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This is an account that should be heard of an important struggle: the struggle of a large group of experts who came together at the beginning of the COVID-19 pandemic to warn the world about the risk of airborne transmission and the consequences of ignoring it. We alerted the World Health Organization about the potential significance of the airborne transmission of SARS-CoV-2 and the urgent need to control it, but our concerns were dismissed. Here we describe how this happened and the consequences. We hope that by reporting this story we can raise awareness of the importance of interdisciplinary collaboration and the need to be open to new evidence, and to prevent it from happening again. Acknowledgement of an issue, and the emergence of new evidence related to it, is the first necessary step towards finding effective mitigation solutions., Competing Interests: Potential conflicts of interest. J. H. reports an unpaid position as secretary/treasurer of BOD of IEQ-GA. F. F. reports an SBIR grant from Franchimon ICM, informal shares and co-founder stock from BlueCanary and iventionair, consulting fees for professional papers and reports from Franchimon ICM, payment for lectures from TVVL (Member Association for HVAC), and payment for webinars from the Dutch Green Building Council. A. B. reports a position at Delft University of Technology and consultancy with a company specialized in indoor environmental quality. J. L. J. reports grants or contracts from NASA, US National Science Fund, and Balvi Filantropic Fund; payment for lectures at Peking University; and unpaid roles on the committee for Testing of Air Cleaning Devices of ASTM and Aireamos volunteer group on ventilation solutions for the Spanish-speaking world. Y. L. reports a collaborative research grant and 3 general research grants from the Hong Kong Research Grants Council (RGC), a grant from the Health and Medical Research Fund (HMRF), and grants or contracts from P&G Singapore. Marcel G.L.C. Loomans reports grants or contracts from TVVL, RVO government program TNO for P3Venti project, a grant from TKI NL Health Holland for the CLAIRE project, and a grant from NOW for the MIST project (all paid to the author's institution); M. G. L. C. L. also serves as an ISIAQ.nl board member, and is a member of Impulgroep TVVL (Dutch Association of Building Services Engineers), Kennisgroep VCCN (Dutch Association on Contaminant Control), and WHO, for which they sit on the Environment and Engineering Control Expert Advisory Panel (ECAP) for COVID-19. L. C. M. reports a contract with the National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH) Department of Health and Human Services (HHSN272201400007C); grants or contracts from the Flu Lab and Wellcome Leap (all paid to the author's institution); consulting fees paid to the author from MITRE Corporation, Freedman Consulting, and The New York Times; payment or honoraria paid to the author from the University of Toronto, University of Hong Kong, Queensland University of Technology, and Virginia Tech Carilion School of Medicine; support for meetings and/or travel from MITRE Corporation and the International Society for Influenza and Other Respiratory Viruses. G. M. reports their role as president of the International Union Against TB & Lung Disease. D. K. M. reports the following grants or contracts—US NIAID U19 AI 162130, for Evaluating Modes of Influenza Transmission (EMIT-2) using Innovative Technologies and Designs in Controlled Environments; US NIAID contract number 75N93021C00014, for the Centers of Excellence for Influenza Research and Response, for Wearable Micro Air Cleaner for Protection of Workers’ Health during Prolonged Use (US NSF Project 75D30121C10610); US NIAID AI148689, A Phase 3, Randomized, Stratified, Observer-Blind, Placebo-Controlled Study to Evaluate the Efficacy, Safety, and Immunogenicity of mRNA-1273 SARS-CoV-2 Vaccine in Adults Aged 18 Years and Older; US NIAID F3038571, Implementing Vaccine and Treatment Evaluation Unit (VTEU) clinical site protocols (CoVPN 3004 Novavax Supplement); US NIAID R21 AI161501, Elucidating Airborne SARS-CoV-2 Infectivity at Single Aerosol Resolution; US National Institute of Environmental Health Sciences (NIEHS) R01 ES034303, Research Employing Environmental Systems and Occupational Health Policy Analyses to Interrupt the Impact of Structural Racism on Agricultural Workers and Their Respiratory Health (RESPIRAR); Bill & Melinda Gates Foundation INVO022295, COVID-19: Quantifying viral RNA in exhaled breath; US NIAID 025806864609, COVID-19 Surveillance and Exhaled Breath Aerosol Assessment Study; US National Science Foundation (NSF) CMMI2034755, EAGER: Protecting University Communities from COVID-19 with Model-Based Risk Management; US NSF 2030502, COVID: Collaborative Research: RAPID: Understanding and Facilitating Remote Triage and Rehabilitation During Pandemics via Visual Based Patient Physiology Sensing, WMATA Bus Air Cleaning Effectiveness with Novel Air Cleaning Systems, College Band Directors National Association Aerosol Generation from Playing Band Instruments and Risk of Infectious Disease Transmission; US Centers for Disease Control and Prevention (CDC) COVID-19: Real-World Tests of Face Coverings as Source Control for COVID-19 and US DARPA, BARDA Contagious Phenotypes of Acute Respiratory Infection: Identification, Characterization, and Biomarkers. D. K. M. also reports payment or honoraria for the American Conference of Governmental Hygienists from the American Dental Association, support for meetings and/or travel from the Flu Lab, and receipt of UVC fixtures from Aeromed, Inc, and FarUV, Inc. C. N. reports the following grants or contracts paid to their institution: Class-ACT (Co-Investigator [CoI]), 2021–22, DHSC-funded project on air cleaning interventions in schools; Far UV projects (CoI), 2021 DHSC funding, 2022 NHS Scotland Assure funding collaboration led by St Andrews with Leeds, Dundee, Ninewells Hospital, Colombia University; PROTECT National Core Study on Transmission (CoI, theme lead), 2020–2023, £21 M program led by HSE; CONTACT (CoI), 2020–2023, NIHR, £1.6 M, Breathing City: Future Urban Ventilation Network (Principal Investigator [PI]), 2020–2023, UKRI SPF Clean Air Programme Network, £500 K; TRACK: Transport Risk Assessment for COVID Knowledge (PI), 2020–2023; EPSRC, £3.4. Multi-partner project with Universities of Manchester, Cambridge, Newcastle, Imperial, UKHSA, Dstl, Department for Transport and multiple transport stakeholders, CECAM, Engineering and Physical Sciences Research Council (EPSRC) £950 K funding to support a new bioaerosol chamber (2021–2025); Centre for Doctoral Training in Fluid Dynamics (Co-Director, PI Prof P. Jimack), 2019–2027, EPSRC, £4.2 M; Cross-faculty programme to train 50 PhD students over 8 years; and EPSRC National Fellowships in Fluid Dynamics coordination hub (CoI), 2022–2025. C. N. also reports honoraria for Royal Institution Christmas Lecture; payment from BBC Life Scientific; accommodation and registration fees for Indoor Air June 2022, Kuopio, Finland, and BOHS annual conference, June 2022, Belfast; travel expenses for the International Symposium on Stratified Fluid, August 2022; travel and registration fees for IHEEM annual conference (October 2022). C. N. served as co-chair for SAGE Environment and Modelling Group (April 2020–May 2022) and participated in multiple UK government advisory groups during COVID, serves as Chair for HSE Science Quality Assurance Group, is a member of the UKRI SPF clean air programme steering board, a member of the WHO Environment and Engineering Control Expert Advisory Panel (ECAP) on ventilation and WHO European High Level Expert Group on COVID-19 leading the ventilation subgroup, and as a Member of the Royal Academy of Engineering COVID-19 Working Group on Infection Resilient Buildings and co-author of reports in July 2021 and May/June 2022. C. N. also received 3 Aranet CO2 sensors from Naltic Industrials. S. M. reports a leadership role as the Vice President for the Academy of Fellows, International Society for Indoor Air Quality and Climate, and on the Aerosol Science and Technology Journal advisory board (both of which are unpaid positions). W. B. reports an unpaid role as chair of the ASHRAE Epidemic Task Force. P. M. B. serves in a leadership or fiduciary role on the KNAW Commission and the Pandemic Disaster Preparedness Centre (both unpaid positions). All other authors report no potential conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed., (© The Author(s) 2023. Published by Oxford University Press on behalf of Infectious Diseases Society of America.)

The coronavirus disease 2019 (COVID-19) pandemic has caused untold disruption throughout the world. Understanding the mechanisms for transmission of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is key to preventing further spread, but there is confusion over the meaning of 'airborne' whenever transmission is discussed. Scientific ambivalence originates from evidence published many years ago which has generated mythological beliefs that obscure current thinking. This article collates and explores some of the most commonly held dogmas on airborne transmission in order to stimulate revision of the science in the light of current evidence. Six 'myths' are presented, explained and ultimately refuted on the basis of recently published papers and expert opinion from previous work related to similar viruses. There is little doubt that SARS-CoV-2 is transmitted via a range of airborne particle sizes subject to all the usual ventilation parameters and human behaviour. Experts from specialties encompassing aerosol studies, ventilation, engineering, physics, virology and clinical medicine have joined together to produce this review to consolidate the evidence for airborne transmission mechanisms, and offer justification for modern strategies for prevention and control of COVID-19 in health care and the community.

During the rapid rise in COVID-19 illnesses and deaths globally, and notwithstanding recommended precautions, questions are voiced about routes of transmission for this pandemic disease. Inhaling small airborne droplets is probable as a third route of infection, in addition to more widely recognized transmission via larger respiratory droplets and direct contact with infected people or contaminated surfaces. While uncertainties remain regarding the relative contributions of the different transmission pathways, we argue that existing evidence is sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit infection risk indoors. Appropriate building engineering controls include sufficient and effective ventilation, possibly enhanced by particle filtration and air disinfection, avoiding air recirculation and avoiding overcrowding. Often, such measures can be easily implemented and without much cost, but if only they are recognised as significant in contributing to infection control goals. We believe that the use of engineering controls in public buildings, including hospitals, shops, offices, schools, kindergartens, libraries, restaurants, cruise ships, elevators, conference rooms or public transport, in parallel with effective application of other controls (including isolation and quarantine, social distancing and hand hygiene), would be an additional important measure globally to reduce the likelihood of transmission and thereby protect healthcare workers, patients and the general public.

Increasing evidence supports the significant role of short-range airborne transmission of viruses when in close contact with a source patient. A full-scale ventilated room (Cleanliness: ISO 14644–1 Class 5) and two face-to-face standing breathing thermal manikins were used to simulate a source individual and a susceptible person. Monodisperse particle generation and measurement techniques were used to evaluate the effect of virus-laden droplet nuclei size on short-range airborne transmission risk. We analysed four particle sizes (1.0, 1.5, 2.5, and 5.0 µm) to simulate the transport of exhaled droplet nuclei within an interpersonal distance of 0.5 m. The results indicated that the size distribution of airborne droplet nuclei could significantly influence transmission, with the inhalation fraction decreasing with increasing droplet nuclei size. Additionally, results showed that proximity to the source manikin could influence transmission. Inhalation fraction decreased with increasing interpersonal distance, fitting well with the 1/ d rule of droplet nuclei concentration decay. Our findings improve the understanding of the mechanism of the disease transmission. [ABSTRACT FROM AUTHOR]

The global spread of the coronavirus disease 2019 (COVID-19) has increased the demand of effective control of the disease transmission between people, especially when they are in close distance between each other. The microenvironment between the people in short distance contains multiple airflow patterns that directly affect the disease transmission. By understanding and respecting this special localized environment, the airborne cross-infection at both short and long distance can be minimized. This paper gives an overview of the flow fields in human microenvironment. The exhalation flow from different respiratory activities, e.g. normal breathing, speaking, coughing or sneezing is considered as a part of the microenvironment. The dynamics of the exhalation flow and the contained droplets or aerosols are summarized from previous studies. The factors influencing the flow fields in human microenvironment are discussed, including both the physiological factors of the occupants and the environmental factors in the ventilated context (macroenvironment). Effective control of these influencing factors can be helpful to mitigate airborne transmission risk between individuals. This paper highlights the importance of better understanding of the dynamics and transmission routes of the expelled virus-laden droplets or aerosols, which are largely affected by complex flow interactions in human microenvironment. [ABSTRACT FROM AUTHOR]

In order to control the spread of Covid-19, authorities provide various prevention guidelines and recommendations for health workers and the public. Personal protection equipment (PPE) and physical barrier are the most widely applied prevention measures in practice due to their affordability and ease of implementation. This study aims to investigate the effect of PPE and physical barriers on mitigating the short-range airborne transmission between two people in a ventilated environment. Four types of PPE (surgical mask, two types of face shield, and mouth visor), and two different sizes of the physical barrier were tested in a controlled environment with two life-size breathing thermal manikins. The PPE was worn by the source manikin to test the efficiency of source control. The measurement results revealed that the principles of PPE on preventing short-range droplet and airborne transmission are different. Instead of filtering the fine droplet nuclei, they mainly redirect the virus-laden exhalation jet and avoid the exhaled flow entering the target's inhalation region. Physical barriers can block the spreading of droplet nuclei and create a good micro environment at short distances between persons. However, special attention should be paid to arranging the physical barrier and operating the ventilation system to avoid the stagnant zone where the contaminant accumulates. • The principles of PPE on preventing droplet and airborne transmission are different. • PPE redirects the exhalation jet and avoids the exhalation flow to enter the target's inhalation region. • Physical barriers block the spreading of fine droplet nuclei and create a good macro environment. • Ventilation shows significant impacts on the exposure risk in the scenario without PPE or physical barriers. [ABSTRACT FROM AUTHOR]

• Three phases of probable aerosol transmission of SARS-CoV-2 from source to receptor are comprehensively reviewed. • Key parameters of two models for SARS-CoV-2 risk assessment are analyzed. • Ventilation plays an important role in mitigating infection risk by intervening both dispersion and inhalation phases. • Novel personalized, intelligent and resilient ventilation strategies should be developed for clear objective of epidemic control. Global spread of COVID-19 has seriously threatened human life and health. The aerosol transmission route of SARS-CoV-2 is observed often associated with infection clusters under poorly ventilated environment. In the context of COVID-19 pandemic, significant transformation and optimization of traditional ventilation systems are needed. This paper is aimed to offer better understanding and insights into effective ventilation design to maximize its ability in airborne risk control, for particularly the COVID-19. Comprehensive reviews of each phase of aerosol transmission of SARS-CoV-2 from source to receptor are conducted, so as to provide a theoretical basis for risk prediction and control. Infection risk models and their key parameters for risk assessment of SARS-CoV-2 are analyzed. Special focus is given on the efficacy of different ventilation strategies in mitigating airborne transmission. Ventilation interventions are found mainly impacting on the dispersion and inhalation phases of aerosol transmission. The airflow patterns become a key factor in controlling the aerosol diffusion and distribution. Novel and personalized ventilation design, effective integration with other environmental control techniques and resilient HVAC system design to adapt both common and epidemic conditions are still remaining challenging, which need to be solved with the aid of multidisciplinary research and intelligent technologies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Personalized ventilation (PV) is proved to be effective in mitigating user's airborne infection risk. • Protective effect of PV is significantly affected by relative orientations between occupants. • A risk reduction ratio of SARS-CoV-2 is achieved of 0.65 over 2 h with dose–response model. • Four risk models were validated with effectiveness in predicting exposure or infection risk with PV. In the context of COVID-19, new requirements are occurring in ventilation systems to mitigate airborne transmission risk in indoor environment. Personalized ventilation (PV) which directly delivers clean air to the occupant's breathing zone is considered as a promising solution. To explore the potentials of PV in preventing the spread of infectious aerosols between closely ranged occupants, experiments were conducted with two breathing thermal manikins with three different relative orientations. Nebulized aerosols were used to mimic exhaled droplets transmitted between the occupants. Four risk assessment models were applied to evaluate the exposure or infection risk affected by PV with different operation modes. Results show that PV was effective in reducing the user's infection risk compared with mixing ventilation alone. Relative orientations and operation modes of PV significantly affected its performance in airborne risk control. The infection risk of SARS-CoV-2 was reduced by 65% with PV of 9 L/s after an exposure duration of 2 h back-to-back as assessed by the dose–response model, indicating effective protection effect of PV against airborne transmission. While the side-by-side orientation was found to be the most critical condition for PV in airborne risk control as it would accelerate diffusion of infectious droplets in lateral diffusion to occupants by side. Optimal designs of PV for closely ranged occupants were hereby discussed. The four risk assessment models were compared and validated by experiments with PV, implying basically consistent rules of the predicted risk with PV among the four models. The relevance and applicability of these models were discussed to provide a basis for risk assessment with non-uniformly distributed pathogens indoor. [ABSTRACT FROM AUTHOR]