Clambering around the ceiling of a big-box store, Jeff Siegel, a mechanical engineer at the University of Toronto, had no idea that he was looking at the post-pandemic future of air-conditioning systems. Siegel studies indoor air quality, and he and his colleagues were testing the air in the store—he wouldn’t say which one. This is the possibly grim future part: While they were up there, they found that one of the six HVAC units (that’s heating, ventilation, and air-conditioning) was installed exactly upside down. Like, 180 degrees from spec. “The door that was used to access the filter couldn’t be fully opened, and the filter couldn’t be replaced,” Siegel says.
When the team went back six months later to test again, “the filter was entirely plugged with dust and other stuff. There was literally no way to change it without a Sawzall to cut the frame,” Siegel says. At the time, Siegel knew what that meant. The air inside the store would be that much crummier. It was, in a way, just like a thousand other HVAC mistakes Siegel has seen: dampers that are supposed to admit outside air into a building rusted open or shut, badly installed filters letting air pass around their edges, forced-air fans running barely 18 percent of the time. In theory, HVAC heats and air-conditions. In practice, it doesn’t always ventilate or filter.
But now that scientists around the world largely agree that the pandemic virus SARS-CoV-2—like a lot of other respiratory viruses—transmits most easily indoors, in crowded and poorly ventilated spaces, that occasional, multipronged failure signifies a much more serious problem. As the number of people infected with Covid-19 in the US breaks records, and cooped-up kids and suffering business owners agitate for a life slightly more normal, the once boring ventilation and filter systems in the guts of homes, schools, offices, and factories have become a focus of debate. People know that if they want to go back inside those buildings—even while masked and 6 feet away from each other—something has to vent potentially virus-infused air. That means a renewed interest in HVAC, and possibly a new future for an often-overlooked field of science. Maybe. “The best HVAC in the world performs poorly when it’s not well maintained, and the usual standard is ‘not well maintained,’” Siegel says. “What we’re seeing now in the pandemic is that people want HVAC to help us, and it’s like, wait a second—you’ve systematically underinvested and not done the kinds of things you should do to have a well-functioning system.”
On the other hand, that might mean a new future for ventilation is emerging, and along with it a new way of seeing the future of building design and engineering—because trying to Covid-proof a home or office might make it better in all sorts of other ways, too.
SARS-CoV-2 is a respiratory coronavirus that almost certainly has among its modes of transmission the ability to move almost like a vapor, in invisible bubbles of snot and spit or dessicated protein that waft on air currents, emitted by people showing no symptoms of illness. Transmission is most common indoors, where the air doesn’t exchange as often as it does outside. So one of the biggest ideas for decreasing transmission but still letting people go back to school and work safely—not to mention places like restaurants, theaters, and bookstores—is ventilation: getting potentially infectious viral particles in the air out, and clean air in. “Those of us in this field have been arguing for decades that we need to pay attention to the indoor environment, and we’re thrilled people are recognizing it’s important. But how to get from here to there will take an infusion of investment,” says Shelly Miller, a mechanical engineer at the University of Colorado Boulder who studies indoor air. “We view outside air and water as shared goods. This is something everybody shares. I don’t really see why it would be any different from the air in a building, because lots of people share the air in the building. We just haven’t looked at it that way.”
The basics of the technology are already there, deployed in specialized environments like hospitals. It’s really the idea of equivalent air changes, or clean air delivery that is the sum of the outdoor air delivered to spaces to dilute contaminants and particulates, filtered air, and disinfected air, says William Bahnfleth, an architectural engineer at Penn State and chair of the Epidemic Task Force of the American Society of Heating, Refrigeration, and Air-conditioning Engineers, or ASHRAE, a standards-setting organization for the field. He and everyone else who works on indoor air has suddenly become very popular as the coronavirus’s airborne ways have become clearer. Everyone wants to shore up their buildings’ defenses.
One way to do that is to increase a metric called the air exchange rate—how often new air from outside replaces old air inside. A basic approach: Open lots of windows. But that only works if they’re there—and if the air outside isn’t even worse, like during a wildfire. A second angle is cleaning the air that’s already inside with a high-grade filter, either built into the central air system or with a stand-alone unit—mounted on a wall or in a portable air purifier anyone can buy (or even make). High-efficiency particulate air (HEPA) filters are probably too expensive and require too much energy to expect their use in typical HVAC systems, Bahnfleth says; that’s why so much advice now focuses on filters with a "minimum-efficiency reporting value" (MERV) of 13. They’re plenty good enough for Covid-capturing.
That makes it sound easy, and it isn’t. Even if your home has central air—and many don’t—the system probably doesn’t replace the air once per hour, much less six times, as some of the new virus-oriented recommendations call for in other indoor spaces. Moving more air calls for bigger fans that run more often. And bigger filters can create something called pressure drop inside a system, slowing down the air throughout. It takes bigger fans to overcome that, too. And the main point of an HVAC system is making all that air comfortable, raising or lowering its temperature. All those things increase costs and the amount of energy the system uses.
A third option is some kind of device, either in the ducts or in rooms, that actually removes microorganisms. Ultraviolet light can blow apart viruses’ genetic material, and at a wavelength of 254 nanometers—that’s so-called UV-C—it can kill germs. Even shorter wavelength "far UV-C" can kill coronaviruses. But nobody’s exactly sure about the best dosages and placement to deal with airborne viral particles. Bipolar ionizing filters and hydroxyl generators put chemically reactive or electrically charged particles into the air, but careful, rigorous studies of their effectiveness don’t exist yet.
None of that has stopped all those options from becoming very popular. “It’s a total Wild West of a marketplace for technologies that purport to clean the air or kill coronavirus. People are flocking to them because they’re really low cost, but don’t introduce additional pressure drop like a filter might. So you don’t have to retune or redesign the HVAC system,” says Brent Stephens, an architectural engineer and indoor air quality researcher at the Illinois Institute of Technology. “But a lot of that stuff is not well tested and not well regulated for effectiveness.”
Another approach, maybe even more expensive, might be to rethink—or more accurately, re-rethink—how people design buildings overall. Arguably, the history of architecture is the history of finding new ways to deal with uncomfortable climates. The shotgun houses of the American south were designed for front-to-back airflow in humid summers; the high ceilings people favored closer to the equator let hotter air rise out of the living space while the lower, closer confines of northern buildings keep that heat in. Thicker walls and interior courtyards are cooling, while fireplaces in every room provide local control over heat when fuel is expensive.
That history has a semi-secret undercurrent. Buildings, especially in the modern era (and the modernist tradition) have been designed with health in mind. At least as early as the 1860s, a physician lectured on the dangers of “man’s own breath” and a lack of ventilation as being somehow responsible for the spread of disease. The foundational story of public health—the London physician John Snow tracing a cholera outbreak to public water pumps—is a story of geography and urban planning. And a newer thread of research traces the large-windowed, airy, smooth style of early modernism to hospitals built to help people recover from tuberculosis and the Spanish flu of 1918—airborne diseases that prefigured Covid-19. By the early 1920s, scientists understood that people who lived in high-quality housing had lower rates of respiratory and gastrointestinal diseases than those who lived in tenements, a conclusion that led to more spacious and well-ventilated housing, but also to racially motivated “slum clearance” under the guise of the sanitation.
In the 1920s and 1930s, “modernism developed in the context of the flu and health concerns, and with the ambition to be a modernizing, socially beneficial force,” says Daniel Barber, an architectural and environmental historian at the University of Pennsylvania and author of Modern Architecture and Climate: Design Before Air Conditioning. “The capacity for a building to render a space more healthy than the neo-Baroque structure next to it, for example, was one of the selling points and reasons that modernism is appealing to its client base.”
Then, around the middle of the 20th century, came air conditioning. “HVAC was available, and mechanical systems could manage the climate of a building so the architect didn’t have to worry about it,” Barber says. In a way, that was liberating—the climate no longer had the power to literally shape the building, and architects could do crazy postmodern stuff. They’d let the engineers take responsibility for all the machinery to keep the inside of the building warm or cool. “The culture of architecture today doesn’t even think about air-conditioning. It’s just not a factor. It’s a given,” Barber says.
The result: nearly-airtight buildings filled with synthetic materials, with windows that sometimes don’t open, and high levels of recirculated, often unfiltered air. Even that’s not consistent, because building codes tend to ask for just minimum adherence to standards. So some buildings are better than others, and some legitimately make people sick. Add that to the general inability of US cities to build enough housing—forcing people to live in crowded conditions—and you have a country of petri dishes with bad air, even before Covid-19 and increasingly stringent (and necessary) energy efficiency requirements. “We treat our water so we don’t get sick. We process our waste so it doesn’t transmit disease. And then we basically stopped, and haven’t done much for the health of our urban society—where we spend 90 percent of our time indoors,” Miller says. “We focused a lot on outdoor pollution because in the 1950s we realized outdoor air pollution kills people. We started to control emissions and put controls on cars and power plants and industry. But we’ve left the indoor environment to the purview of the owner or renter or occupant and said, ‘That’s your personal domain, do what you want.’”
Fixing all that will require more than just new air purifiers. Like so many other problems revealed by the Covid-19 pandemic, the way people think about air in buildings needs fundamental changes. Even seemingly straightforward retrofits of existing HVAC systems won’t be easy. Like, here’s some HVAC nerdiness: In most standard air-conditioning systems, the rack designed to hold a filter is just 2 inches deep. But a MERV-13 filter, the one folks like Bahnfleth are recommending to capture viral particles, really has to be 4 inches thick to maintain its performance. Or you can use a 2-inch one manufactured to enhance its filtering abilities with an electrostatic charge. “But it wears off,” Bahnfleth says. “You have to change them pretty often if you’re going to keep them operating at that level.” That gets back to the kinds of problems Siegel has seen with maintenance. And refitting for 4-inch-thick filters means increasing overall pressure drop, which means spending money to amp up the whole system.
Covid-related changes to ventilation systems will be complicated, perhaps, and almost certainly expensive, but they might not require new technology. They might need more power, and that’d be a bummer, because buildings are the biggest users of power in the US and are responsible for as much as 40 percent of greenhouse gas emissions, the cause of global climate change. Some officials, like in New York City, are already trying to put more stringent emission and energy efficiency standards on some commercial buildings. The global “passive house” movement advocates heavier insulation and other innovations to moderate indoor climate and air quality with less power—very much like the old pre-air-conditioning ideas, actually, but with fancier materials. “You build a house that’s very well insulated, very tight, very good windows, and you hardly ever need to heat or cool it, but you’re always bringing in outside air, filtering it, using a heat exchanger,” says Miller. (That’s a device that lets air at different temperatures trade heat without mixing—on a cold winter day, outside air pulled into the house gets warmed by indoor air exhausting out, saving the energy of heating it from scratch. It works vice versa too.) “The only issue they need to address is how to exhaust the kitchen emissions when you’re cooking.”
These kinds of systems-level changes might do much more than help deal with Covid-19. Before the pandemic, indoor air quality researchers were worried about all kinds of other issues. Airborne diseases like influenza were on their minds, of course, but so were even more insidious problems. Particle pollution and allergens can increase childhood asthma rates. Most people understand that carbon monoxide, a byproduct of incomplete combustion and sometimes faulty stoves and heaters, is dangerous enough to have warning alarms for. But even the carbon dioxide that all of us humans give off when we breathe can accumulate indoors, and if levels of it get too high it may actually slow us down cognitively. And it isn’t the only gas in our poorly ventilated living and work spaces—human bodies give off all kinds of weird stuff, and not all of it’s just the usual vaguely humorous, odorous gas. “Very few studies show an impact of CO2 by itself. Many studies show no impact of CO2 by itself,” Siegel says. “But many studies across the board show an impact of reduced ventilation.”
For now, though, the standards that end up in municipal codes for ventilation often come straight, or nearly straight, from recommendations made by groups like ASHRAE. And those, says Bahnfleth, are still sort of de minimis rather than extreme. The suggested rules for homes and offices don’t even have the same philosophy as the suggestions for health care facilities, where you can imagine people really care about things like viruses. Home and office standards measure ventilation rates per person and per square foot, and they’re more focused on the perception of air quality, on bad smells. Hospital standards focus on complete air exchanges and filtration of an entire room or building, and health care facilities may build in the equivalent of as many as 20 changes per hour in high-risk areas like operating rooms. “This air change concept really hasn’t penetrated the current design standards for residential and commercial buildings, and it’ll be interesting to see if that’s the way to go,” Bahnfleth says. “I would like to believe the experience we’re having with Covid-19 is going to create the motivation to improve standards for non-health-care buildings to provide better health care control. But how far we’ll go with that, I don’t know.”
The technology, then, isn’t as tricky as the sociology and the politics. Legend says John Snow was able to remove the handle of a cholera-contaminated pump in London, rendering the pump unusable and arresting the epidemic; he might’ve had a much harder time if he'd tried to go into the homes of everyone with indoor plumbing to confiscate their water faucets.
More pragmatically, no one knows who’d pay for any of these changes. Landlords? A government fund? The incentives among government, building owners, and renters to make any of these changes aren’t exactly in alignment. The people who might spend the money aren’t necessarily the ones who’d accrue the direct benefits. It might be true that better ventilation and filtration could improve cognitive outcomes and affect, increase productivity, reduce absenteeism from illness—but those things are all good for the people who work in the office and rent the space, not the people who own the skyscraper and have to install a more powerful, more energy-efficient, more magical HVAC system. New rules that mandate those systems are great for the people who build, sell, and maintain them, and maybe not so great for the people who might be forced to buy them.
That said, the pitch is compelling: New systems to bring fresh air into buildings where people spend most of their time could beat back not only this pandemic, but maybe the next one too—while making people generally healthier and also fighting global warming. That’s the change that’s in the air.
11/12/20 6:10 PM PST Updated to clarify William Bahnfleth's role at ASHRAE and the nature of that organization, as well as the mixture of air sources in a summed total air change and the use of UV-C rather than "far UV-C" as in the original story.