IAQ Solutions to Improve Schools – Now and in the Future

A version of this article, co-authored by Paul Hutton, FAIA, NCARB, LEED® Fellow BD+C, initially appeared in Learning By Design’s Spring 2021 issue

During the COVID-19 pandemic, many parents, teachers, administrators, and even students have become experts in matters relating to Indoor Air Quality (IAQ), including MERV filters, ultraviolet germicidal irradiation, and bipolar ionization. While this increased knowledge base and awareness of IAQ in schools is welcomed, there remains more work to be done. The pandemic has exposed a critical fact about schools: in general, they are poorly ventilated for both optimum health and academic performance. As such. school designers and administrators have an obligation to improve air quality beyond the measures touted in response to COVID-19.

Ventilation in Schools

Many standards set limits for indoor carbon dioxide (CO2), indicating how well ventilated an indoor environment is or isn’t. The U.S. standard for building codes, American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) 62.1, suggests an allowable CO2 level of 700 parts per million (ppm) above the outside air ambient condition. Given the current average ambient CO2 level of over 400 ppm, that implies approximately 1,100 ppm indoors.

Unfortunately, the current reality is that most classrooms are inadequately ventilated, leading to higher concentrations of CO2 than ASHRAE 62.1 recommends. It is important to note that ASHRAE states “indoor carbon dioxide concentration in the range of 700 ppm above outdoors are not related to any health impact from the carbon dioxide itself, only to odor perception.”1 In short, odor perception is still the governing factor in IAQ, as opposed to human cognition. Because it is easily measured and a simple indicator of the amount of fresh air inside, CO2 is often used as a general proxy for IAQ. However, odor perception is inadequate. It should not be the guiding standard for health outcomes and certainly not the resource to determine what conditions would allow students to do their best work.

More recent health-related standards set CO2 levels based on numerous research efforts in health and IAQ, such as the WELL Building Standard and RESET Air. These programs are evidence-based and as a result have set a much lower allowable limit for CO2, in the order of 600–800 ppm total CO2 (allowable CO2 and outside air conditions).

Most schools fail to even meet basic CO2 limits outlined by standards and codes. Multiple widespread studies2-6 of K-12 environments have shown time-average concentration levels exceeding 1,000 ppm, with maximum values ranging from 1,400 ppm to 5,200 ppm.7 When assessing existing schools, the actual CO2 measurements align with these findings.

How is it that so many schools fail to meet the most widely-accepted standard for ventilation? The top four reasons are as follows:

  • No Mechanical Ventilation System: Many currently operating schools were built during a time that predates a code requirement for outside air. These “leaky” buildings with operable windows were expected to “breathe” on their own. Over time, many districts have replaced the operable windows with fixed windows or secured them shut. And, of course, opening a window in winter isn’t practical in many climates.
  • Design Deficiencies: These are common in many schools and in particular those with variable air volume (VAV) systems, one of the most common mechanical systems used in K-12 environments. These systems often fail to provide sufficient outside air in some areas (particularly in the more densely occupied areas such as classrooms) while simultaneously providing excessive outside air in other areas, resulting in a double penalty of poor IAQ and unnecessary energy use.
  • System Degradation: Over time, a system can develop issues that compromise IAQ, even if installed correctly. This is exacerbated by a lack of adequate maintenance capacity and training that afflicts many school districts.
  • Energy Efficiency: There is a lack of emphasis on the connection between IAQ and occupant performance. Even in school districts with more robust facilities staff, the emphasis is typically on energy efficiency rather than IAQ. In many cases, we have even seen operational measures put in place that cause harm to IAQ in pursuit of energy savings.

Given our new collective understanding about the effect of ventilation on student performance, we can and must do better. Healthy indoor environments need to be integral to the 131,000 existing schools already in operation, and not just those that are yet to be built.

The COVID-19 pandemic is greatly impacting how schools are renovating and remediating these ventilation challenges. Prior to and after this pandemic, viruses have and will continue to impact absentee rates. Because direct measurement of viral particles in the air is very difficult, we use IAQ measurements such as CO2, VOCs, and PM as proxies to infer conditions related to the virus. So how can designers create spaces that have good ventilation and also reduce viral transmission?

Design Response to COVID

Recommendations for responding to COVID-19 published by leading institutions such as the CDC, WHO, and ASHRAE focus primarily on filtration and removal of harmful particles in indoor air. MERV 13 filters, UV-c irradiation, and bipolar ionization all tackle the issue from this perspective. Some sources have advocated opening windows and doors when possible, but this strategy is limited by modern school design as well as weather. These same sources have also recommended increasing ventilation, presumably because more fresh air will effectively dilute viral particles. Most school buildings don’t have enough spare capacity to increase ventilation, especially during extreme conditions of cold or hot weather, which is rarely mentioned.

With limited budgets and mechanical system capacity, school districts across the country have still managed to improve filtration and increase ventilation where possible. Even with these updates, most schools endured long and repeating closures due to inadequate ventilation.

The pandemic has heightened awareness of IAQ and how it is measured. When school districts are not transparent with teachers, parents, and students concerning CO2 and VOC (IAQ metrics) levels, occupants measure and share this data themselves on social media.

It is a reasonable expectation that COVID-19 will be largely controlled by the 2021–22 academic year. However, the likelihood of future pandemics, and the ever-present threat of influenza and other communicable diseases creates a demand for better, healthier indoor air. It is expected that teachers, parents, and students will demand healthy environmental conditions, especially now that there is a growing awareness around the learning environment.

Looking Forward

Equipped with more knowledge about viral transmission, the public will continue to be interested in how the built environment is working to protect their health. How do school districts concurrently make their buildings healthy and communicate those changes with the public, all while working to keep operational costs within their budgets? A huge challenge for school districts lies ahead.

Numerous strategies that undoubtedly increase both energy use and maintenance have been suggested to help during the pandemic. One example that causes us concern is the recent increase in advanced filtering technology systems, particularly those based on needle ionization. Although some tests indicate these systems reduce bio-effluents (odor causing biological contaminants) and perhaps viral particles, we do not rely on these devices as the primary solution to IAQ. They do not reduce the concentration of CO2 inside, which may provide a false sense of security about IAQ. In any installation involving these devices, CO2 levels should be monitored and kept below the new and more stringent standard of 800 ppm. When the pandemic recedes, these types of strategies are the most likely to be abandoned.

So what strategies, technologies, and solutions are here to stay? Here are the following “trends” that will have lasting value in a post-COVID environment.

Retro-commissioning Existing Systems.

As discussed earlier, the biggest opportunity to impact the greatest number of occupants is in the quality of our existing schools. Within these buildings, the most consistent opportunity is to focus on improving the performance of their existing systems. If it has not been performed recently, most schools would benefit from a process called retro-commissioning — a systematic approach to improve the performance of existing building systems, including both the mechanical equipment and controls systems that are largely responsible for maintaining IAQ. Over time, existing building systems will experience failures or degrade in such a way that compromises IAQ, as well as energy performance. The retro-commissioning process finds these issues and proposes corrective measures.

Even experienced districts will often benefit from a formal retro-commissioning process. For example, a common issue includes failed damper actuators in air-handling units. Relying on a building automation system alone, the units appeared to be providing fresh air during occupied hours. In reality, the building had operated for months, if not years, without bringing in fresh air, certainly increasing the likelihood of viral transmission in those buildings. Before considering any types of expensive upgrades or technologies, building owners should first make sure that their existing systems work as intended, and maximize the opportunities to improve their performance.

IAQ Monitoring.

Once systems have been commissioned/retro-commissioned and are working properly, it is important to know that they are working to perform their intended function. IAQ monitoring is one measure that provides better insight into the health of a space. The sensors and monitors that are currently available range from low cost with limited detection capabilities, to expensive lab-grade units. Basic consumer units that detect CO2 can be found online for less than $100, and we have even seen teachers bring these into their classroom as they now understand the correlation between CO2 rates and viral transmission.

The sweet spot for school districts seems to be in the $500–$700 range for “Grade B” sensors. These are more robust than consumer models, reasonably accurate, but far less costly than lab-grade equipment. In addition to CO2, these units typically detect other contaminants such as particulate matter (PM2.5,10), VOCs, ozone, or formaldehyde in addition to basic criteria like temperature and humidity. More advanced units can integrate into the building automation system, but even standalone systems can provide a district with more insight into the state of their air quality.

When deployment of these sensors is combined with on-going monitoring, a school district can proactively address issues that occur. In the example previously provided of a failed outside air damper, this zone of the school would have experienced high CO2 levels, along with potentially high levels of total volatile organic compounds (TVOCs) and particulate matter. Had IAQ sensors been deployed and monitored, the district would have realized that an issue was occurring that required further investigation. This likely would have led them to discover the source of the issue and resolve it in short order, eliminating a much longer exposure to poor air quality.

IAQ Certifications.

While occupants may know a little bit about the importance of indoor air quality on health, their knowledge is typically limited. They may have read that CO2 may be a proxy for viral transmission, but don’t know if 700 parts per million is good or bad. In these cases, a third-party rating system can be helpful. These provide occupants with an independent “seal of approval” that the environment they are learning and working in has been deemed to be healthy by somebody other than the district.

Examples of these programs include the WELL Building Standard (including its new Health-Safety Rating) and the RESET Air Standard. It is important to note that these are considered “performance-based” programs, meaning that the buildings have to be tested or measured and proven to actually meet the IAQ requirements.

Modeling for Viral Transmission.

In response to the pandemic, several tools have been developed that allow for modeling of viral transmission in buildings. One such tool developed by BranchPattern is called the Facility Infection Risk Estimator™, which can currently model both SARS-CoV-2 and Influenza A. It allows the user to create a representative model of an environment, such as a classroom, gym or cafeteria. By changing the parameters such as ventilation, filtration, relative humidity, exposure time, and vaccination rates, the user can predict the likelihood of viral transmission over a day, or in the case of the Influenza A, over an entire flu season, which can then be aggregated over the whole building to show the impact of the environment on things such as total teacher and student days lost.

One of the primary reasons for developing this tool was to allow users to make smarter decisions about where to invest in strategies that reduce viral transmission. Some of these strategies often require additional energy use to function. For example, both increasing the amount of outside air used for ventilation and selecting higher rated filters will typically have energy penalties associated with them. The questions of “how much outside air should I bring in?” and “what level of filtration should I use?” can then be answered for the specific building and environment through modeling.

When used in concert with energy modeling tools, a number of strategies can then be evaluated from the standpoint of health benefits, cost, and energy impact. This approach provides a more holistic view of the strategies being considered and makes for more informed decisions.

Operable Windows.

Operable windows were a standard feature of all schools through the 1950s. Due to the adoption of widespread air conditioning and increased security concerns, schools began eliminating operable windows in the 1960s and by the beginning of the millennium, operable windows were the exception rather than the rule. The lack of operable windows has left most schools unable to simply and inexpensively increase fresh air intake during the pandemic.

Building codes specify how many operable windows are needed to classify a classroom as “naturally ventilated,” which is 4% of the net floor area. A 900-square-foot classroom would therefore need 36 square feet of actual openings. While many schools have been designed to this standard, there is a significant drawback to relying solely on window size. During inclement weather occupants are most likely to close those windows, depriving themselves of fresh air. And even if those windows were always open, that amount of opening may still be inadequate depending on the type of use and number of occupants in the space.

Designing for effective natural ventilation is a far more complex issue than simply providing adequately sized operable windows. The height, width, spacing, and location of those windows make a critical difference, as well as the remainder of the air flow path. See sidebar to learn about a highly successful example of natural ventilation combined with high energy efficiency and excellent IAQ.

Balancing IAQ with Energy Consumption.

As school districts have implemented improvements to ventilation related to COVID-19, they have generally experienced increases in energy use and utility costs. This has reinforced the incorrect impression that good IAQ implies higher energy use. Both can be achieved in the same building using smart design combined with effective maintenance.

HVAC system overhauls are good opportunities to realize excellent IAQ and high energy efficiency. A strategy we recommend to our clients is Dedicated Outside Air Supply (DOAS) in which heating and cooling systems are decoupled from ventilation and energy recovery. This approach delivers the promised combination of good IAQ and high energy efficiency. This principle can be applied to new construction and existing school HVAC system renovations.

IAQ bond projects.

Communities struggling to pass bond initiatives may find more success reframing renovation projects (like HVAC system replacements) with more emphasis on conversations around IAQ and healthy learning environments. One result of the pandemic could be more school bond issues passing that target HVAC systems and IAQ improvements.

Given that the average age of U.S. school buildings is more than 40 years, it is inevitable that school districts are constantly renovating and upgrading their facilities. Much of this effort focuses on aging HVAC systems. Until recently, most HVAC replacement projects have been exactly that — putting newer versions of the same equipment in the same locations. The increased awareness of IAQ has begun to change this dynamic, resulting in more thorough analysis of existing IAQ conditions and installation of HVAC systems that provide healthier conditions for all.


Final Thoughts

While it looks like the pandemic will soon be in our rear view mirror, lessons we learned on making schools safer from a viral transmission standpoint can be applied to better IAQ that promotes health and cognition. Now that the public is armed with more information, school districts are tasked with transparently communicating newly implemented trends, like those suggested above, to building occupants. To bring about these strategies without increasing energy use will require a concerted effort from architects, engineers, maintenance staff, and school administrators.


  1. Interpretation IC 62-2001-07 of ANSI/ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air Quality
  2. Allen JG, Macomber JD. Healthy Buildings: How Indoor Spaces Drive Performance and Productivity. Harvard University Press, Cambridge, MA; 2020.
  3. Fisk WJ. The ventilation problem in schools: literature review. Indoor Air 2917;27:1039–51. Available at: https://doi.org/10.1111/ina.12403.
  4. Canha N, Mandin C, Ramalho O, et al. Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France. Indoor Air 2016;26:350-65. Available at: https://doi.org/10.1111/ina.12222.
  5. Corsi R, Torres VM, Sanders M, K. Kinney. Carbon Dioxide Levels and Dynamics in Elementary Schools: Results of the Tesias Study. Indoor Air 2002 – 9th International Conference on Indoor Air Quality and Climate. http://www.irbnet.de/daten/iconda/CIB6553.pdf.
  6. Mendell MJ, Eliseeva EA, Davies MM, et al. Association of classroom ventilation with reduced illness absence: a prospective study in California elementary schools. Indoor Air 2013;23(6):515-28. Available at: https://doi: 10.1111/ina.12042.
  7. Fisk, WJ. The ventilation problem in schools: literature review. Indoor Air 2017;27:1039–51. Available at: https://doi.org/10.1111/ina.12403.
  8. Facility Infection Risk Estimator. Available for free use at https://branchpattern.com/research/facility-infection-risk-estimator-v2-0/