Ventilation: The NPI That Works for Airborne Viruses
In commercial aircraft—one example of a highly ventilated indoor space—virus transmission has been low despite crowded conditions. This has been attributed to forced masking of passengers (unfortunately including children with disabilities). However, pressurized aircraft cabins experience 20-30 air changes per hour (ACH), meaning all of the air within a cabin will be filtered and replaced every 2-3 minutes. With this level of ventilation, the ability of a person to expose an individual to an infectious dose even in the next seat is quite limited. A classic example of a commercial aircraft outbreak was in 1977, where a single passenger with influenza infected 72 percent of other passengers on the flight. This extraordinary event required a complete failure of the ventilation system and a three-hour delay on the tarmac. If the ventilation system had been functional, the outbreak would have never happened.Although this table was created to illustrate the potential for high-quality respirators to decrease time to infectious dose (for workers in high-risk situations), you can also see the advantages of increased ventilation. As ACH increases, the time to infectious dose increases much more significantly than with face coverings, with differences that can be measured in hours for ventilation compared to minutes for masking.
A Sterile Air Pipe Dream
But, as with any mitigation strategy, what can be done can also be overdone (this might be an unofficial motto of the U.S. pandemic response). In September, the Atlantic published an article titled “The Plan to Stop Every Respiratory Virus at Once.”“It’s not just about COVID-19. The scientists who recognized the threat of airborne coronavirus early did so because they spent years studying evidence that—contrary to conventional wisdom—common respiratory illnesses such as the flu and colds can also spread through the air. We’ve long accepted colds and flus as inevitable facts of life, but are they? Why not redesign the airflow in our buildings to prevent them, too?”Building engineers believe that measures used during the current pandemic aren’t enough: “The pandemic has already prompted, in some schools and workplaces, ad hoc fixes for indoor air: portable HEPA filters, disinfecting UV lights, and even just open windows.” But in their eyes, these aren’t long-term solutions. “Modern buildings have sophisticated ventilation systems to keep their temperatures comfortable and their smells pleasant—why not use these systems to keep indoor air free of viruses too?”
There are some obvious barriers to engineering every building with virus-scrubbing ventilation: “The pandemic has made clear that Americans do not agree on how far they are willing to go to suppress the coronavirus. If we can’t get people to accept vaccines and wear masks in a pandemic, how do we get the money and the will to rehaul all our ventilation systems?”
I believe—in agreement with building engineers and indoor environmental scientists—that it’s possible to design ventilation systems with the ability to scrub the air of all airborne respiratory viruses. I also agree that this might have a measurable benefit in one aspect of human health, as the disease burden of respiratory viruses is significant.
The Example of Polio
In addition to influenza and smallpox, polio was also considered a major viral pathogen of the 20th century. However, before that time, it wasn’t thought to be a major threat. There weren’t serious outbreaks. It wasn’t as if the virus didn’t exist. People had been infected with polio for thousands of years. But with the advent of the industrial revolution, our relationship with the virus changed, and so did the disease it caused.Polio is a highly contagious virus that causes an asymptomatic (yet still transmissible) or very mild symptomatic infection in 90 percent of people. The virus infects the gastrointestinal tract of humans, although in some cases, the virus spreads to the nervous system, and can result in paralysis or death. Because so many people are asymptomatic, the virus can infect many others before a paralytic case is identified, making it difficult to contain. The virus is transmitted by the fecal-oral route, through water and other fecal contaminated material.
For most of human history, people could not avoid being infected with polio virus. Most were infected during infancy. And since almost everyone was infected, they had antibodies, including new mothers who were passing those antibodies on to their children through the placenta prior to birth and via breastfeeding afterwards. Infantile paralysis, although still possible, was relatively rare.
The polio virus was no longer an endemic virus, and had ceased to coexist with humans the way it had evolved for thousands of years. And where it lost its endemic status, it gained epidemic potential. And these epidemics just so happened to hit wealthy people, since clean conditions were a sign of wealth, and thus the rich were less likely to have immunity to polio from infancy.
A Consequence of Sanitized Indoor Air: Increased Immune Ignorance
The subsequent rise of polio epidemics with improved sanitation suggests that just because an advance in public health has immediate and obvious benefits, doesn’t mean there won’t be a cost that isn’t immediate or obvious.Children reared in environments that expose them to a variety of bacteria appear to have immune systems that are “educated” to tolerate those bacteria and other biological microparticles, while those in “clean” environments have immune systems that could be described as “ignorant” and thus more likely to overreact.
Many working parents have experience with heterologous immunity, and sometimes a lack of it, often with out realizing it. When a firstborn child goes into daycare, weeks to months old, all microbial hell breaks loose (or at least it seems like it). The baby is sick all of the time, and so are the parents. This can go on for months, and is especially bad during the cold and flu season.
However, the next year, it gets better. And with a second child, everyone doesn’t get as sick, and that’s not just a figment of the parents’ imagination. That’s because immune memory has been acquired by the parents and the older sibling that protects against the same cold and flu viruses, and also offers a degree of cross-protection against other viruses with similar properties. The second baby doesn’t get as sick either, as he/she gets some protection from mother’s antibodies and less infections from their own immune-strengthened family.
This raises some important questions about the downsides of a complete overhaul of current building design to eliminate common respiratory virus infections. Even if widespread adoption of these plans was feasible, would the potential loss of cross-immunity in healthy individuals be worth it? Much like non-pharmaceutical interventions have only delayed viral spread in the current pandemic (if they’ve had any effect), wouldn’t these measures just delay the inevitable?
Even if many buildings have clean, virus-free air, there will always be places that don’t, mainly in older buildings concentrated in older areas with higher poverty rates. Like with polio, this could actually increase epidemics among the wealthy once certain respiratory viruses evolve ways to overcome modern building engineering.
In our current safety culture-driven pandemic response, any risk of infection is deemed unacceptable, and those who highlight potential costs of mitigation measures are labeled irresponsible and dangerous. However, safety worship and building engineers might override old-fashioned common sense, but they can’t override our own biology. The old adage survives despite our best efforts; in the case of seasonal cold and flu viruses, what doesn’t kill you still makes you stronger.