections or intervention [
Protocol: EPOS trial: the effect of air filtration through a plasma chamber on the incidence of surgical site infection in orthopaedic surgery: a study protocol of a randomised, double-blind, placebo-controlled trial.
A quasi-experimental study analyzing the effectiveness of portable high-efficiency particulate absorption filters in preventing infections in hematology patients during construction.
Protocol: EPOS trial: the effect of air filtration through a plasma chamber on the incidence of surgical site infection in orthopaedic surgery: a study protocol of a randomised, double-blind, placebo-controlled trial.
The European Polyp Surveillance (EPoS) study is a large multi-national project financed by multiple sources in the participating countries.
that was conducted at seven hospitals from 2017 to 2022 to assess the implementation of air-cleaning devices and the incidence of surgical site infections in orthopaedic surgery. Description of the impacts measured, study design, follow-up, and outcomes reported by the studies are summarized in Table IV.
Table IVGeneral characteristics of the included studies in which rates of infections or interventions were assessed
Protocol: EPOS trial: the effect of air filtration through a plasma chamber on the incidence of surgical site infection in orthopaedic surgery: a study protocol of a randomised, double-blind, placebo-controlled trial.
Improvement of health outcomes for patients admitted with respiratory distress in the paediatric hospital setting
Non-randomized prospective study of 562 patients
3 months
Non-significant association with the decreased overall length of stay in the hospital and ICU, intubation, nebulizer, and non-invasive ventilation use. However, the authors reported that these reductions were clinically meaningful with a significant impact on the healthcare system.
A quasi-experimental study analyzing the effectiveness of portable high-efficiency particulate absorption filters in preventing infections in hematology patients during construction.
Decreased rate of infections in patients being treated for haematologic malignancies during construction near the hospital
Non-randomized prospective study
12 months
Significant association with decreased overall rates of infections. The preventive effect was more pronounced in patients with acute lymphocytic leukaemia, patients undergoing consolidation therapy, and patients with moderate neutropenia.
Most of the 24 studies included in this review (71%) were conducted after the COVID-19 outbreak from 2020 to 2022. Only one study assessed SARS-CoV-2 RNA in air samples collected in a ward and an intensive care unit adapted for COVID-19 patients [
In summary, 20 out of 24 studies demonstrated significant potential to prevent and mitigate the impact of bioaerosols in healthcare environments, regardless of the scenario and methodology used. These results were especially prominent in settings with poor ventilation, such as dental clinics, where economic issues or lack of guidelines may limit the installation of an appropriate ventilation system. According to the US Occupational Information Network (O∗NET), which calculates risk levels for different occupations, dentists and clinical dentistry professionals are at the top of the risk scale when comparing ‘exposure to disease and infection’ versus ‘physical proximity to other people’ [
How often are dental care workers exposed to occupational characteristics that put them at higher risk of exposure and transmission of COVID-19? A comparative analysis.
]. Across the nine different healthcare facilities of the included studies in this review, 28% of the studies that fulfilled our inclusion criteria were performed in dental clinics, followed by patients' wards (16%), operating rooms (16%), and intensive care units (12%).
This review has limitations. Variability in aerosol measurement remains a challenge. Active air samplers exhibit high variability, yielding different results in the exact location simultaneously. A calibration following validated standards is strictly necessary, but it was not mentioned in all studies in this review.
Although the ACH sums up all methods of aerosol removal – natural or mechanical (e.g. unknown leakage, settling, opening windows, HVAC system, etc.) – which could significantly impact the measured outcome, only seven studies calculated the pre-existing ACH in the rooms. Also, temperature, relative humidity, and air velocity directly influence aerosols' mechanics, but only seven studies controlled these parameters in their statistical analyses.
Methodologies and outcome measurements were not standardized in current research, compromising the overall quantitative measure of the magnitude of the effect. Although several studies assessed airborne microbial counts, the conclusions of these studies cannot be extrapolated to species that require specific growing conditions or different sampling requirements. Some species cannot be detected with conventional culture counting methods, requiring additional analysis that includes molecular identification methods.
Although the included studies assessed the outcomes using well-known methodologies, most of them lacked the complexity associated with analysing indoor air quality in healthcare settings. Standardization of methods is necessary to obtain a body of evidence with less heterogeneity, which would allow for establishing the size of the effect, making recommendations, direct comparisons, and cost/benefit analyses of the implementation of portable air cleaners in healthcare settings. Given the global economic pressure on clinical settings and the rapid evolution of practices to live with COVID-19, the device manufacturers should focus on efficiency and being affordable for their implementation in low- or medium-income countries.
Future research should assess (a) active airborne microbial sampling (at least overall fungi and bacteria) or quantitative PCR analysis, (b) airborne particle concentrations (<5 μm), and (c) indoor air parameters (mainly ACH, temperature, relative humidity, and air velocity) to be controlled in statistical analyses, including the flow of people and the procedures performed during sampling. Most importantly, studies need to evaluate the influence of portable air cleaners on rates of infections through prospective randomized or non-randomized trials with long-term follow-up and large sample sizes.
We recommend calculating the sample size for microbiological sampling, preferably based on a pilot study for assessing the variability of the setting since physical and biological variables may affect the aerosol mechanics or viability of micro-organisms. A full description of these technologies should be supplied – including their design, ease of use, noise level, maintenance cost, and the energy consumption – enabling us to compare the cost-effectiveness of the different devices tested.
Acknowledgements
A.G. and G.M. thank FACEPE, CNPq, and CAPES for the continuing support of their research. Authors also thank to the National Institute of Photonics project, grant CNPq 403233/2017-8.
Author contributions
M.A. designed the study and drafted the paper with input from all authors. M.A. and J.D. performed the searches and data extraction. M.A., J.D., and B.L. analysed methodologies. G.M. and A.G. revised the manuscript critically for important intellectual content and final approval of the published version.
Conflict of interest statement
None declared.
Funding statement
This study was supported by the Fundação Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil, Finance Code 001; and did not receive any specific grant from other funding agencies in the public, commercial, or not-for-profit sector.
Appendix A. Supplementary data
The following is the Supplementary data to this article:
The Centers for Disease Control and Prevention STRIVE initiative: construction of a national program to reduce health care-associated infections at the local level.
The effectiveness of an air cleaner in controlling droplet/aerosol particle dispersion emitted from a patient’s mouth in the indoor environment of dental clinics.
Protocol: EPOS trial: the effect of air filtration through a plasma chamber on the incidence of surgical site infection in orthopaedic surgery: a study protocol of a randomised, double-blind, placebo-controlled trial.
Indoor air quality evaluation using mechanical ventilation and portable air purifiers in an academic dentistry clinic during the COVID-19 pandemic in Greece.
A quasi-experimental study analyzing the effectiveness of portable high-efficiency particulate absorption filters in preventing infections in hematology patients during construction.
Indoor air quality evaluation using mechanical ventilation and portable air purifiers in an academic dentistry clinic during the covid-19 pandemic in Greece.
How often are dental care workers exposed to occupational characteristics that put them at higher risk of exposure and transmission of COVID-19? A comparative analysis.
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