From MERV to UV: Mastering building health with ASHRAE’s latest guidelines

Dive into the revolution of indoor air quality (IAQ) as ASHRAE 241 unveils groundbreaking strategies — from high-tech filtration to pandemic-ready building designs — promising a breath of fresh, purified air for a safer and healthier tomorrow

By Consulting-Specifying Engineer February 22, 2024
To help eliminate excess leakage of contaminated air, taping air handling unit (AHU) filter edges to prevent as much outside air (OA) leakage as possible helps ensure that all that air is passing over filter media to capture as much as possible that's at a building level. Courtesy: Consulting-Specifying Engineer

Air filtration insights

  • The information discusses ASHRAE 241’s focus on improving indoor air quality in buildings through ventilation, filtration and cleaning technologies.
  • Filtration options, measured by MERV ratings, play a crucial role, with higher ratings indicating better particulate trapping.
  • Secondary air cleaning options, such as UV and bipolar ionization, are recommended for additional purification, but precautions are necessary due to potential ozone generation.

While watching the webcast HVAC: IAQ and IEQ is a much more complete overview, reviewing this transcript of the presentation helps define the topic better. It has been edited for length and clarity.

The expert presenters covered this topic in depth. They are:

  • Garett Karalus, PE, CCP, Director of Engineering/Lead Commissioning Agent, IEA Inc., Brooklyn Park, Minnesota
  • Emmy Riley, CEM, BEAP, WELL Performance Testing Agent, Energy Engineering Team Leader & Account Manager, Cyclone Energy Group, Chicago

Emmy Riley: Let’s consider filtration and secondary cleaning options for buildings, which are two parts of ASHRAE Standard 241: Control of Infectious Aerosols. The main options that we must improve indoor air quality are ventilation, filtration and cleaning technologies. And in talking about filtration, usually what we’re talking about is a MERV rating. ASHRAE 241 has a minimum MERV rating of MERV 11. It’s reporting a filter’s ability to capture larger particles between 0.3 and 10 microns, derived from ASHRAE test methods. As a rule of thumb, a higher MERV rating means that the filter is better at trapping particulates.

The highest you can go — and it’s not high-efficiency particulate air, or HEPA — is MERV 16. And then after that you would be using what you would call a HEPA filter, whether it’s like 17, 18, 19, 20 and those can theoretically remove at least 99.97% — almost all of dust, pollen, mold, bacteria that has a size of 0.3 microns or greater.

Otherwise, you would see the applications of those higher-filtration options in places like clean rooms, in surgical suites, in manufacturing applications, things like that.

Garrett Karalus: Guidance during the pandemic from ASHRAE included a couple of secondary air cleaning options. Ultraviolet (UV) or germicidal irradiation is one of the technologies. It works on the principle that when UV light enters a microorganism, its energy will damage the microorganism cellular function so that it will not be able to grow.

A prominent application is in heating, ventilation and air conditioning (HVAC) systems and mostly in HVAC units, but sometimes in ductwork as well. They’re located downstream of the cooling coil and the condensate pan to essentially clean and disinfect the coils and limit microbial growth from those sections of the air handling units (AHU). And then that further prevents the spread throughout the system and into the spaces.

UV applications can be within rooms themselves — upper room UV — and that is to directly disinfect the surfaces of the spaces that they’re installed in. One note about any UV, is it disinfects what it touches. But in some spaces, you have desks and chairs and tables and other wall partitions and walls and things; that’s not necessarily always going to disinfect everything.

Bipolar ionization is the other kind of emerging technology that was a hot topic during the pandemic. The goal of bipolar ionization is to add mass to the airborne particles. It charges ions or atoms so that they can attract small particles and then enlarge and get caught in either filter media or also sometimes drop out of the air completely.

There is a secondary function of bipolar ionization where it can target the hydrogen within the airborne pathogens and microorganisms as well. But the primary function is really to enlarge the particles to the point where it would be filtered out more easily or again, fall out of the air.

There are some precautions that need to be taken for these secondary air cleaning options and technologies. Bipolar ionization is still considered emerging technology by the Environmental Protection Agency, whereas UV has significantly more research and studies verifying the technology’s efficacy. Ultimately, a lot of these technologies can have the potential to generate ozone and other harmful byproducts.

Ozone is harmful to human health. Just making sure that devices meet the safety standards and that they are also making sure that the designer understands what they’re trying to clean and how the device is going to clean their system because unlike HEPA filters, UV air purifiers cannot effectively remove volatile organic compounds (VOCs) or other gases from the air. They would need a slightly different setup for that. Safety and effectiveness are a very important part of any sort of secondary technology that’s going to be used for the air cleaning.

Emmy Riley: In terms of risk mitigation strategies, using higher filtration and manipulating the amount of outside air that you’re using, depending on where the contaminant is, are two good and highly recommended strategies. If your contaminant is indoors like it was during the pandemic, you would be trying to have more outside air come into your space to dilute indoor pollutants. And if it’s outdoors like it was during the wildfire, or if you’re in, for example, Los Angeles and there’s smog, you would probably want to be reducing your OA volume when the air quality indices are high, over 100 ppm would be what we’re meaning with that.

Another strategy that we recommended a lot during the pandemic and during wildfire season was taping AHU filter edges just to prevent as much outside air (OA) leakage as possible, making sure that all that air is passing over filter media to capture as much as possible that’s at a building level (see Figure 4).

To help eliminate excess leakage of contaminated air, taping air handling unit (AHU) filter edges to prevent as much outside air (OA) leakage as possible helps ensure that all that air is passing over filter media to capture as much as possible that's at a building level. Courtesy: Consulting-Specifying Engineer

To help eliminate excess leakage of contaminated air, taping air handling unit (AHU) filter edges to prevent as much outside air (OA) leakage as possible helps ensure that all that air is passing over filter media to capture as much as possible that’s at a building level. Courtesy: Consulting-Specifying Engineer

At an individual level, using a personal air purifier with a HEPA filter where you’re sleeping or working can be effective. And then if a contaminant is inside, you could also try opening windows or closing them if the contaminant is outside.

Garrett Karalus: There are a few other commercial building strategies and concepts such as different modes of controls. It has been a little bit more common recently for designers to include toggle drop-down menus for building operators to adjust the mode of operation from a normal to a low outdoor air for wildfire smoke or maximum purge outdoor air for a pandemic or for other situations for cleaning.

There are some different buildingwide and control system strategies that might be available, might be present in newer systems and probably should be incorporated into newer designs when there’s the ability to. But then ultimately with ASHRAE 241, it’s a standard that was in response to the pandemic. And ultimately, having a building readiness plan that allows a building owner or operator to have a step-by-step plan of what they need to do to get the building ready and changes that are needed to meet the infection risk mode, management mode requirements and ultimately provide a better indoor environment and m