Engineers, contractors, and facility owners have all received a tremendous amount of information within this past year regarding how to improve the air quality within their indoor environments. Some of this information comes from organizations such as ASHRAE, the U.S. Centers for Disease Control and Prevention (CDC), U.S. Environmental Protection Agency (EPA), the U.S. Department of Education, and other well-respected and knowledgeable organizations. Much of the information is disseminated by vendors emphasizing the benefits of their products, often summarizing their interpretation of various studies or selecting specific sections of a study that seemingly support their product. There is so much information being provided, it is difficult to sort through it all to see what makes sense for various applications and what might be an exaggeration to the point it could be construed as misleading. As an engineer, when asking a supplier for specific information about a product, in lieu of getting direct responses to our questions, we are frequently given 10-20 pages of reports and product specifications that we are told include responses to our inquires.

In the past, much of the discussion about the quality of an indoor environment predominantly centered around temperature control, humidity control, and the volume of outside air delivered to the space. If these parameters were adequately addressed, most occupants and owners were satisfied that the design of the indoor environment was acceptable. In recent years, we have come to understand that acceptable IAQ is much more than simply the control of the indoor temperature and humidity levels. Many may feel the COVID-19 pandemic started this whole discussion on how to address viruses and other indoor contaminants, such as volatile organic compounds (VOCs), pathogens, and fine particulates, within spaces. However, the coronavirus may have brought these concerns to the forefront, but engineers and organizations, such as ASHRAE, have been reviewing various technologies to improve IAQ for many years prior to the current pandemic.

Some technologies being discussed to help address IAQ concerns have been around for a long time, such as ultraviolet germicidal irradiation (UVGI) and high-efficiency particulate absorbing (HEPA) filtration. Other technologies, such as bipolar ionization, sometimes called ionizers, have not been around nearly as long and have not yet firmly established themselves as viable options for use in ventilation systems. With these newer technologies, what approach should engineers take, assuming they don’t automatically accept what they’ve read? Engineers look at what information is provided in numerous reports and white papers but also look for what information might be missing from those documents. The CDC, Department of Education, ASHRAE, and others advise that newer technologies should be looked at carefully and evaluated by engineers and owners based upon a thorough review of information available from a wide variety of sources. This often is time-consuming but necessary to make the appropriate decisions for a specific application, as there is a tremendous amount of information available on each technology.

FIGURE 1: Aerosol treatment technologies.

The design approach to provide acceptable IAQ within a building can vary significantly depending upon whether or not the installation is for new construction or the renovation of an existing building’s mechanical system. For the design of a new system in a new facility, engineers are able to select a specific amount of outside air, total air changes, duct, and air handler velocities, filter ratings, space airflow patterns, temperature, and humidity ranges, and other characteristics of the system. However, with the retrofit of an existing system, engineers may not very easily (and without significant cost) implement some of the features desirable in a new facility.

ASHRAE defines ventilation air as the portion of supply air that is outdoor air plus any recirculated air that has been treated for the purpose of maintaining acceptable IAQ. However, in various studies, white papers, etc., the term “ventilation” is sometimes used a little differently. Ventilation air, in the most simplistic terms, is basically the air being supplied to the space. ASHRAE states this air has been treated for the purpose of maintaining acceptable IAQ, but we know, in many installations, maybe even in the majority of existing installations, the air being supplied to the space is unable to meet the current definition of acceptable IAQ.

It's also important to understand how ventilation (outside air/total airflow) changes with different types of HVAC systems. There may be more modern HVAC systems in existence that do not have constant airflow than those that do, and that often affects how IAQ needs to be addressed. Some of the common HVAC systems and their outside air and total airflow characteristics within a space include the following:

  • Constant volume system: outdoor air and total airflow are constant.
  • Single-zone variable air volume (SZVAV): Outdoor air can be constant or vary and total airflow varies.
  • VAV system: Outdoor air varies and total airflow varies.
  • Fan-powered VAV systems: Outdoor air varies and total airflow may or may not vary.
  • Demand control ventilation: Outdoor air varies and total airflow may or may not vary.

UVC – Air Handler Installation

UVC, when installed in HVAC air-handling systems, has basically two functions. The first, and easier installation, is to provide cooling coil and surface irradiation. Since the light is fixed on these components, mold, bacteria, and other organisms residing on these surfaces are deactivated (i.e., killed).

The second, requiring much greater attention to a number of factors, is the inactivation of airborne bacteria and pathogens. To accomplish this, lamp intensity, residence time, fixture spacing, and other criteria must be met in order to have a significant impact on airborne microorganisms. It's important to note that UVC has virtually no effect on VOCs or particulates in the space or airstream.

UV systems produce light in predominantly two wavelengths: 253.7 nm (UVC) and 185 nm (vacuum UV or UVV). Wavelengths below 200 nm generate ozone. Since ozone above 5 ppb (UL 2998) is generally not acceptable in the HVAC industry, manufacturers use one of two methods to block the 185 nm wavelength. The first method is where the lamps are “doped” quartz, which is where a thin layer of titanium dioxide is applied to the inside of the quartz. The second method currently being used is to produce the lamps from “soft glass,” which is glass made from sodium barium silicate.

Although UVC systems in the air handler do help keep the coils clean, a major consideration for use of UV in an HVAC system should be its ability to deactivate airborne bacteria and pathogens. The predominate factor in determining the effectiveness of UVC in air-handling systems is its residence time. Residence time is simply the length of time the airstream and whatever is in the airstream — virus, bacteria, spores, etc. — is exposed to the UV light. With a velocity of 500 feet per minute (fpm), which is fairly common across a cooling coil in an air handler, the residence time is 0.96 second in an 8-foot section of air handler or duct exposed to UV light. If that length is reduced to 2 feet, the residence time is reduced to 0.24 seconds. As the residence time is reduced, the lamp intensity must be correspondingly increased. At very short residence times, the lamp intensity cannot realistically be increased enough for a significant kill rate. It’s not uncommon to find UV lights that have been installed into very tight sections of air handlers. It is doubtful that much more is being done in those installations other than keeping surfaces and coils cleaned in these installations.

FIGURE 2: Ultraviolet germicidal irradiation (UVGI). Image courtesy of UV Resources

The life of a typical UV lamp is approximately 9,000 hours. Manufacturers recommend the lamps be energized 24/7, even without the unit running to help keep coils clean which, in turn, reduces fan brake horsepower (BHP). At the end of one year, the lamp intensity has dropped to roughly 90% of its initial capacity and should be replaced. When reviewing the time of operation and loss of intensity characteristics of UV lamps with several manufacturers, they stated that many owners do not replace lamps on an appropriate schedule, resulting in poorer performance then intended. Reps from another manufacturer felt that maybe up to 80% of the systems they had seen in operation were operating with lamps that far exceeded their useful (90% intensity) life.

There are other guidelines that need to be followed when considering installation of UV equipment in an HVAC system. Many duct liners and filters will deteriorate rapidly with continuous UV exposure. Even if the UV is installed downstream of the heating and cooling coils, there is potential, depending upon the characteristics of the coils installed, for reflectance of the UV light through the coil and onto the filters.

Particulate Filtration

The quality of the air in a space is at least partially dependent upon the number of particulates in the airstream we are exposed to. In a typical building, there can be 18 million particles in each cubic foot of air. It is important for mechanical systems to remove as many particles of various sizes as practical. Some mechanical equipment has minimal filtration capabilities, basically roughing filters or maybe a 1-inch filter. Other equipment may have 2-inch, 4-inch, and deeper filters of various capacities and efficiencies. Often, an existing piece of equipment can physically hold a higher efficiency filter but not without a penalty of reduced airflow.

Since the start of the COVID pandemic, it seems that almost every organization addressing IAQ is suggesting a MERV-13 or MERV-14 filter should be the minimum filter used in a mechanical system. While, for many engineers, this is a simple change in design philosophy from what was the previous norm, the concern is often more related to the maintenance of the systems. There are many owners whose systems are designed and installed based upon the use of high-efficiency filters. After taking over the operation of the systems, owners frequently replace these high-efficiency filters with less costly filters, such as MERV-8 filters. This is an issue that can easily be addressed.

A second issue will take a little more time and owner education to resolve. There are several approaches taken by manufacturers to produce filters — specifically filters with higher MERV ratings that are frequently discussed in conjunction with improved IAQ. The MERV rating, that is the initial MERV rating of a filter, can be achieved by one of three fabrication methods. The first, and usually the most expensive way, is by utilization of a fine filter media of sufficient arrestance to achieve the desired MERV rating. The second method is to utilize a coarse filter media that is then electrostatically charged during the fabrication of the filter media. The last method is similar to the second method, except the filter media is not charged until after fabrication. This last method is the least expensive method to fabricate a filter to meet the desired MERV rating. The filters, which achieve an initial MERV-13 rating as the result of electrostatically charging the filter media after fabrication, quickly lose efficiency once they are installed in air-handling systems and are exposed to particulates. They lose efficiency either by loss of charge or by the filter fibers being insulated with captured particulates. Filters produced in this manner can drop from a MERV 13 to essentially a MERV 9 after about three weeks of service. The vast majority of MERV-13 filters sold, as you can probably guess, are the least expensive filters to fabricate.

Filters are evaluated and rated based upon testing required by ASHRAE 52.2 developed in 1999. Later, a secondary test method was developed and designated as ASHRAE 52.2, Appendix J, in which filters are tested to demonstrate how filter efficiencies might change in actual field conditions. The difference in MERV ratings for a filter tested to ASHRAE 52.2 and ASHRAE 52.2, Appendix J, is dramatic, particularly for many electrostatically charged filters. The following chart shows how dramatic a change occurs to a filter’s MERV rating when tested to each standard.

FIGURE 3: A MERV-13 filter media test. Image courtesy of Camfil

Another concern with filters is the rapid deterioration of the media when used in systems that also utilize UV technology. The media used in some filters does not hold up well when exposed to UV. Synthetics, cotton, polyester, and polypropylene all degrade at different rates when subjected to UV. Glass fiber media holds up better than these other materials.

One last comment is related to the cost of the filters and the benefits of high-efficiency filters, except where higher filtration efficiencies are required for certain particle sizes, there is often a diminishing benefit to utilizing higher-rated, more costly filters — rated above a MERV 13 or 14 — in a typical office or school environment.

Another recommendation of some of these same organizations addressing IAQ is the use of portable HEPA filtration units in a space such as a classroom. The definition of a HEPA filter is a filter capable or removing 99.97% of all airborne particulates larger than 0.3 microns in diameter. There are many portable air-cleaning devices sold that are promoted as a product containing a HEPA filter. This does not necessarily mean those filters are true HEPA filters. A true HEPA filter has been tested and certified to meet the performance requirements of a HEPA filter. Some of the filters, but not all, marketed with portable air filtration devices are sometimes labeled as HEPA, like 99% HEPA or just HEPA, but have not been tested or certified to be actual 99.97% HEPA filters. Often, there is no way of knowing the actual efficiency of these noncertified HEPA filters.

The second concern with the portable filtration units is knowing exactly how much benefit is achieved by locating these in a space such as a classroom. While there is no doubt these units remove particulate from the space, it is difficult to find good data identifying how much the space is benefiting from these units. In a classroom, for example, we often see these units located in the front or rear corners of the room. It’s very difficult to determine what the benefit is to the occupants as there are so many variables — capacity of unit, filter rating, location of unit in space, size of space, etc. As a minimum, the portable HEPA filters should be selected for the size of the space in which the unit is to be installed as recommended by the manufacturer with the appropriate adjustment for the actual volume (height) of the space.

Bipolar Ionization

Bipolar ionization is maybe one of the more controversial technologies discussed when considering design options to implement or improve IAQ measures. The characteristics of bipolar ionization commonly considered include the number of ions generated, types of ions produced, life of an ion, ion density in a space, maintenance, controls, monitoring, power, installation, and ozone generation. The primary issues and concerns expressed by those critical of this technology include: Is ionization effective in increasing particulate size; can ionization deactivate some pathogens; are negative ions generated by ionization harmful; does ionization generate ozone or VOCs; and, if ionization does work as promoted, what levels of ions are recommended for a space such as a classroom?

To determine if an ionization device is viable for a facility, the first things that must be considered are the types of technologies available to produce the ions. Ions can be generated by an ionic process inherent to corona discharge, electrostatic precipitation, and bipolar ionization. One ionic process utilizes dielectric barrier discharge tubes to generate nonthermal plasmas. The two most commonly used devices currently being applied in HVAC designs are needlepoint and dielectric barrier discharge bipolar ionization devices.

FIGURE 4: Air treatment technologies to improve IAQ.Image courtesy of the U.S. Environmental Protection Agency

FIGURE 5: Reactive oxygen species. Image courtesy of AtmosAir

The ozone generation issue should be simple to address. Many articles group bipolar ionization with other technologies and caution about the amount of ozone generated by these devices or at least the potential amount of ozone that can be generated by these devices if the device is not properly maintained. Some manufacturers have their devices tested and certified to meet UL 2998. UL 2998 tests the device every second for 24 hours by measuring the ozone produced at 2 inches from the device to ensure levels do not exceed 5 ppb. While it’s true that devices that do not control the voltage output to the ionizing device properly can have varying amounts of ozone generated, which exceed UL 2998, devices where the voltage is accurately and consistently regulated by a quality voltage regulator don’t face the same problem caused by varying voltages.

Similar to the ozone concern is the issue of potential VOC generation. Some bipolar ionization devices have been tested by independent laboratories to determine if any measurable levels of VOCs are generated by that device. Results are available from each manufacturer to verify if a device does or does not generate VOCs in any measurable quantity.

In an article written by Dr. Marwa Zaatari and Dr. Marcel Harmon titled, “Open Letter to Address the Use of Electronic Air Cleaning Equipment in Buildings,” a good portion of the article examines the negative impact on the health of occupants caused by the generation of ozone by ionization devices and other electronic air cleaning devices. It's mentioned that some of these devices have been certified to meet UL 2998 but that there have been no studies or testing done to verify “ozone emissions that do not start or do not increase in a variety of installation conditions over time.” The article further recommends that schools should “strongly consider turning off or disabling these electronic air cleaners to prevent unintended harm to building occupants.” To support this statement, the article refers to studies that stated bipolar ionization could generate ozone and VOCs, to another study where the authors “found increases to exposure to negative air ions resulted in systematic oxidation stress, which can lead to cell and tissue damage,” and to a final document that stated, through a study of school children, that “exposure to negative air ions resulted in a negative impact on heart rate variability.” In reviewing these issues online and with others in this profession, it was hard to find much researched support for these statements.

There are many studies that challenge the position that negative air ions (NAIs) are harmful to humans or to anywhere near the degree expressed by Drs. Zaatari and Harmon. One detailed study, published in the International Journal of Molecular Sciences (September, 2018), reported there were many benefits to humans being exposed to NAIs. They reported that various studies showed that NAIs could help boost the immune system, had positive effects on the cardiovascular system, and showed highly significant increases with various tested tasks. In addition, the article stated that various studies showed that NAIs attach themselves to particulates, such as dust, mold spores, and other allergens, that allow particulate clusters to precipitate or deposit faster than particles not exposed to NAIs. NAIs were also shown to inhibit the growth of bacteria.

FIGURE 6: Dielectric barrier discharge bipolar ionization. Image courtesy of AtmosAir

The Johns Hopkins School of Public Health recently published a paper (May 2021) that very briefly mentions bipolar ionization. In that paper, it was stated that bipolar ionization and other technologies “have not been shown to be safe and effective,” and “The effect of these cleaning methods on children has not been tested and may be detrimental to their health.” As with the articles written by Dr. Zaatari and Dr. Harmon, we have not found much evidence to support John Hopkins School of Public Health’s position that negative ions create an unsafe environment.


The CDC, EPA, ASHRAE, and other respected organizations each have a wide range of recommendations to provide an acceptable indoor air environment in schools and other facilities. Some are operational recommendations, such as the desire to operate the mechanical systems 24/7, operate systems two hours prior to the start of the school day and run them for two hours after the end of the school day, increase outside ventilation air to the extent possible, open windows when possible, increase total airflow to the space, disable demand control ventilation systems if any are installed, and allow fans to run continuously in lieu of intermittently. Other recommendations include installing higher filtration in the HVAC systems (MERV 13 or 14 minimum), installing UV-C devices, consider modifying the air distribution for directional airflow, and installing true portable HEPA filtration units appropriately sized for the space.

Perhaps the best guidance given to engineers and owners by the various organizations is that “consumers are encouraged to exercise caution and do their homework” regarding IAQ and especially technologies which are considered to be emerging.