With the onset of the COVID-19 pandemic and the identification of airborne droplets as the primary path of infection transmission, professionals designing, operating, and maintaining all types of facilities are focusing on the importance of indoor air quality (IAQ). The quality of the indoor environment, and air quality in particular, has always been a top priority in health care, where people responsible for designing and operating facilities work to address the potential for infection transmission and prevent nosocomial infections, or health care associated infections (HAI), that cause a substantial number of illnesses and deaths every year. As the COVID-19 pandemic continues to evolve, and our understanding of the disease and the behavior of the virus that causes it develops, the focus on IAQ and its role in infection transmission has expanded to all types of facilities, including schools and other public buildings.

In general, we know IAQ has a major impact on occupant health, comfort, productivity, and quality of life. Occupants take an average of 20 breaths every minute. Each breath contains more than life-sustaining oxygen. Occupants also inhale aerosols and particles, spores, dust, and allergens.
Unfortunately, viruses and bacteria are present among the airborne matter that gets circulated through buildings and transferred from occupant to occupant through breathing and other forms of contact.

A range of solutions and technologies, many already common in health care environments, are being adapted and implemented in buildings elsewhere. Such pathogen mitigation solutions and technologies fall under the umbrella of Engineered Infection Prevention (EIP). EIP mitigation strategies usually include dilution (ventilation), filtration, humidification, and air cleaning and disinfection. Filtration is the most prevalent method employed by straining the air and capturing particles. Filtration has its place in an EIP and IAQ strategy but only treats air that moves through the filter media. In order for pathogens to be captured by the filters in an HVAC system, they must get entrained in the return airstream and find their way to the filter banks. Filtration also adds a restriction (static pressure) that must be overcome through additional fan energy. Along with proper filtration, it is important to address the risk of infection transmission directly in the space where occupants work, learn, play, and live. Therefore, EIP strategies incorporating a mix of technologies are most effective in mitigating the risk of infection transmission.

Any EIP solution, whether consisting of one technology or a mix of technologies, comprises a risk mitigation strategy. With each element of the strategy, risk of infection is reduced but not eliminated. Combined with proven basic personal measures of masks and social distancing, and with careful application, EIP strategies can make indoor environments safer, and, in general, address both airborne and surface transmission, where examples are listed in the table below. For the purposes of our discussion here, we are going to focus on the EIP strategies that address the potential for airborne infection transmission. Think of them as building blocks in an overall airborne EIP strategy. Fundamental building blocks include ventilation, filtration, and humidity. Supplemental building blocks address additional air cleaning and disinfection with ionization and ultraviolet germicidal irradiation.

Table 1

Table 1: Methods to reduce risk of infection transmission.



First and foremost, proper ventilation is key to a healthy indoor environment. Today’s buildings are designed to comply with codes that require minimum ventilation based on ANSI/ASHRAE Standard 62.1-2019, “Ventilation for Acceptable Indoor Air Quality” (or a prior version of this standard). However, many existing buildings, even if they were designed to meet a prior version of this standard, may not be properly ventilated. Therefore, an assessment of ventilation is a recommended first step in the development of any EIP approach. Ventilation assessments often involve the measurement of CO2 (carbon dioxide) over time, when the building is operated and occupied under normal circumstances. Since people exhale carbon dioxide, it serves as a proxy to determine if adequate fresh air is being introduced into an indoor environment. 

While recent studies have drawn correlations between increased ventilation and better cognitive performance, it is primarily important for ventilation to be adequate to dilute pollutants and pathogens indoors. The tradeoff for increased ventilation is added energy cost to heat, cool, dehumidify, and distribute the ventilation air.

In the case of inadequate ventilation, some buildings’ systems may be adjusted to increase ventilation, while others may require changes to controls and/or mechanical systems. Older buildings may not offer the possibility for increasing ventilation without substantial and costly changes and are good candidates for air cleaning and disinfection approaches.



Typical application of improved ventilation involves adjustments and/or changes to controls and HVAC systems. General application considerations include:

  • Potential for existing systems to accommodate increased ventilation;
  • Viable retrofit solutions to improve ventilation; and
  • Increased energy usage and utility costs.



Filtration is a necessary element of any HVAC system, but there is a range of filtration capability to address a broad variety of applications. MERV is a reporting value for a filter evaluated according to ANSI/ASHRAE Standard 52.2. Basically, as MERV increases, a filter’s ability to remove a greater number of particles of increasingly smaller size from an airstream increases. But this comes with a tradeoff of increasing pressure drop, or restriction to airflow, which in turn translates into more required fan power. 

A common question arises when looking at options to reduce risk of infection transmission: “Can we change our HVAC filters from MERV 8 (common in commercial/institutional) to MERV 13 (common in health care)?” The answer depends on several factors including the existing HVAC system specification and configuration, maintenance practices, and operations budget. Simply changing existing filters with more stringent filters can place undue stress on fans and motors and more stringent (and more expensive) filters may need to be replaced more often. Therefore, the matter of filtration selection comes down to applying the appropriate filtration for the application and combining it with other measures that go further to reduce risk of infection transmission in indoor environments.


Table 2

Table 2: Advantages and disadvantages of ventilation.


Typical application of filtration involves installation of filtration equipment in HVAC systems. Application considerations include:

  • Available space for filtration banks;
  • Placement within equipment, including location relative to other components/accessories (fans, and coils);
  • Accessibility for maintenance;
  • Instrumentation for pressure loss across filter banks; and
  • Processes for regular inspection and maintenance and filter replacement.



Like filtration, humidity is another basic building block in the IAQ equation. In most buildings, humidity is generally not controlled directly but allowed to fluctuate with indoor and outdoor temperature and relative humidity conditions. In the cooling season, when outdoor humidity levels are high, buildings use mechanical cooling to both cool and remove moisture, preventing indoor relative humidity from reaching unhealthy levels — too much moisture indoors is uncomfortable and can lead to microbial and mold growth and degradation of building materials. And, in the heating season, when outdoor humidity levels are lower, buildings use heating that lowers indoor humidity levels further. The human body meanwhile is happiest when relative humidity levels remain between 40%-60%. And, to maintain building materials and indoor environments, indoor relative humidity should be maintained below 60%. Fortunately, it turns out that infectious diseases and particularly viruses are more virulent when humidity levels are low. So, controlling indoor relative humidity offers multiple benefits of greater comfort and lower risk of infection transmission.

Actively controlling humidity is common in some health care and industrial environments, where humidity is essential to maintaining a critical indoor environment. Such buildings’ systems remove moisture when relative humidity is high and add moisture when relative humidity is low. While dehumidification is common in many buildings, it is the addition of moisture that most buildings are missing as they were never designed with humidification in mind. Humidification systems come with additional initial and operational cost and complexity, and they must be vigilantly maintained to work properly. The three primary methods of humidification include:

  • Steam generation and dispersion;
  • Adiabatic fogging; and
  • Ultrasonic humidification.

Perhaps the most important question when considering humidification involves the building’s original design and, in particular, the building envelope. If the building envelope was not designed for indoor humidification, then condensation on and in building materials may cause mold growth and material degradation.

Table 3

Table 3: Filters and MERV.



Typical application of humidification involves installation of humidification equipment in HVAC systems. Application considerations include:

  • Electrical power or other utilities (steam, natural gas) availability and installation;
  • Placement within equipment, including location relative to other components/accessories (filters, fans, and coils);
  • Accessibility for maintenance and cleaning;
  • Integration, for example with building automation systems;
  • Instrumentation, for feedback on relative humidity levels in ducts and spaces;
  • Water treatment — utilization of reverse osmosis (RO) water significantly reduces required maintenance on humidification unit; and
  • Processes for regular inspection, maintenance, and cleaning.


Air Cleaning and Disinfection

While our existing buildings may offer constraints on potential for improving ventilation and filtration or adding humidification, various air-cleaning technologies and approaches can be applied in existing environments. Air cleaning is achieved by employing different technologies, each with strengths and limitations that must be carefully considered for the given application. Beyond filtration, the common methods include ionization, and ultraviolet germicidal irradiation (UVGI). These technologies, when combined with a building’s existing filtration and/or humidity control systems, produce multilayered EIP strategies.



Ionization technology uses an electronic charge to generate a high concentration of positive and negative ions intended to eliminate particulates, gases, and odors and neutralize pathogens. As these ions travel with the airstream, they attach to the particles, pathogens, and gas molecules. 

  • The ions help to agglomerate fine sub-micron particles, making them filterable; 
  • The ions neutralize pathogens by robbing them of life-sustaining hydrogen;  
  • The ions breakdown harmful volatile organic compounds (VOCs) into harmless compounds like O2, CO2, N2, and H2O.

Ideally, this applies to VOCs with an electron volt potential under 12 (eV<12) to avoid making reactive oxygen or ozone. The ions produced flow with the airstream into the occupied spaces, cleaning the air everywhere the ions travel. In recent laboratory testing by one manufacturer, the technology was demonstrated to be very effective at neutralizing SARS-CoV2, with a 99.4% elimination rate over 30 minutes. Note that the effectiveness of ionization depends on the ion density achieved at the point of origin along the ductwork and in the space, where the ions last approximately 60 seconds from the point of origin. There are two dominant types of ionization technology:

  • Bipolar ionization tubes: These devices employ a corona discharge bipolar ionization tube to generate ions. The tubes must be periodically replaced; and 
  • Needlepoint bipolar ionization: These devices utilize carbon fiber emitters to generate ions, and the emitters can last up to and more than 20 years; however, the emitters must be periodically cleaned.

Note that a potential byproduct of ionization is ozone, which is an irritant to people. Be sure to look for systems that do not generate ozone and meet UL 867 and 2998.


Table 4

Table 4: Advantages and disadvantages of filtration.



The cost of bipolar ionization varies over a range depending on the application and the complexity of the installation, but the following guidelines will get you in the ballpark for installed cost:

  • $0.30-$0.60 per cfm;
  • Plus, two to eight hours per HVAC unit for a typical installation.

Modular ionization devices address large central air-handling units (AHUs) or rooftop units, while HVAC systems that are largely unitary and distributed, such as water-source heat pumps or unit ventilators, can be addressed with small ionization devices at each HVAC unit. Maintenance costs for ionization systems consist of time for cleaning. Products with ionization tubes require periodic tube replacement.



Typical application of ionization involves the installation of ion generators in air distribution equipment, including AHUs and/or terminal units, such as fan coils, VAV boxes, or unit ventilators. In residential applications, a compact unit may be installed in a furnace or split system. Application considerations include:

  • Electrical power availability and installation;
  • Placement within equipment, including location relative to other components/accessories (filters, coils, humidification);
  • Airflow rates and coil sizes, where larger coils may require more ion generating equipment;
  • Accessibility for installation, maintenance, and cleaning;
  • Integration, for example with building automation systems; 
  • Instrumentation, for measurement of ion density and/or levels of contaminants/pollutants; and
  • Optional features, such as self-cleaning.

For commissioning of ionization systems, time series measurements before and after implementation can be used to determine how well the system is working:

  • Primary measurement — Measure ion density in the ductwork and/or in the space;
  • Secondary measurements — Measure VOCs and particulate (particle count). 

Filters may also be visually observed before and after implementation. If ionization is working, it is likely that filters will collect considerably more airborne particles following implementation.


Table 5

Table 5: Advantages and disadvantages of ionization.


Ultraviolet germicidal irradiation (UVGI) has been used successfully for many years because of its proven ability to inactivate microorganisms, VOCs, and viruses. Some of the most susceptible microorganisms, from most to least susceptible, include:

  • Vegetative bacteria;
  • Viruses;
  • Mycobacteria;
  • Bacterial Spores; and
  • Fungal Spores.


UVGI utilizes one or more ultraviolet lamps with a specific wavelength discharge, typically 250-260 nm. Several key applications of UVGI include: 

  • In the airstream;
  • In-space upper zone;
  • In-space with filtration; and
  • In-space fixtures, fixed and portable.


UVGI in the Airstream

UVGI lamps located in the airstream are typically placed in return ducts, or in AHUs before or after the cooling coil. The UV light inactivates microorganisms passing through the duct or the AHU. If the UV lamps are placed downstream of and immediately adjacent to the cooling coil, they have the added benefit of cleaning the coil; thus, avoiding degradation of heat transfer and increasing pressure drop caused by fouling of the coil with biofilms. 


In-Space Upper Zone UVGI

UVGI can be located directly in the space effectively placing it closer to the source of contaminates. In this application, known as Upper Zone UVGI, the UV units are mounted at the ceiling and cast the light outward to treat the air as it crosses its path. Baffles around the UV lamp prevent occupants in the room from being exposed to the UV light. Typical applications include:

  • Spaces with no central ventilation system; and
  • Areas with high occupant density, such as cafeterias, waiting rooms, and lobbies.


In-Space UVGI with Filtration

UVGI can be used to filter and purify the air in spaces with wall-mounted or portable air purifiers. These units may be placed directly in the space since the UVGI light is built into the equipment. These units typically draw air across the UVGI light, through a HEPA filter, and disperse the treated air back into the space.


In-Space UVGI Light Fixtures, Fixed and Portable

UVGI can be used in spaces to clean the air and sanitize surfaces. When the space is unoccupied, the UV disinfecting lights are automatically enabled for the required disinfecting time for that space. Portable units can be placed in rooms on a rotating basis or as needed, and autonomous portable units can circulate automatically. Examples of applications include:

  • High traffic variable occupancy spaces;
  • Health care patient rooms, restrooms, and operating rooms;
  • Educational rooms, nurses offices, infirmaries, and restrooms;
  • Equipment sanitation for gym equipment, laptop/notebook computers, etc.;
  • Police station interview rooms; and
  • Fire station equipment and personnel gear.



The cost of UVGI varies based on the application and is typically estimated on a per fixture basis plus installation cost, where larger spaces require multiple fixtures to provide complete coverage. Maintenance costs for UVGI systems consist of lamp replacement and time for cleaning.


Table 6

Table 6: Advantages and disadvantages of UVGI.



While installing UVGI consists primarily of installing the fixtures, getting the design right is important so the amount of UV energy distributed in the environment is sufficient to neutralize pathogens. In the airstream, this means adequate energy and time to kill pathogens as they pass by, and for in-space applications, it means distributing adequate energy for the area and volume of the space.

Then there is safety: “Don’t look at the light.” When applying UVGI light, regardless of the application, precautions must be taken to ensure installers, occupants, and maintenance personnel are not exposed directly to the light. Warning labels should be placed on duct and AHU access doors and panels. The UV lamps should be turned off prior to servicing the equipment.

For upper zone UVGI applications, equipment must be mounted at least seven feet above the floor and should be supplied with deflectors to direct the light away from occupants. On a recent installation, I discovered an electrician turned on the power while completing installation on a ladder and peered into the UV upper zone device, resulting in sunburn to his corneas. In addition, this solution should not be applied in locations where there may be reflective surfaces nearby, like TV screens or mirrors.

For fixed UV light fixtures, units should be installed with multiple occupancy sensors that have full space coverage and redundancy, and door sensors immediately shut units off upon entry to the space.

Also note that when utilizing UVGI light for disinfection some materials may be degraded with prolonged exposure to UV light. Work with the manufacturer and engineer to ensure that UVGI is appropriate for the materials in the space.


Is EIP Here to Stay?

Even as we move past the COVID-19 pandemic, EIP strategies will be here to stay as we better understand the importance of making buildings and indoor environments safer and healthier for everyone. EIP strategies address concerns that go well beyond COVID-19:

  • Other potential pathogens and illnesses, such as influenza;
  • Asthma;
  • Particulates;
  • VOCs; and
  • Odors.

Addressing these concerns yields benefits of greater comfort and productivity, reduced absence from work and school, and, perhaps most importantly, increased peace of mind that comes with knowing we are doing what we can to take care of the people in our buildings.




• ANSI/ASHRAE Standard 52.2-2017: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency By Particle Size

• ANSI/ASHRAE Standard 62.1-2019: Ventilation for Acceptable Indoor Air Quality


• UL 867: Standard for Electrostatic Air Cleaners

• UL 2998: Environmental Claim Validation Procedure (ECVP) for Zero Ozone Emissions from Air Cleaners

ES Magazine:

• https://www.esmagazine.com/articles/100243-ashrae-resources-available-to-address-covid-19-concerns

• https://www.esmagazine.com/articles/100883-high-efficiency-air-purifier-enverid-systems?oly_enc_id=0740A0812012C5U