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Finding Contamination Hot Spots

Why ventilation and air flow matter

Indoor air quality could be the most important factor in building health right now. Something we cannot see that has the potential to kill us now lurks in the air we breathe, and while indoors, we are twenty times more likely to come into contact with our new mysterious assassin. So yeah, the air indoors is kind of a big deal. In order to expose and flush out this invisible killer, we need to learn more about it, like how it travels, how long it stays active, and how we can reduce the likelihood of ever meeting it. From ASHRAE to the CDC to every HVAC specialist, folks are on a mission to get to know the virus known as SARS-CoV-2 on a very intimate level by examining how infectious aerosols, or bioaerosols, are transmitted.  

Size matters

A recent white paper from Enverid that examines different ventilation and filtration solutions with comparisons of cost and effectiveness defines bioaerosols as “airborne particles smaller than five microns that contain viruses.” To put size into perspective, one human hair is approximately 70 microns. And five microns is the maximum size to remain airborne. Bioaerosols can be as small as 0.3 microns, which is the equivalent of splitting a human hair 233 times. Understanding just how small these invisible particles are is important because their size dictates ease of transmission. The heavier the particle, the less likely it can float in the air. Lighter, smaller particles, on the other hand, can be easily picked up by air currents and travel throughout a space. This is also the reason air dilution and system purges have become increasingly important for indoor spaces—to flush out bioaerosols with fresh air intake.

However, key findings from Enverid’s white paper conclude that “increasing OA [outside air] ventilation adds material cost without significantly reducing the risk of airborne transmission,” arguing that this technique makes it more difficult to control humidity and pollution levels from outside air, hence increasing operational costs. But if healthy building experts like John Macomber have stressed anything, it’s the importance of bringing fresh air indoors. Even at the beginning of the century, doctors had realized that Spanish flu patients started getting better faster once they were placed in outdoor tents, rather than inside the hospital. Since then, research has continued to show the benefits of fresh air in relation to human health. So how important is airflow and ventilation when it comes to bioaerosol transmission? Can having just higher levels of air filtration suffice, or is ventilation required as well? 

Logically, air that is not moving or stagnant cannot be adequately filtered. Air needs to be pressurized in order to be pushed through filtration, and the higher the level of filtration, the more energy that is required to push air through it, according to Jiarong Hong, a mechanical engineer from the University of Minnesota who recently published a study showing the importance of having optimized ventilation. Aside from the increased energy usage required to pressurize air for filtration, another drawback of relying solely on filtration is that it may not be enough to capture the smallest bioaerosols. Only filters with the rating of MERV 14 and 16 have the ability to capture bioaerosols smaller than 1 micron. Size really does matter. According to Hong, “Filtration can certainly reduce the risks, but how much can it reduce it in comparison to replacing the air with fresh air coming from outdoors? That’s the big question.”

Amatuer “airheads”

Even non-experts (like myself) are able to understand the concept that stagnant air is not conducive to indoor air quality, but is recycled air that is moving any better? Craig Mod is a self-taught air ventilation aficionado, and his recent article in The Atlantic, which quotes Hong, explains how living on the humid coast of Japan forced him to learn ventilation techniques, lest his shoes continue to “bake their own bread” and his “doors grow their own skin.” Mold was a real challenge for Mod, and simply closing the windows and cranking up the air conditioner still left pockets of moisture in places with stagnant airflow throughout his home, allowing mold to grow in a matter of days. In addition to at home ventilation experiments, Mod spoke with Hong, who told him, “If an AC unit just blows air in circles, aerosols can stick around,” meaning that without air exchange, particles will just move about the space according to whatever airflow pattern is occurring. 

This stagnancy creates opportunities for “hot spots,” according to Kevin R. Hart, Co-founder and CEO of TZOA, the company responsible for Haven, a data collection tool that measures indoor air quality and the mechanical systems that support them. “You get these hot spots with any sort of source of air pollution—like a kitchen will have a higher concentration of particles from combustion than other parts of the home,” said Hart. Similarly, in commercial spaces with inadequate airflow, a hot spot could occur in any place of stagnancy, allowing particles to linger. 

While filtration without ventilation is possible, according to Hart, having both is ideal. Ventilation, or the process of bringing outdoor air inside, is what enables dilution. “It’s not that you’re changing the total amount of contaminants in the air, you’re just reducing them from being concentrated to being mixed and therefore diluted,” said Hart. In addition to stagnant air, hot spots are also more likely to happen with less dilution which occurs with less ventilation. Being close to a source of infection when air is stagnant (or not mixing with fresh air) increases the likelihood of transmission because the particles in the air are not being diluted. In fact, Oxford and MIT researchers found that proximity to the source and dilution play a huge role in the likelihood of getting COVID-19. 

Meanwhile, Hong’s research shows that it’s not only the amount of ventilation but how a space is being ventilated that matters. Hong told me, “Developing measurement and computational tools that provide accurate assessment and guidance on building ventilation is vital to ensuring occupant health.” The abstract of Hong’s study states, “Our results show that the design of ventilation is critical for reducing the risk of particle encounters. Inappropriate design can significantly limit the efficiency of particle removal, create local hot spots with orders of magnitude higher risks, and enhance particle deposition causing surface contamination.” Based on the average amount of bioaerosols that could be distributed from an asymptomatic carrier’s breathing or speaking in a given setting, the study was able to determine how many of those particles would be present in the air through different ventilation simulations. 

In the classroom setting simulation, the carrier was a lecturer standing at the front of the class, speaking for the entire 50 minute time period. When the vent was located on the back corner ceiling of the classroom, this created a hot spot in the back, particularly near the vent, “such that a student sitting in a hot spot in the back could inhale several times more particles than a front student at a safe spot.” When the vent was located in the front of the class near the carrier, the risk of transmission was significantly reduced. 

These simulations were under the assumption of 30 air exchanges per hour, essentially replacing the air in the room once every two minutes, which is “the upper limit that we have for HVAC systems now,” explained Hong. He continued, “if you don’t apply an appropriate ventilation strategy, even under that high air exchange rate, only ten percent of particles were removed during the 50 minute lecture.” This indicates a need for multiple sources of ventilation in large spaces like a classroom because we have no idea who or where the carrier may be. Without the measurements and simulations like those done by Hong’s research team, buildings are playing a dangerous guessing game. 

Back in Japan, multiple sources of ventilation is exactly how Mod was able to inadvertently create what Hong called a pressure gradient, which basically means Mod was able to achieve proper air flow throughout his home. He accomplished this by experimenting with various dehumidifiers, fans, and self-installed in-window ventilation units that essentially suck in outdoor air from one entry point and push out indoor air from a different exit point. By creating this pressure gradient, Mod found that not only did he solve his mold problem, but that this constant flow of fresh air is more conducive to overall human health—a consideration that Hart says is lacking in today’s property industry. 

IAQ is up to you

“Who is responsible for indoor air quality for human health?” This the question Hart’s been asking since he entered the industry. The answer he’s found is no one. “Homes are not being built for the health of occupants. They’re being built for someone’s bottom line. They’re being bought as an investment. But they’re not being built for human health.” According to Hart, there is a lack of education in terms of both buyers and builders, and there is a lack of governance and regulation when it comes to indoor air quality. Now, because of advancements in construction, building envelopes are getting tighter, which means that mechanical ventilation is more important than ever in order to ensure fresh air can enter these spaces.

One of Hart’s investors purchased a brand new $10 million home in Montana, and Hart flew out to install Haven (their sensor-based air quality data collection tool) at the property. Hart installed the equipment and immediately noticed the sensors were detecting high chemical levels. “I talked to the mechanical contractor who just built this home and many other homes in the community, and because they used geothermal heating, there’s no gas combustion. So they didn’t think that outdoor air was important, and now there’s no ventilation,” Hart explained. 

Hart was immediately concerned because even without gas combustion for heating, there are still plenty of contaminants in the air simply from occupants and their belongings. Volatile organic compounds (VOCs) are given off by carpets and paintings and furniture. Cooking and cleaning also produce chemicals that can linger in the air. With no fresh air entering the home, its occupants will begin to suffer, and they did. Eventually, the owner paid to have the mechanical contractor come back in and install an energy recovery ventilator (ERV). The average property owner would assume that their brand new home or building has adequate ventilation, but apparently that’s not necessarily the case. This anecdote highlights the importance of measuring ventilation and indoor air quality in general. 

Property owners have no idea what kinds of issues they need to fix or what kinds of updates their systems may need without objective air quality data. I spoke with Jeff Hendler, CEO of Logical Buildings, a company that integrates hardware with multiple data sources in order to monitor buildings’ air quality and ventilation, among other things. One of the ways they measure ventilation is through “sensors that sense vibration and movement of equipment at different angles” and these are placed “on air dampers that are used to modulate ventilation of a large building,” Hendler explained. He said they also deploy sensors “that have the capability of picking up how many particles per thousand are in the air.” By placing sensors strategically throughout a building, it can give property owners a better understanding of where hot spots may be and what areas need better ventilation and air flow. According to Hong, “it’s not just the amount of ventilation that matters, it’s really how you ventilate, how you distribute the ventilation in and ventilation out.”  

During my conversation with Hart, I realized that if nothing else, COVID-19 has gotten everyone talking about indoor air quality. Perhaps in the past, things like ventilation and filtration were topics only discussed by actual “airheads,” or when an individual noticed mold growing in their home, but now, our invisible foe has created quite a bit of visibility for an industry that was otherwise not often noticed. Now, the average person may be walking the office to ensure their desk is not located near a hotspot. Now, tenants may be inquiring about the ventilation system a building has before signing a lease or even asking for data to verify functionality. The likelihood is that people will continue to ask questions and demand information about indoor air quality even after a vaccine is widely available, and this is because we can’t unknow how indoor air quality affects our health. We’ve discovered that there are invisible assassins hiding in the air we breathe, but we’ve yet to uncover all of their identities.

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