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Clean air, year round.

The Covid-19 pandemic was a wake-up call for everyone that breathes air indoors. We are all now aware that indoor air cannot be taken for granted and must be monitored and cleaned of indoor contaminants like viruses. The Lancet COVID-19 Commission is a group of scientists, doctors, and researchers working with the Lancet, a scientific journal that has focused on medical breakthroughs for the past 200 years. The commission is dedicated to understanding Covid-19, strengthening our defenses against it, and forging relationships between global health organizations to better prepare society for not just current and future pandemics, but also climate change and sustainable development. 

At the end of 2022 they published recommendations on how to better avoid infection by airborne respiratory diseases, and provided non-infectious air delivery rates, or NADR, for different settings. NADR is a new term coined specifically to deal with pandemics. You aren’t likely to find a NADR number on the side of any air purifiers or HVAC equipment any time soon, but that doesn’t mean we can’t look at the commission’s recommendations and learn a little about what these experts think is the best way to protect our air, and how to go about it.

Two large HVAC exhaust fans spinning

What is the best NADR?

The commission grades NADR in three levels based on the amount of virus-free air provided to a room and the people in it. This is very similar to CADR, or clean air delivery rate, used to gauge dust, smoke, and pollen capture for air purifiers.

NADR table for good, better, and best Clean air exchanges per room each hour, Cubic feet of clean air per person per minute, and Cubic feet of clean air per square foot per minute over ASHRAE minimum

So to score a “good” NADR, the commission recommends at least one of the following:

  • Four air exchanges per hour. This means a volume of non-infectious air equal to the whole room size is blown in every 15 minutes. To get an idea of what this amount of ventilation feels like, a stationary car with one open window exchanges air about 6.5 times per hour, but a room in a home with one open window adds somewhere in the neighborhood of 1 air change per hour, depending of course on the size of the window.
  • 21 cubic feet of clean air per person per minute. This depends on the occupancy of the room. 21 cubic feet per minute isn’t a huge amount of air, Molekule Air Mini+ puts that out about 4 times per minute at top speed, so would be able to meet the good level at top speed in a room with 4 people. With 2 people it wouldn’t need to be on top speed, and with 8 people at least 2 Mini+ purifiers would be needed.
  • 75 cubic feet of clean air per square foot per minute over the ASHRAE minimum. This one is a little harder to calculate because you need to know the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) minimum for the room, which is usually 0.35 air changes an hour for the whole room. For a small bedroom (10 feet by 10 feet by 10 feet), that ends up being 6 cubic feet per minute (0.35 times 1,000 cubic feet is 350, divided by 60 minutes to a rounded up quotient of 6), plus 75 cubic feet a minute (0.75 times 100 square feet) for a total of 81 or just a little bit less than Air Mini+ at full speed. 

While these are nice whole numbers, it’s not quite clear how to achieve these recommendations at home or in the office, or if clean air exchanges, clean air per person, or clean air per square foot is the most effective. The commission admits this is still under debate, but also points out that the current indoor clean air standards are not enough. 

To make matters even more confusing, there isn’t an agreed-upon industry standard for NADR testing, so it’s not like you can just buy a system that blows out a NADR of 35 and carry it around with you.

Woman coughing out simulated virus particles

You don’t need to count viruses to understand NADR

Most of us do not have the equipment or funding to capture pathogens from the air to know how many are present in real-time. Because carbon dioxide (CO2) always builds up in poorly ventilated and occupied rooms, which are the same places where viruses most commonly infect us, we can look for a level of CO2 that represents poor ventilation and recommend staying below that level. CO2 meters are relatively inexpensive, and are included along with some air purifiers, like the Molekule Air Pro.  All occupied rooms fill up with CO2 as people breathe, so rooms where pathogens are building up will also have a lot of CO2. To this end, the Covid-19 commission points to the Federal Public Service (FPS) Health, Food Chain Safety, and Environment of Belgium. The FPS requires all public spaces to have CO2 monitors, and sets the Code of Wellbeing at Work to the following CO2 levels:

  • 900 ppm for Standard A level


  • 1200 ppm for Standard B level 

In the absence of airborne virus sampling, CO2 can be a good stand-in. Keep in mind that many ways to reduce viruses will not change CO2 levels, as we will discuss next.

Woman changing Mini+ filter

Stack protective layers for a good NADR

The commission mentions three specific ways to decontaminate the air- ventilation, filtration, and disinfection. Each one comes with its own drawbacks and benefits, and not all can provide a measurable level of protection, instead offering a qualitative impact. Using more than one technique will accumulate NADR for a more effective total solution. 

How to ventilate for a good NADR

There are three types of ventilation, each one with a different level of possible control. Infiltration is air that seeps into a room through cracks and other tiny openings, and cannot be controlled easily. Natural ventilation refers to windows, doors, or elsewhere that air can get into a room, and can be turned on or off by opening or closing, but its speed can’t be controlled. Mechanical ventilation is HVAC systems and other powered devices that can be balanced against each other and offer full control of air moving through them. All of these types of ventilation will also lower CO2.

Mechanical ventilation is the best option to improve NADR, when it’s affordable. Upgrading HVAC systems can be prohibitively expensive or even impossible depending on what kind of space is available. Further investment in energy recovery systems like heat exchangers allow for the conservation of energy and money spent on heating, cooling, or humidity control.

Natural ventilation can be a good solution, but can vary considerably depending on wind and other weather conditions outside. Keep an eye on your CO2 meter to know if prevailing conditions have made the air more or less stagnant. 

How to filter for a good NADR

Air filters don’t move air on their own, so their performance is directly dependent on the fan or blower that powers them. The Covid-19 commission points out that many lower-efficiency filters are not effective in removing viruses from the air. They say that HEPA filters have a very high efficiency and will be best to achieve a higher NADR.

The best use of air filters is as a portable unit, like Molekule air purifiers or any other portable air purifier. This is because HVAC systems are rarely or never designed to clean the air. They are not active at all times and instead only kick on about half the time to maintain temperature. In addition, HVAC filters are often in place to protect machinery from being damaged by large particles, not to remove infectious biological agents. Filtration of dust, smoke, or pollen is usually an afterthought in an HVAC system and they often need to be retrofitted to use high-efficiency filters like HEPA.

The commission doesn’t address additional filtration technologies such as plasma, ionization, or PECO. However, if an additional technology has performance data showing that it can help to diminish a pathogen's ability to infect, then it will also help to improve the NADR.

There is no air purifier that can remove CO2, so a CO2 monitor will only be able to tell you that the room has a potential ventilation problem, not if or when the infection risk has been solved.

How to disinfect for a good NADR

There is really only one scientifically recognized method of disinfecting air, and that is with UV-C light. The most effective UV disinfection method is upper-room disinfection, where many lights are placed just below the ceiling to create a “disinfection zone” above everyone’s heads. There are also UV lights in HVAC ducts or in portable units, but the commission notes that the most important factor in UV disinfection is the amount of time a pathogen spends in front of the light. The air in ducts and portable units is moving quickly, which could present a challenge with exposure time. They think the best NADR comes from upper-room disinfection where the air isn't moving very fast.

Like purification, disinfection does not change CO2, which can only be relied on to tell if a room has insufficient ventilation. 

If the commission does find a way to determine the best way to suggest an acceptable NADR for everyone, it will involve passing laws and regulations for public buildings. In the meantime we can only do our best to optimize the NADR in our homes and offices. 

Please note, no air purifier can prevent transmission of a virus. Molekule recommends use of PPE and following directions of the CDC and other government authorities.

Header image by Monstera Production and image of fans from PictorialEvidence via Wikimedia Commons.

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