Bridging the Gaps
IHs Can Connect Ventilation Engineering, IAQ, and Infection Control
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Lessons learned from the COVID-19 pandemic have increased awareness that the air we breathe may be contaminated with infectious aerosols. Whether those aerosols become inhaled, land on mucosal surfaces, or settle out and enable subsequent fomite transmission, engineering controls can impact their ability to effectively transmit infection.
While ventilation is preferably not the only layer in a facility’s infection prevention strategy, current awareness is largely focused on ventilation strategies that incorporate contaminant dilution, directional air movement, and air cleaning or treatment to reduce infectious aerosol concentrations. Implementation of effective ventilation intervention strategies is sometimes complicated when engineering design professionals are insufficiently trained in the characteristics of infectious disease transmission. From the public health perspective, industrial hygienists are well suited to play a strategic role in bridging gaps in knowledge, terminology, implementation, and evaluation between building owners and occupants, ventilation engineers, and clinical professionals. This article summarizes some of the ventilation strategies and related resources that can aid the industrial hygienist in this important role.
A Note on the Hierarchy of Controls Following the hierarchy of controls, source elimination through self-identification or triage is the preferred intervention for infectious aerosols. But when cases are sufficiently high and symptoms not always obvious, reliance on elimination is unrealistic. Social factors that influence the willingness or ability to isolate also hamper elimination’s effectiveness. Some might consider direct source control (source masking, for instance) to be a form of elimination but for this approach, awareness of infection and compliance with the selection and use of well-fitted face coverings, when not limited by supply issues, introduce an inconsistent human element. Since substitution is not applicable, we look to engineering controls as our next-preferred intervention, as discussed in this article.
FIRST THINGS FIRST Where should you start in evaluating and improving your heating, ventilation, and air conditioning (HVAC) system? The steps below highlight ways to obtain a better understanding of your system design, how well it is working, and how it compares to current consensus ventilation standards.
1. Inspect your system and make sure all components are operating properly. Check the equipment and controls for operating problems. Note what types of air filters you have and their rated efficiency. This is commonly reported as a minimum efficiency reporting value (MERV), which is a measure of how well the filter captures particles in each of three particle size ranges. For the most infectious aerosols, the two size ranges that are 3 µm and smaller are most relevant. Higher MERV ratings indicate better filtration efficiencies, especially for those small aerosols that can float in air or become inhaled. It’s also important to check the filter’s fit and condition within the filter rack to verify that air flows properly through the filter.
2. Check the building code or consensus standards and see how much outdoor and total airflow your HVAC system should be providing. Often, the applicable HVAC code will be related (in total or in part) to ASHRAE Standard 62.1, Ventilation and Acceptable Indoor Air Quality. Note that the design airflows should correspond to the standards when the building was built or when it was last upgraded. If you have a recent testing, adjusting, and balancing (TAB) report, it should contain this information. If you don’t have a recent TAB report, consider obtaining one first so that you have a current understanding of how your HVAC system performs. When selecting a qualified (and preferably certified) TAB contractor, verify their familiarity with TAB procedures and specifications from consensus groups such as ASHRAE; the Associated Air Balance Council (AABC); the National Environmental Balancing Bureau (NEBB); the Testing, Adjusting and Balancing Bureau (TABB); and the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA).
3. Ensure your system meets the minimum requirements of the building code or consensus standards. ASHRAE 62.1, Ventilation and Acceptable Indoor Air Quality, prescribes methods to identify the minimum delivery rates of outdoor air for several facility types. In addition, it provides a list of minimum filtration and maintenance requirements as well as a schedule for checking and maintaining your HVAC system. For healthcare facilities, refer to ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities, to see minimum recommended ventilation and filtration requirements for designated spaces within these buildings.
Once you have verified that your HVAC system is performing as designed, consider upgrading its performance to current standards or better. In a November 2022 report (PDF), the Lancet COVID-19 Commission Task Force on Safe Work, Safe School, and Safe Travel suggests that the ASHRAE 62.1 outdoor airflow rates are too low for effective control of infectious aerosols, since they were not developed for this purpose. As you move forward, consider improving your building ventilation using the approaches discussed in the following sections.
IMPROVE VENTILATION BEYOND CODE REQUIREMENTS Once you are meeting code requirements for ventilation and filtration, there are additional measures you can take to improve ventilation and reduce the potential for infectious aerosols to accumulate in indoor air. One such measure is to evaluate the ability to introduce outdoor air beyond code minimum requirements by adjusting the HVAC system’s outdoor air dampers. This will likely require consultation with an experienced HVAC professional as its implementation is subject to system capacity and environmental conditions. This measure will potentially impact energy consumption. Use of an energy recovery ventilator can lessen the potential energy and system implications of increased outdoor air.
Providing clean air through the HVAC system is effective only if it can enter the breathing zones of all room occupants. For general ventilation, good room air mixing is important to avoid the development of ventilation “dead zones” where infectious aerosols might accumulate. This is preferably achieved through good design and strategic placement of supply and return air louvers. Improved room air mixing might be also achieved through increasing total airflow into the room or by redistributing the supply and exhaust. The airflow should be consistent, which means that during periods of occupancy you may need to disable any demand-controlled ventilation controls that reduce air supply volumes based solely on temperature demand or occupancy sensors. If necessary, room air mixing can be facilitated through the use of in-room fans. When there are clear demarcations of higher and lower exposure risk, a ventilation design that provides clean-to-less-clean airflow can provide added protection (see FAQ number 4 on CDC’s webpage on ventilation in buildings to learn more).
After adjusting the HVAC system, consider (with expert consultation, if needed) increasing central air filtration to as high as possible without significantly reducing design airflow. This is especially helpful when enhanced outdoor air delivery options are limited. ASHRAE suggests using at least MERV 13 filters because they remove 85 percent or more of 1–3 µm particles.
If the HVAC system cannot handle the filter upgrade to MERV 13 or if you would like to further improve air cleanliness through ventilation, some supplemental treatments can be considered. High-efficiency particulate air (HEPA) fan/filtration systems are tried and proven tools to enhance air cleaning. These HEPA systems can be portable units, or built-in ceiling or wall systems. A HEPA filter is defined to be at least 99.97 percent efficient at capturing particles 0.3 µm in diameter, which approximates the most penetrating particle size (MPPS) through the filter. This means that HEPA filters are even more efficient at capturing particles larger and smaller than the MPPS and are thus at least 99.97 percent efficient at capturing infectious aerosols. A common criteria for HEPA air cleaner selection is the system’s clean air delivery rate. CADR is a performance rating that incorporates both airflow and system design into an effective air cleaning rate, typically reported in cubic feet per minute (see the fact sheet “Selection and Use of Portable Air Cleaners to Protect Workers from Exposure to SARS-CoV-2” as a PDF). Although these HEPA air cleaners do not bring in outdoor air, they are effective at cleaning air within spaces to reduce the concentration of infectious aerosols. Thus, they provide effective air exchanges without the need for conditioning outdoor air and can be combined with HVAC filtration to achieve MERV 13-equivalent or better levels of overall air cleaning. Note that portable air cleaners with filters less efficient than HEPA also exist and can contribute to room air cleaning. However, it may be difficult for the consumer to identify exactly what level of air cleaning is being accomplished. To reduce confusion, these air cleaners should be clearly labeled as non-HEPA units.
Ultraviolet germicidal irradiation (UVGI)—also known as germicidal ultraviolet, or GUV—is another proven supplemental air treatment. Although UVGI does not physically remove airborne particles, it effectively reduces infectious aerosols by inactivating them. Depending on the building type and occupancy, you may consider upper-room UVGI systems to provide air cleaning within occupied spaces. In-duct UVGI systems can also help supplement air cleaning inside central ventilation systems (see FAQ number 7 on CDC’s ventilation in buildings webpage to learn more). UVGI systems can be especially helpful when dealing with inadequate HVAC system designs. The design, installation, and testing of UVGI systems requires an experienced HVAC professional, a reputable UV-system manufacturer, or both to ensure that systems are effective and safe. Once the systems are professionally installed and their performance validated for safety and efficacy, an industrial hygienist with appropriate training and equipment could be well-suited to perform periodic monitoring of UVGI irradiance levels. The UVGI fixture manufacturer should provide information on measurement protocols. In addition, a discussion on UVGI measurements for efficacy and safety purposes can be found in a NIOSH document focused on environmental control for tuberculosis (PDF).
How much ventilation is needed to reduce infectious aerosol concentrations depends on many factors, including how many other prevention actions are in place.
The supplemental treatments mentioned previously can be categorized into two general applications: permanent installations used to supplement HVAC system performance or provide enhanced protection to identified higher-risk areas, or interim measures used to address an increased exposure threat brought on by temporary circumstances. Permanent installations might be chosen for areas where there is a perceived higher risk of exposure potential (think medical reception, dental office, workout facility, band or choir room, or dining hall). Temporary measures are more likely to invoke portable interventions such as portable air cleaners and be applied to temporal circumstances such as high community incidence rates, HVAC malfunction, or temporary overcrowding of a space beyond its HVAC design intent. One temporary intervention that has received a lot of attention during the COVID-19 pandemic is the use of do-it-yourself (DIY) air cleaners assembled from box fans and HVAC filters. These air cleaners can contribute to room air cleaning, including removing infectious aerosols, but there is less research supporting their long-term effectiveness. For DIY air cleaners, important considerations to ensure safe and effective operation include using a newer box fan (made since 2012) with a UL (Underwriters Laboratories) label, selecting MERV 13 or higher filters, and ensuring high quality construction and an effective design, among other recommendations summarized by EPA.
Besides these supplemental treatments, additional steps such as opening (even slightly) windows and doors can enhance natural ventilation and dilute indoor infectious aerosols in temporary circumstances of increased exposure risk. You may consider further augmenting an open window by safely and securely placing a window fan to exhaust room air to the outdoors because this will help draw outdoor air into the room via other openings without generating strong room air currents. Unless it is part of a protective directional airflow design, use caution to avoid placing fans and air cleaners in a way that could potentially cause contaminated air to flow directly from one person to another. See FAQ number 11 on the CDC ventilation in buildings webpage for more detail on the use of fans.
WHEN HAVE YOU DONE ENOUGH? Ventilation by itself is not enough to guarantee people won’t get sick. Improving ventilation (to include the use of UVGI) can reduce the concentration of infectious aerosols in indoor air, thereby reducing the potential for exposure that leads to subsequent disease transmission. How much ventilation is needed to reduce infectious aerosol concentrations depends on many factors, including how many other prevention actions are in place. Simple interactive ventilation tools are available from CDC to comparatively model the reduction in particle levels under different ventilation scenarios (a home ventilation tool and one for schools can be found on the agency's website). The Wells-Riley model provides the capability to predict infectious disease transmission in indoor air. Adaptations of this model have evaluated the effect of multiple prevention strategies, including ventilation, on the transmission of tuberculosis, measles, and more recently for COVID-19, but the model doesn’t account for proximity to the infectious source.
An article in the October 2022 issue of The Synergist highlights the COVID-19 Exposure Assessment Tool (CEAT), which addresses the proximity issue, is free to download, and can be used to determine relative risk given a dozen different factors to include room volume, ventilation rate, and type of filtration, among others. The advantage of using exposure modeling tools such as CEAT is that ventilation can be uniquely evaluated in combination with other prevention strategies to optimize an approach.
An opposite approach from the customization provided by modeling is to have a single health-focused ventilation rate broadly applied to most indoor spaces. Although a single target value is appealing due to its simplicity, it can result in too little ventilation in some spaces or too much ventilation in others. Too little ventilation can lead to increased exposure to infectious aerosols. Too much ventilation can result in increased capital costs and wasted energy, perhaps without the intended exposure reductions.
For those who prefer not to perform exposure modeling and do not want the negative consequences of a single target value, where can they turn to determine when they have done enough? The most obvious place professionals look for ventilation advice is ASHRAE, which in December 2022 announced its commitment to expeditiously develop a national indoor air quality pathogen mitigation standard. In the meantime, professionals can look to the Lancet report “Proposed Non-Infectious Air Delivery Rates (NADR) for Reducing Exposure to Airborne Respiratory Infectious Diseases” (PDF), which outlines a “good,” “better,” and “best” strategy rather than a single target value. The Lancet NADRs are intended to reduce respiratory disease by improving ventilation and filtration above current bare-minimum building ventilation codes in non-healthcare and non-residential settings. The NADRs can be met using volumetric flow rates based on room volume, per person, or per floor area using engineering controls such as ventilation, filtration, or air treatment technologies that have been shown to quantifiably remove infectious aerosols in real-world settings.
CAUTIONS REGARDING EMERGING TECHNOLOGIES Throughout the history of infectious disease, there have been interventions marketed with promising claims to reduce the concentration of infectious aerosols in indoor air. Given the sizeable resources available during the COVID-19 pandemic, the number of intervention technologies seems to have increased dramatically. Some of these are truly new while others are new applications of existing technologies. Collectively, they are referred to as “emerging technologies.”
Providing proof of performance under real-world, as-used conditions as well as safety validation for all people who could potentially be exposed to generated products or byproducts can be a difficult challenge stemming from emerging technologies. This challenge is not always sufficiently addressed. For example, from the perspective of the consumer, it is often difficult to identify when a technology and its claims are legitimately (and safely) worth the investment. Public media sources have reported cases where resources were spent in the pursuit of interventions that ended up providing little to no protection and, in some cases, potentially exposing occupants to new contaminants of concern. Unlike well-established intervention technologies where safety and efficacy standards exist, emerging technologies often represent uncharted waters.
Government agencies are insufficiently resourced to provide safety and efficacy studies for every new emerging technology. They are also hesitant to engage in such evaluations when an emerging technology has just a single or very few sources, out of fear of appearing to influence the “winners and losers” for intervention funding. For similar reasons, professional associations may also be hesitant to conduct such safety and efficacy testing. In the absence of a resource to aid consumers in determining whether marketed claims of performance and safety are applicable to their unique situation, a potential user of emerging technology must perform their own determination. CDC has addressed the topic of emerging technologies in FAQ number 8 on its ventilation in buildings webpage. This FAQ discusses the questions and data requests consumers might incorporate into their decision-making when considering an emerging technology. CDC encourages consumers to exercise caution and to research the technology. Independently determined data on performance efficacy under intended-use conditions are important. For instance, data collected in a small static-air box may not be applicable to large-volume rooms with moving airflows. Safety determinations should be applicable to all people likely to enter the space. As with the other topics discussed previously, the industrial hygienist is especially suited to assist building owners and occupants in evaluating performance and safety claims for these emerging technologies.
INDUSTRIAL HYGIENISTS’ ROLE Recently heightened awareness of indoor air as an infectious aerosol exposure pathway provides opportunities for industrial hygiene practitioners and others to adopt a broad range of proactive intervention strategies. While actual protection levels will be disease- and circumstance- specific, reducing the concentration of infectious aerosols will generally lower the associated risk of disease transmission, regardless of the clinically-defined transmission route (for example, droplet, short- or long-range airborne, or fomite). Thus, proactive interventions can be at least partially protective, regardless of debates over terminology, definitions, or routes of transmission. But increased awareness of the indoor air environment and its potential to influence disease transmission is of little value without action.
Building-specific air protection plans to reduce the spread of infectious aerosols in indoor air should be informed by ventilation guidance and tools from government agencies, professional organizations, and other entities. Industrial hygienists’ knowledge of ventilation engineering, indoor air quality, and infection control position them well to navigate ventilation guidance, conduct risk assessments, pilot exposure modeling tools, or conduct due diligence when considering emerging technologies. Building owners, ventilation engineers, infection control professionals, and others should engage industrial hygienists to help develop comprehensive strategies to improve indoor air quality and prevent the spread of infectious diseases.
CHAOLONG QI, PhD, PE, is an engineer consultant in the Engineering and Physical Hazards Branch, Division of Field Studies and Engineering at NIOSH.
DUANE R. HAMMOND, MS, PE, is a team leader and mechanical engineer in the Engineering and Physical Hazards Branch, Division of Field Studies and Engineering at NIOSH.
KEVIN H. DUNN, ScD, CIH, is a research mechanical engineer with the NIOSH Division of Field Studies and Engineering.
KENNETH R. MEAD, PhD, PE, is chief of the Engineering and Physical Hazards Branch, Division of Field Studies and Engineering at NIOSH.
Disclaimer: The findings and conclusions in this article are those of the authors and do not necessarily represent the official position of the National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention.
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ASHRAE: ANSI/ASHRAE/ASHE Standard 170-2017, Ventilation of Health Care Facilities (2017).
ASHRAE: “ASHRAE Commits to Developing an IAQ Pathogen Mitigation Standard” (December 2022).
ASHRAE: Standard 62.1-2022, Ventilation and Acceptable Indoor Air Quality (2022).
CDC: “COVID-19: How to Protect Yourself and Others” (January 2023).
CDC: “Interactive Home Ventilation Tool” (February 2022).
CDC: “Interactive School Ventilation Tool” (May 2022).
CDC: “Scientific Brief: SARS-CoV-2 Transmission” (May 2021).
CDC: “Upper-Room Ultraviolet Germicidal Irradiation (UVGI)” (April 2021).
CDC: “Ventilation in Buildings” (June 2021).
EPA: “Air Cleaners, HVAC Filters, and Coronavirus (COVID-19).”
The Lancet COVID-19 Commission Task Force on Safe Work, Safe School, and Safe Travel: “Proposed Non-Infectious Air Delivery Rates (NADR) for Reducing Exposure to Airborne Respiratory Infectious Diseases” (PDF, November 2022).
National Institute of Environmental Health Sciences (NIEHS): “Selection and Use of Portable Air Cleaners to Protect Workers from Exposure to SARS-CoV-2” (PDF).
NIOSH: “Environmental Control for Tuberculosis: Basic Upper-Room Ultraviolet Germicidal Irradiation Guidelines for Healthcare Settings” (PDF, March 2009).
The Synergist: “A Dashboard for COVID-19 Risk: The Story of the COVID-19 Exposure Assessment Tool” (October 2022).