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Something in the (Indoor) Air: The Importance of Indoor Air Quality

The largest school system in the U.S., the New York City public schools, reopened on Monday, September 13th. To support this reopening, the schools implemented a large number of mitigation measures. On a dedicated webpage titled “Health and Safety in Our Schools,” the district shared 12 different mitigation measures, with ventilation at number seven on the list.

The ventilation strategies employed by the school system vary. They include HVAC maintenance and the use of “fully operational ventilation through either natural, mechanical, or a combination of means.” In addition, and as what the school system calls “an added precaution,” every classroom in New York City has received two air purifier units. These classroom efforts and efforts in larger areas like cafeterias all focus on adding air circulation and increasing ventilation in classrooms and schools to “meet or exceed guidance from the CDC.”

These COVID mitigation practices employed in New York City’s public schools reflect a growing attention to air quality in shared indoor spaces. Closer attention to air quality and ventilation can address one of the key ways in which COVID and other viral diseases spread: via airborne transmission. According to numerous public health experts, increasing efforts to improve air quality in common settings like schools and workplaces is an essential part of stopping the spread of COVID-19 and other common ailments like the flu.

The Fundamentals of Indoor Air Quality (IAQ) and Ventilation

The U.S. Environmental Protection Agency (EPA) defines indoor air quality or IAQ as “the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants.” The EPA explains that IAQ issues originate from three causes:

  • Indoor pollution sources that release gases or particles into the air
  • Inadequate ventilation that does not bring sufficient outdoor air indoors for dilution
  • High temperatures and high humidity levels that can increase pollutant concentrations

Understanding and controlling these common causes of indoor air quality issues can help reduce the immediate and long-term health effects from indoor air pollutants, which can emerge shortly after or even years following exposure.

In their indoor air quality guide, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) details why IAQ is important yet often overlooked. The ASHRAE guide explains, “IAQ affects occupant health, comfort, and productivity, and in some cases even building usability, all of which can have significant economic impacts for building owners and occupants.” However, although IAQ requirements are part of broader building codes, these standards are often met with minimum acceptable compliance. 

So what makes for good IAQ? ASHRAE suggests that good IAQ is achieved by taking the following steps:

  • Providing air in occupied spaces in which there are no known or expected contaminants at concentrations likely to be harmful
  • Avoiding conditions likely to be associated with occupant health or comfort complaints
  • Managing both indoor air pollution levels and thermal environmental parameters

A key caveat in pursuing good IAQ is that air quality can sometimes be subjective. ASHRAE cautions, “The limits of existing knowledge regarding the health and comfort impacts of specific contaminants and contaminant mixtures in nonindustrial environments, coupled with the variations in human susceptibility, make it impossible to develop a single IAQ metric that can provide a summary measure of IAQ in buildings.” To offset this subjectivity, ASHRAE consequently recommends “diligent compliance” with standards as well as “technically sound and well-executed efforts to meet or exceed” the requirements outlined in building codes.

The EPA highlights effective ventilation, or bringing outdoor air indoors, as a key way to ensure IAQ. Ventilation can occur by infiltration, natural ventilation, and mechanical ventilation. These first two methods (infiltration and natural ventilation) result from air movement caused by differences in air temperature between the indoors and the outdoors. Infiltration occurs when outdoor air enters indoor spaces through openings such as joints, cracks in walls, floors, ceilings, and spaces around windows and doors. By contrast, natural ventilation sees air move through intentionally opened windows and doors. The third method, mechanical ventilation, relies on devices such as fans and ducts to remove indoor air and distribute filtered or conditioned outdoor air into a building. The rate at which outdoor air replaces indoor air is described as the air exchange rate or air changes per hour (ACH).

The Spread of Airborne Pathogens and Indoor Air Quality

As an airborne pathogen, COVID has brought IAQ to the forefront of environmental health conversations. According to a recent article in The Atlantic, the basics of airborne pathogenic spread are rooted in the reality that humans breathe about eight to ten liters of air per minute. When that inhalation and exhalation occurs in a shared indoor space with other occupants, “we are constantly breathing in one another’s lung secretions.” Those lung secretions include small droplets or particles called aerosols from deep inside the lungs. Aerosols become suspended in air like liquid sprayed from a nebulizer. Because these droplets originate from the depths of the respiratory tract, they often carry the most virus. Furthermore, their small size allows them to stay suspended in the air for longer and to travel deeper into the lungs of surrounding people as they inhale. 

Catherine Noakes, who is an IAQ researcher at the University of Leeds in England, explained to The Atlantic why these qualities of aerosols make them particularly threatening in the context of viral transmission. She commented, ““We want to feel we’re in control. If something is transmitted through your contaminated hands touching your face, you control that. But if something’s transmitted through breathing the same air, that is very, very hard for an individual to manage.” Environmental health expert Joseph G. Allen, director of the Harvard Healthy Buildings program, has elaborated on how aerosol contamination poses threats indoors. In an interactive New York Times feature that modeled air flow and contamination in a classroom, Dr. Allen acknowledged that while it is difficult to pinpoint what level of contamination presents the greatest risk of infection, “exposure is a function of concentration and time.”

Another complicating factor when discussing airborne spread of disease has been a long-standing debate about particle size and what qualifies as an “aerosol.” In an April 2021 editorial for The British Medical Journal (BMJ), a group of researchers explained, “The confusion has emanated from traditional terminology introduced during the last century. This created poorly defined divisions between ‘droplet,’ ‘airborne,’ and ‘droplet nuclei’ transmission, leading to misunderstandings over the physical behaviour of these particles.” Despite these debates, the authors advise that the definition of an aerosol should be updated to be more encompassing. They assert, “If you can inhale particles—regardless of their size or name—you  are breathing in aerosols.” 

The difficulty of managing airborne transmission is one reason why the COVID pandemic and other, more seasonal viruses like influenza have presented such a mitigation challenge. Against the backdrop of the current COVID pandemic, the BMJ co-authors argue that one key takeaway from coronavirus should be that indoor air quality deserves more attention. They state, “If we accept that someone in an indoor environment can inhale enough virus to cause infection when more than 2 m away from the original source—even after the original source has left—then air replacement or air cleaning mechanisms become much more important. This means opening windows or installing or upgrading heating, ventilation, and air conditioning systems” because the likelihood of infection is much higher “in a room with windows that can’t be opened or lacking any ventilation system.”

Many other experts agree with this assessment that ventilation is critical. In May of 2021, 39 public health, indoor air quality, and engineering researchers and experts published an editorial in the journal Science advocating for a “paradigm shift” in how we think about IAQ as it relates to respiratory infection transmission. When introducing their topic, the authors decried how “airborne pathogens and respiratory infections, whether seasonal influenza or COVID-19, are addressed fairly weakly, if at all, in terms of regulations, standards, and building design and operation, pertaining to the air we breathe.” They underscored the importance of acknowledging the impact of airborne transmission by explaining, “Although the highest exposure for an individual is when they are in close proximity, community outbreaks for COVID-19 infection in particular most frequently occur at larger distances through inhalation of airborne virus-laden particles in indoor spaces shared with infected individuals. Such airborne transmission is potentially the dominant mode of transmission of numerous respiratory infections.” 

Ultimately, the authors conclude that while airborne transmission of pathogens will not cease to occur, how we manage it can evolve dramatically by investing in better ventilation and IAQ standard enforcement. They assert, “In the 21st century, we need to establish the foundations to ensure that the air in our buildings is clean with a substantially reduced pathogen count, contributing to the building occupants’ health, just as we expect for the water coming out of our taps.” In essence, these experts are posing a provocative and insightful question: what would happen if we applied the same rigorous standards for the cleanliness of our water to the cleanliness of our air?

Why IAQ Will Continue to Matter

Elevating the standards we apply to air quality will be essential to living with and managing the risk of ongoing viral and environmental realities. In a recent article for The Atlantic, former FDA Commissioner Scott Gottlieb wrote that eliminating COVID will be “as politically unrealistic as it is biologically implausible” because the virus is now so widespread across the globe that it cannot be eradicated. Rather, it will become what Gottlieb calls “an endemic virus, settling alongside the other four strains of coronaviruses that circulate widely among us.” When combined with other ongoing viral threats like influenza, Gottlieb predicts that “the harm to public health and economic productivity will be too great for society to simply shrug off. The dual viral threat will compel better defenses. We won’t have a choice.”

Gottlieb suggests that these “better defenses” will include an evolving understanding of ventilation. He notes, “Buildings will also undergo renovations to improve airflow and filtration in order to reduce the spread of aerosols. Modern buildings are sealed tight, sometimes as part of an attempt to make them greener. Now they should go blue—improving airflow and air quality by adding hospital-grade air-handling and -filtration systems.” 

Seeing buildings “go blue” through better ventilation is part of the future envisioned by the 39 co-authors of the Science editorial piece mentioned earlier. They contend that “going blue” needs to be reframed so that ventilation upgrades are seen not as a cost that is too high but instead as an investment in our future that we can’t afford to defer any longer. The co-authors predict, “Economic costs of infections can be massive and may exceed initial infrastructure costs to contain them.” To emphasize this point, they cite the following costs associated with widespread viral transmission: “The global monthly harm from COVID-19 has been conservatively assessed at $1 trillion, but there are massive costs of common respiratory infections as well. In the United States alone, the yearly cost (direct and indirect) of influenza has been calculated at $11.2 billion; for respiratory infections other than influenza, the yearly cost stood at $40 billion.”

With COVID-19 and other airborne illnesses here to stay, there is an opportunity to learn from the challenges of our present pandemic to protect air quality and the occupants of indoor spaces in the future. Because aerosol transmission is now widely acknowledged as the principal way that COVID-19 has spread, ventilation has increased in importance. Unfortunately, the ventilation systems for most non-medical settings are not designed to achieve the air changes per hour that are recommended for airborne infection control. However, when existing ventilation systems are enhanced by bringing outdoor air in through windows and by applying the germicidal power of UV light in indoor spaces, IAQ can improve dramatically. Said another way: by combining the diluting power of outdoor air with the applied technology of UV light, we can all breathe a little more easily and look forward to a safer future indoors.

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