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Indoor Air Changes and Potential Implications for SARS-CoV-2 Transmission

Air Purification

Buildings have been associated with spread of infectious diseases, says Joseph Allen

Buildings have been associated with the spread of infectious diseases, such as outbreaks of measles, influenza, and Legionella. With SARS-CoV-2, the majority of outbreaks involving 3 or more people have been linked with time spent indoors, and evidence confirms that far-field airborne transmission (defined as within-room but beyond 6 feet) of SARS-CoV-2 is occurring.

Controlling concentrations of indoor respiratory aerosols to reduce airborne transmission of infectious agents is critical and can be achieved through source control (masking, physical distancing) and engineering controls (ventilation and filtration). With respect to engineering controls, an important flaw exists in how most buildings operate in that the current standards for ventilation and filtration for indoor spaces, except for hospitals, are set for bare minimums and not designed for infection control. Several organizations and groups have called for increasing outdoor air ventilation rates, but, to date, there has been limited guidance on specific ventilation and filtration targets. This article describes the rationale for limiting far-field airborne transmission of SARS-CoV-2 through increasing outdoor air ventilation and enhancing filtration, and provides suggested targets.

To reduce far-field airborne transmission of SARS-CoV-2 in small-volume indoor spaces (eg, classrooms, retail shops, homes if guests are visiting), the suggestions include targeting 4 to 6 air changes per hour, through any combination of the following: outdoor air ventilation; recirculated air that passes through a filter with at least a minimum efficiency rating value 13 (MERV 13) rating; or passage of air through portable air cleaners with HEPA (high-efficiency particulate air) filters.

Despite the dose-response for SARS-CoV-2 being unknown, and continued scientific debate about the dominant mode of transmission, evidence supports these suggestions. First, SARS-CoV-2 is primarily transmitted from the exhaled respiratory aerosols of infected individuals. Larger droplets (>100 μm) can settle out of the air due to gravitational forces within 6 feet, but people emit 100 times more smaller aerosols (<5 μm) during talking, breathing, and coughing. Smaller aerosols can stay aloft for 30 minutes to hours and travel well beyond 6 feet.1 Second, high-profile and well-described SARS-CoV-2 outbreaks across multiple space types (eg, restaurants, gyms, choir practice, schools, buses) share the common features of time indoors and low levels of ventilation, even when people remained physically distanced.

Third, these suggestions are grounded in the basics of exposure science and inhalation dose risk reduction. Higher ventilation and filtration rates more rapidly remove particles from indoor air, thereby reducing the intensity of exposure and duration that respiratory aerosols stay aloft inside a room. Fourth, this approach is consistent with what is used in hospitals to minimize risk of transmission (eTable in the Supplement). Fifth, reviews on the relationship between ventilation and infectious diseases found that the weight of evidence indicates ventilation plays a key role in infectious disease transmission, citing observational epidemiological studies showing low ventilation associated with transmission of measles, tuberculosis, rhinovirus, influenza, and SARS-CoV-1. All 3 reviews note the limited number of research papers on this topic and limitations of observational data. Sixth, more recently, the National Institute of Allergy and Infectious Diseases cited the importance of adequate ventilation in the suite of COVID-19 control measures, and the Centers for Disease Control and Prevention and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) support higher ventilation rates and enhanced filtration as components of holistic risk reduction strategies.

Increasing air changes per hour and air filtration is a simplified but important concept that could be deployed to help reduce risk from within-room, far-field airborne transmission of SARS-CoV-2 and other respiratory infectious diseases. Healthy building controls like higher ventilation and enhanced filtration are a fundamental, but often overlooked, part of risk reduction strategies that could have benefits beyond the current pandemic.

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