Clean Air: Why, What and How

Authors: Katelyn Miyasaki, Yaneer Bar-Yam

A few months ago, WHN held our first in a series of panels on clean air topics, so we thought it would be a good time to release a newsletter on clean air basics. You can view the panel discussion here: https://youtu.be/EXaP-1jkUvM?si=nkcowznAdyTa98kw 

We breathe around 20,000 times a day, yet many people rarely think about what’s in the air we share. Clean air is one of the most powerful tools for good health—reducing infections, improving learning, and even enhancing our ability to think. When air is polluted or poorly ventilated, viruses, allergens, and other tiny particles build up, spreading illness, harming lungs, and getting into our blood to affect our hearts and brains. COVID-19 brought into sharp focus that the quality of the air we share affects not only our infection risk but our overall health. Clean air isn’t a luxury—it’s a basic necessity for health.

Clean air means more than the absence of smell or visible smoke. Indoor air contains exhaled particles—which can carry infectious pathogens—as well as environmental pollutants and carbon dioxide (CO₂). Good indoor air quality depends on two essentials: ventilation that brings in fresh air and reduces CO₂ and indoor pollutants, and filtration that removes particulates and infectious aerosols. Together, these create air that is both safe to breathe and comfortable to live and work in—helping our bodies and minds function at their best.

Air quality can be seen through simple measurements. CO₂ levels reveal how much exhaled air is accumulating indoors; outdoor air is about 400-500 parts per million (ppm) CO₂, and poor ventilation can result in 800 ppm, 1000 ppm, or even higher levels of indoor CO₂, which have been shown to negatively impact cognition. Fine particle sensors (PM2.5) track airborne pollution and smoke. Detecting specific pathogens is more difficult, so standards for ventilation and air filtering are set by estimates of how much air cleaning is needed to remove them. The previous standard of 6 Air Changes per Hour (ACH) was designed to dilute indoor air pollutants, not to achieve sufficient removal of infectious aerosols for infection control. It is now recognized as inadequate, particularly because it does not account for how many people are in a space. Hospitals have long required 12 ACH, and new guidance from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 241 specifies clean air delivery per person and per floor area. In a typical classroom, this corresponds to 10–12 ACH or more of equivalent clean air, with higher levels needed where people gather densely.

FAQ

Why does clean air matter?

• Air is a key determinant of health. The air we breathe can hold both particulates (pollution, dust, allergens, other material particles) which can cause harm and disease in the respiratory system, as well as infectious microbes which cause respiratory and systemic disease. Ensuring we are breathing clean air is therefore crucial to health, and we can improve air quality through cleaning the air.
• Air quality is particularly important for people with preexisting respiratory conditions such as asthma, which affects ~6.9% of children and ~9.8% of adults in the United States [1]. Clean air can also minimize the impact of seasonal allergies.
• Humans breathe in oxygen and breathe out carbon dioxide (CO₂), so low ventilation and crowded buildings can lead to high levels of CO₂, which negatively affects cognition [2].

How do infectious diseases spread through the air? How large are the particles?

• Airborne viruses and bacteria spread when infected people exhale particles that contain pathogens. These particles come from breathing, talking, singing, coughing, etc. They range from submicron aerosols (<1 µm) up to tens to hundreds of microns in diameter (~50–100 µm, or more). The smaller particles stay suspended in the air like smoke for hours and travel across rooms and between rooms, which is why indoor air quality is central to prevention.
• Larger particles, or droplets, can also form, but don’t spread as far, falling to the ground relatively quickly. This is where the six foot rule came from [3]. However, COVID-19 and many other diseases are spread primarily through small, airborne particles which can linger in the air for hours, which means that the six foot rule is insufficient for protection against infection. 

How do we clean the air? How does ventilation with outdoor air, filtration, inactivation with germicidal UV, and other technologies work?

1. Ventilation (bringing in clean outdoor air): Clean outdoor air dilutes CO₂ as well as other particles people breathe out. The more air exchanges per hour, the faster the particles are cleared out and the fewer particles are in the air at any one time.
2. Filtration (capturing particles with filters): Portable High Efficiency (HEPA) filters and Corsi-Rosenthal boxes physically remove virus-carrying particles from the air as air travels through them. HEPA filters remove a very high fraction of particles in a single pass, so air blowing directly from a HEPA purifier contains very low particle levels. Corsi-Rosenthal boxes use filters with lower single-pass efficiency but move larger volumes of air, achieving high overall clean air delivery at lower cost.
3. Inactivation (killing pathogens in the air): Germicidal ultraviolet light (UV-C) can damage and inactivate infectious pathogens while they’re in the air. Upper-room UV-C, an older technology, sanitizes air when air circulates away from people without exposing them to harmful UV light. A newer technology, far UV-C, uses wavelengths that have limited penetration of skin and eyes and is currently being developed for use in occupied spaces without requiring upper-room specific installation.

• The amount of each clean air technology that we use is dependent upon the needs of the building and the occupancy. When higher ACH is needed, circulating air from the outside can raise the costs of heating and cooling and the equipment needed to do so. It is often more cost effective to filter the air using portable filters, which can be remarkably low cost. 

How do we measure air quality?

• Indoor air quality can be assessed using several straightforward tools. CO₂ monitors measure carbon dioxide levels (in ppm), which serve as a proxy for ventilation and the amount of shared air in a room. For good ventilation, CO₂ levels should generally remain below a targeted level, often set to be ~800 ppm; higher levels signal insufficient airflow and increased risk of airborne transmission, as well as reduced cognitive clarity.
• Particulate sensors measure levels of PM2.5, PM10, and sometimes PM1, indicating how much particulate matter—such as dust, smoke, or aerosolized pollutants—is present. Many home monitors also report VOCs (volatile organic compounds), which track chemical pollutants released into the air as gases [4].
• Air movement through HVAC systems can be evaluated using airflow measurements and filter specifications. Clean-air performance is typically expressed in air changes per hour (ACH), combining natural ventilation with filtration from HEPA units or Corsi–Rosenthal boxes. While historical pollution control standards used around 6 ACH, and 12 ACH is recommended for higher protection environments, newer infection control frameworks base targets on the number of occupants and the amount of clean air per person. For example, a standard school classroom should provide the equivalent of at least 10–12 ACH of clean air.

How are filters classified and rated?

• MERV ratings were developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers and are often used to rate furnace filters. Higher MERV ratings indicate higher filtration capabilities. MERV ratings of 13 and above are typically recommended to capture airborne viral particles [5]. However, older HVAC systems may not be compatible with MERV 13 filters. 
• ISO 16890 ratings are an international air filter rating system [5]. The ePM₁ rating under ISO 16890 is recommended for capturing airborne viral particles.
• HEPA filters under U.S. standards are required to filter >99.97%, while under European standards (EN 1822) they are required to remove ≥99.95% of particles [6].
  • While HEPA filters have a high filtration efficiency, they also are hard to push air through, which may lead to a less efficient air purifier. Joey Fox, an HVAC engineer, discusses why HEPA filters are not always necessary to protect against viruses here: https://itsairborne.com/hepa-filters-are-not-needed-67e952792481 [7].

How do filters work to capture particles?

• Air filters and masks remove particles from the air via the same physical and electrostatic mechanisms. Larger particles tend to crash into the fibers (inertial impaction), while smaller ones tend to get caught when they brush past (interception) or move in zigzag patterns and collide with fibers (diffusion). Some particles settle due to gravity (gravity sedimentation), and others are pulled in by the fiber’s electrostatic charge (electrostatic attraction). Because these mechanisms affect different sizes of particles differently, particles of 100-300 nanometer diameter tend to be the ones hardest to filter out. Filtration standards therefore test efficiency either in this range or at the filter’s “most penetrating particle size.” These combined methods make high-quality masks and air filters very effective against all sizes of particles, including both tiny viruses and larger particles carrying viruses.

What is the role of DIY air cleaners?

• DIY air cleaners such as Corsi–Rosenthal (CR) boxes have demonstrated that lower-efficiency filters operated at higher airflow can reduce indoor air contaminants at substantially lower cost than commercial HEPA units. These air filters can be constructed cheaply at home using duct tape, MERV-13 furnace filters, and a box fan, with instructions available here:  https://corsirosenthalfoundation.org/instructions/ [8]. This establishes that cost-effective clean-air delivery can be achieved by prioritizing total airflow and filter surface area rather than relying exclusively on high-efficiency filters.
• Over the long term, DIY devices can continue to serve as:
  • A low-cost supplement to existing HVAC systems when additional clean-air delivery is needed.
  • A flexible option for spaces where ventilation upgrades are impractical.
  • A rapid-deployment tool for temporary occupancy or changing room layouts.
  • A foundation for community-level clean-air strategies, especially where budgets are constrained.
• As standards evolve, the key lesson is that multiple methods exist to achieve high clean-air delivery, and DIY solutions provide a scalable, economical approach that complements rather than replaces engineered systems.

How do we measure clean air output?

• Air purifier performance is typically measured using its Clean Air Delivery Rate (CADR), which reflects how much clean air the device supplies per minute. CADR is calculated by the rate of air passing through a filter (e.g. in cubic feet per minute, or CFM) multiplied by the percentage of particles that are removed when air passes through the filter, giving the rate of particle-free air that is supplied by the purifier. CADR values are standardized for different particle sizes and provide a practical basis for determining the size of the room a purifier can effectively serve.
• From CADR, we can estimate the air changes per hour (ACH) contributed by the purifier:
  • Air changes per hour is the number of times the air in a room is filtered each hour. ACH = CADR (in CFM) x 60 minutes / Room volume (in ft³)
• These values allow different devices—HEPA units, DIY CR boxes, or HVAC upgrades—to be compared on a consistent basis.
• While the precise long-term reduction in exposure to contaminants is difficult to quantify directly, ACH benchmarks provide practical guidance:
  • 6 ACH — historically used as a baseline for particulate pollution control; however, it is not calibrated for infection-control needs. 
  • 10–12 ACH — typical target for classrooms and many occupied spaces when aiming for robust contaminant reduction.
  • Higher ACH levels — may be appropriate for crowded areas, high-turnover spaces, or when rapid reduction of contaminants is needed.
• In practice, the goal is to ensure that the combined clean-air delivery—from HVAC, HEPA devices, and/or DIY units—meets the target ACH or equivalent clean air per person for the space.

What legislation regulates clean indoor air? What benchmarks should future clean air policies aim for?

• At present, only a few jurisdictions worldwide have legislation governing clean indoor air. Most indoor spaces are regulated only indirectly. Building codes set basic ventilation requirements, but these are generally aimed at comfort rather than contaminant reduction or infection control. ASHRAE standards provide guidance but carry no legal force unless a jurisdiction adopts them. OSHA regulates exposure to certain workplace chemicals, not overall ventilation or airborne pathogen control. Meanwhile, the Clean Air Act applies exclusively to outdoor air and does not regulate indoor air quality more broadly.
• Clean-air policy should establish clear performance-based standards for how much clean air a room must receive, through ventilation, filtration, or both. Regulations should include routine CO₂ monitoring as an operational check, aiming to keep indoor levels below 800 ppm. High-occupancy or higher-risk settings—such as schools, healthcare facilities, and public transportation—should meet stronger clean-air delivery benchmarks. Compliance pathways should be simple and affordable, recognizing both commercial and DIY filtration options, so that clean indoor air becomes a universal expectation rather than an optional upgrade.

What can we do as individuals to push for clean air in our communities?

• Start by considering the air in the spaces you use every day—your home, school, workplace, clinics, and community facilities. At home, look at what ventilation and filtration you already have and where it could be improved with better filters, portable purifiers, or increased airflow. Because outdoor ventilation is often limited by very hot or cold temperatures, it is especially important to augment ventilation with air filtration to maintain clean indoor air year-round.
• In shared spaces, identify the key contacts—such as building managers, principals, workplace safety officers, or facility directors—who can answer questions about ventilation and filtration. Asking how air is being cleaned, whether CO₂ levels are monitored, and whether improvements are planned can help raise awareness and encourage practical upgrades.
• Look for opportunities to collaborate with others in your community, whether through parent groups, neighborhood associations, workplace committees, or local health advocates. Working together can amplify visibility and make it easier to request improvements. You can also promote awareness among local policymakers—small outreach efforts, letters, or public comments can make a difference. Recent initiatives such as the London clean air campaign, in which the city committed to rolling out air filters to 200 schools as part of a major clean air effort, illustrate how coordinated community voices can drive meaningful policy action [9].
• Individual conversations, small requests, and clear information often play an important role in advancing clean air in the places where we spend our time.

Sources

1. American Lung Association. Current asthma by state, 2023. [cited 29 Dec 2025]. Available: https://www.lung.org/research/trends-in-lung-disease/asthma-trends-brief/data-tables/asthma-current-state

2. Fan Y, Cao X, Zhang J, Lai D, Pang L. Short-term exposure to indoor carbon dioxide and cognitive task performance: A systematic review and meta-analysis. Build Environ. 2023;237: 110331. doi:10.1016/j.buildenv.2023.110331

3. Silver M. Coronavirus FAQ: Is the 6-foot rule debunked? Or does distance still protect you? NPR. 23 Jun 2024. Available: https://www.npr.org/sections/goats-and-soda/2024/06/21/g-s1-5705/coronavirus-faw-if-youre-still-trying-to-stay-covid-safe-does-the-6-food-rule-matter. Accessed 29 Dec 2025.

4. Us Epa O. What are volatile organic compounds (VOCs)? In: US EPA [Internet]. 19 Feb 2019 [cited 29 Dec 2025]. Available: https://www.epa.gov/indoor-air-quality-iaq/what-are-volatile-organic-compounds-vocs

5. Filtration and Disinfection FAQ. [cited 29 Dec 2025]. Available: https://www.ashrae.org/technical-resources/filtration-and-disinfection-faq

6. High-efficiency filters and filter media for removing particles in air — Part 1: Classification, performance testing and marking. In: Svenska Institutet för Standarder [Internet]. [cited 29 Dec 2025]. Available: https://www.sis.se/api/document/preview/913826/

7. Fox J. HEPA Filters are NOT Needed. In: Medium [Internet]. 25 Feb 2024 [cited 29 Dec 2025]. Available: https://itsairborne.com/hepa-filters-are-not-needed-67e952792481

8. How to build your own Corsi-Rosenthal Box. In: Corsi-Rosenthal Foundation [Internet]. 11 Mar 2023 [cited 29 Dec 2025]. Available: https://corsirosenthalfoundation.org/instructions/

9. Cleaner air for London’s children as air filters to be rolled out to first 200 schools in the capital in ground-breaking initiative. In: London City Hall [Internet]. [cited 29 Dec 2025]. Available: https://www.london.gov.uk/media-centre/mayors-press-release/Cleaner_air_for_London’s_children_as_air_filters_to_be_rolled_out_to_first_200_schools_in_the_capital_in_ground_breaking_initiative