Air Changes Per Hour is a Good Start. Clean Air Per Person Is the Metric We Like Better (and why you should too)

Air changes per hour help describe a room. Clean air per person describes a gathering. Here’s how we think about it for accessible event planning.

WHAT YOU NEED TO KNOW

• Air changes per hour, or ACH, tells you something useful about a room: how quickly air is replaced or cleaned.

• But ACH doesn’t tell you the whole story about the clean air in the room.

• For events and shared spaces, we think metrics that describe clean air per person are better starting points for accessible event planning.

• The basic idea is simple: take the total amount of clean air coming from ventilation, air filtration, and other tools, then divide it by the number of people sharing the space.

• For Positive Deviance NYC, clean air is not a substitute for masks, testing, or staying home when sick. It is one layer of a better indoor culture. We use portable HEPA purifiers, Far-UVC where appropriate, occupancy planning, and KN95+ masking together because each layer helps reduce a different part of the risk.

• We also want clean air to be easier to understand and easier to access. Later in this piece, we share how we think about choosing air purifiers, including why we use Smart Air Blast purifiers at home and at events.

• And if you are already planning to buy from Smart Air, our code PDNYC10 gives you 10% off and sends 10% back to Positive Deviance NYC to support our clean-air work.

At Positive Deviance NYC, we spend a lot of time thinking about air as infrastructure.

When we share indoor air, we also share what’s in that air. Respiratory pathogens like flu, RSV, tuberculosis, COVID and other coronaviruses spreads through aerosols. Those aerosols can be released by breathing, talking, singing, or coughing, and the risk rises with time spent indoors breathing contaminated air.

But when people ask, “How good is the air in this room?” the answer often starts and ends with one familiar number: air changes per hour, or ACH.

We think ACH is useful. But for accessible event planning, we should also ask a more specific question:

How much clean air is actually available when you take people into account?

That question is often useful for events, classrooms, offices, restaurants, rehearsal rooms, studios, and social spaces. It helps us plan around both rooms and the people in them.

What ACH tells us

Air changes per hour tells us how many times the air in a room is replaced or cleaned in one hour.

The basic formula is: ACH = clean air delivery rate ÷ room volume

If a room has a known volume and a known amount of clean air flowing through it, ACH can help estimate how fast airborne particles will clear after a source leaves. That matters in hospitals and other settings where you need to know how long to wait before re-entering or reusing a room.

ACH is also easy to picture. Six ACH sounds better than three ACH. Twelve ACH sounds better than six.

But ACH has an important limitation: people. More people means more breathing, more talking, more shared air and so more possible infectious sources. And a room can have the same ACH with four people or fifty people. The ACH may not change, but the risk profile does.

The problem with ACH

Imagine two rooms.

Room A is small and packed with 12 air changes per hour.

Room B is larger, with the same clean air delivery and fewer people.

Room A may have a higher air changes per hour because its volume is smaller. But that does not make it lower risk. It may have more crowding, and therefore less clean air per person.

This is why ACH can mislead us. It can make small, crowded spaces appear safer than they are. It can make large, high-ceiling spaces look worse than they are.

ACH describes the room. Clean air per person describes the gathering.

The basic formula is:

Clean air per person = total clean airflow ÷ number of people

That clean airflow can (and should!) come from multiple places:

• Outdoor air from ventilation.

• Filtered air from HVAC filters or portable HEPA purifiers.

• Inactivation from properly designed germicidal UV systems, including Far-UVC, when it can be safely deployed.

• Occupancy control, because fewer people means more clean air per person.

Typically, the concentration of airborne pathogens, and therefore inhaled dose, is inversely proportional to equivalent ventilation rate. That is why clean air per person is a more complete infection-control metric than air changes alone.

What air purifiers add to the equation

At Positive Deviance NYC, one of the main tools we use to keep the air clean is portable HEPA filtration.

We use Smart Air Blast purifiers because one unit can add a large amount of clean air at a relatively quiet rate. A single unit adds a lot of clean air to a room while staying quiet enough to talk over. The number we actually care about is its clean-air delivery rate — how much fresh, filtered air it adds each hour.

But that total isn’t the whole story. What matters at an event is clean air per person, and the math is simple:

Clean air per person = total clean air ÷ number of people

Let’s assume that one Blast in a room of 10 gives each person a generous share of about 95 cubic meters per hour each. Put that same machine in a room of 50 and everyone’s share drops to about 19 — same purifier, very different level of protection.

That’s why “clean air per person” is the number worth thinking in. A single purifier can be plenty or nowhere near enough depending on how many people share the air, how long they stay, what they’re doing (talking and singing release more virus than sitting quietly), what other ventilation the room has, and what other protections are layered in.

If the expected crowd doubles, the clean air per person is cut in half. If the crowd is smaller, each person gets more clean air.

In other words, ACH tells us something about the room. Clean air per person tells us something about the gathering. The American Society of Heating, Refrigerating and Air-Conditioning Engineers, or ASHRAE, uses a related concept: equivalent clean airflow rate per person, or ECAi. That clean airflow can come from outdoor air, filtered recirculated air, and pathogen inactivation technologies.

Where Far-UVC fits

HEPA and other filters remove pathogens. Far-UVC inactivates pathogens.

A HEPA purifier pulls air through a filter and removes particles from the room air. Far-UVC uses light, typically around 222 nm, to inactivate airborne pathogens. Both can reduce the amount of infectious aerosol in a room, but they work in different ways.

We use Far-UVC as a tool on top of HEPA filtration, not a substitute for it.

The efficacy of the lamp depends on the device, the spectrum, the optical filtering, the radiant power, the placement, the ceiling height, the dose distribution, the air mixing, the pathogen, and the room geometry. We also recommend ensuring ventilation is sufficient and working before adding Far-UVC.

That being said, the strongest peer-reviewed benchmark was a room-sized chamber study: five filtered 222 nm sources reduced an aerosolized pathogen by 98.4% and added 184 equivalent air changes per hour in a 32 m³ chamber at 3 ACH. By our math, that translates to about 1,636 L/s of equivalent clean-air delivery, or about 327 L/s per device in that specific setup.

A later occupied-room study using four ceiling-mounted 222 nm fixtures found a 99.8% reduction in infectious airborne murine norovirus in an active animal-care room while operating within current recommended regulatory limits. The authors caution that airflow patterns, residence time, and dose distribution add uncertainty, which is why we avoid overclaiming for our setup. For example, in reviews of room-sized bioaerosol chamber studies, reported far-UVC performance ranged from about 4–7 L/s per device at the low end to 327 L/s per device. That is not a small spread. It means device output, spectrum, placement, beam spread, and air mixing meaningfully drive the result. In other words, far-UVC can be powerful, but the real impact is installation-specific.

At Positive Deviance NYC, we treat Far-UVC as an added engineering layer because we understand the limitations of the current tech and the marketing surrounding it. It can help clean the air continuously and quietly, especially in indoor settings. But we still start with the basics: reduce risk at the source, filter the air, improve ventilation where possible, manage occupancy, and keep high-quality masks available.

Planning a Far-UVC setup? OSLUV offers a free germicidal UV design tool, Illuminate, that can help you estimate how much Far-UVC to use and where to place fixtures in a specific space: https://illuminate.osluv.org/

Where masks fit

The science is clear that respirator masks work. When correctly and consistently worn, masks reduce respiratory transmissions. Period. High-quality respirators work both by reducing what an infectious person might emit and reduce what a susceptible person might inhale.

For Positive Deviance NYC, KN95+ masking serves as source control, personal protection, and access infrastructure.

Clean air tech like Far UVC and HEPA filters reduce the room’s shared burden. Masks reduce exposure at the source. The combination is stronger than either layer alone. The combination is even stronger if we add high-quality testing to the mix.

What clean air per person does not solve

Clean air per person is not a guarantee. But it gives organizers a clearer starting point than ACH alone. It helps us compare both spaces and configurations. It helps us plan gear and make informed choices about reducing risk.

Clean air per person shifts the question to “What are we providing for the people inside the room?”

Want to improve your own air?

Clean air should not be reserved for people with the time to become amateur HVAC engineers.

Portable HEPA filtration is one of the most practical ways to add clean air to your home, studio, office, rehearsal, classroom, or gathering space.

At Positive Deviance NYC, we use Smart Air Blast purifiers in our home and at our events because they move a lot of filtered air and can run quietly enough for music, conversation, and life to go on. When comparing air purifiers, look for three things together: clean air delivery rate, noise at the speed you will actually use, and the real cost of filters over time. Dr. Marwa Zaatari’s air cleaner comparison chart is a helpful place to start because it lets you see how different purifiers compare across tradeoffs: more clean air is better, quieter is better, and cheaper is better.

As you’ll see, the “best” purifier is not always the biggest or most expensive one. A Corsi-Rosenthal box or smaller HEPA unit may be a great fit for a bedroom, office, classroom corner, or rehearsal room, while a high-capacity unit may make more sense for larger gatherings where many people are sharing the same air. If you’re ever on the lookout for an air purifier, we’re happy to help you choose a purifier you will actually run and size it for the people in the room. If you’re just trying them out in the NYC area, you can take a look at our rental options.

For more permanent solutions, you should feel free to use the code PDNYC10 for 10% off your Smart Air order. Smart Air will also contribute 10% to Positive Deviance NYC and so when you use our Smart Air code, you help build the next phase of our work. Every use of PDNYC10 helps us build a space where clean air, masks, access, and care are the norm.

That support goes toward our campaign to create a permanent COVID-conscious space in NYC: a home for mask-positive events, classes, co-working, family programming, and community gatherings.

If this piece helped you think differently about what safer indoor culture could look like, please support Positive Deviance NYC’s campaign to build a permanent COVID-conscious space in New York: a home for mask-positive events, classes, co-working, family programming, art, music, meals, and care. Become a member, make a donation, or gift a membership:

Every recurring member helps us move from carrying clean-air infrastructure from room to room toward building a space where clean air, access, and care are built in from day one.