Indoor air quality (IAQ) is critical during the COVID-19 pandemic. With all of the options to improve indoor air quality, we break down the costs and benefits of each. 

A little history: during the Bubonic Plague of the 14th century, people believed that foul-smelling air caused infection with the “Black Death”. They were wrong – the plague was spread between rats and humans through flea bites. However, in spite of this error, some of the technologies employed during the Bubonic Plague to attempt to improve air quality helped to reduce transmission through flea bites: vacations in the countryside, walling up quarantined homes, or covering oneself from head to toe in waxed leather and a beaked mask full of flowers (like the plague doctors did). 

Plague Doctor

During the “Spanish Flu” of 1918, policies were put in place for isolation of infected individuals, quarantining of those exposed, and social distancing measures. The Fresh Air Movement encouraged people to be outside and leave windows in their homes and businesses wide open to bring in fresh air. In northern cities, many old buildings still have steam radiators under the windows designed to warm air coming in from wide open windows during the dead of winter. Unfortunately, the coal smog in the air of those cities didn’t help those struggling to breathe with flu-infected lungs. 

radiator to warm air from ventilation (IAQ improvement)

I think we can safely say that with the possible exception of those two historical periodsthe world has never been as concerned about indoor air quality (IAQ) as it is now, just over 1 year into the COVID-19 pandemic. From all of the infection prevention safety assessments I’ve conducted in Chicagoland schools and businesses during the pandemic, and all of the research into IAQ products I’ve doneI’ve seen a huge range of ways to try improving the air in buildings. 


We got it right in 1918 with the opening of windows. Opening windows in public spaces helps to replace inside air with outside air without having to purchase any new equipment. Fans can help facilitate this ventilation. Many building managers essentially achieve the same effect by increasing the outdoor air intake in their HVAC (Heating, Ventilation, and Air Conditioning) systems. In any case, outdoor air should have much lower concentrations of any airborne pathogens than indoor air has. The problems we face are that in most places, outdoor air isn’t usually the temperature we’d like it to be indoors, and outdoor air can be heavily polluted (especially in or near large cities), contributing to allergy and respiratory issues. HVAC systems can heat or cool the incoming outside air ($$$) to a degree, and can help to filter out some pollutants. 

HVAC Filtration 

HVAC systems have built-in filters that filter out some particles coming into the building, as well as filtering out some particles in room-to-room recirculation of air within a building. These filters are rated using the MERV (Minimum Efficiency Reporting Value) scale. MERV 8 filters are the most common in the US. This filter rating is efficient at capturing mold spores, pollen, and dust without restricting airflow to an extreme degree. During the coronavirus pandemic, many buildings are upgrading to MERV 13-16 filters, which can filter a majority of particles down to 0.3 micronsThat captures a lot of bacteria and respiratory aerosols. Not every HVAC system can handle that sort of added airflow restriction, so upgrading MERV ratings can mean buying more powerful blowers for a building. The monetary cost of this upgrade comes in three ways: the initial investment in blowers, the more costly filters, and the added electrical consumption to move the same volume of air. There are many situations where those added costs are justified for the improved IAQ. However, when it comes to the spread of airborne pathogens, within-room transmission appears to be far more common than room-to-room transmission (1). 

MERV Filter breakdown to understand impact on IAQ 

HEPA Filtration 

Air filters better than MERV 16 are HEPA filters. True HEPA filters are rated to filter 0.3 micron particles with at least 99.97% efficiency, and can effectively capture particles as small as viruses. Because of their large airflow restriction, HEPA filters are not generally used in HVAC systems except in extreme scenarios (such as clean rooms for working with sensitive materials)Despite that, the pandemic has created unprecedented record demand for HEPA filters, primarily in the form of portable HEPA air purifiers for removing germs from the air within a roomWhen strategically placed in areas of high use, these devices can reduce the concentration of virus in the air within a single room, where transmission is most likely. 

Carbon Filtration 

Another popular filter type for portable air purifiers is the carbon filter. Carbon filters aren’t particularly great at removing small particles like viruses, but they’re good at adsorbing (collecting gas and liquid vapor molecules on the surface of the carbon granules). For this reason, the Bubonic Plague sufferers would’ve loved them for their odor-removing powers. These days, carbon filters can help to remove smog from the extra outside air being brought into a building. Since they tend to come standard with portable HEPA air purifiers, many models were on backorder last summer and fall as westerners bought them to reduce smoke inhalation during the wildfire season, while doubling as COVID protection. 

UV Sanitation 

Ultraviolet light has some great uses in sanitation. When you can clear a room and block any windows and doors, flooding the room for long periods of time with high powered UV lights will do a decent job of killing most of the germs that the light touches on surfaces and in the air. Where people get conned, though, is through the lack of a simple rating system for UV devices. Effective UV sanitation is all about bright light, close up, for a long contact time. When I’m done working with dirty microbiology samples in my lab’s biosafety cabinet, I clean it, close it up, set the large built-in UV light on a 30 minute timer, and leave. Most UV sanitation devices for sale to the public these days are weak (for safety reasons), and rarely give clear guidance on distance and time for decent sanitation. UV devices in HVAC systems or portable air purifiers often don’t get enough contact time on the air that rushes past them to do very much sanitation. Another potential concern with UV is that the same energy that destroys germs can cause the formation of trace amounts of ozone. Ozone build-up in a building can cause respiratory irritation. Good building circulation and specially designed UV bulbs can minimize this risk. 


Ionizing air purifiers are another common product marketed against airborne pathogens like coronavirus. Like UV, ionization creates ozone while fighting germs. The difference being that ionizers work largely by creating charged oxygen molecules. Charged oxygen molecules are vergood at breaking down all sorts of substances in the air, but they are also very good at becoming ozone molecules. That’s perfectly fine as long as people aren’t also in the room. There are all sorts of designs of ionizers and plasma generators with different tricks to try to reduce the amount of ozone generated while still providing some degree of air purification. Shoppers should carefully check the claims on these units. Ionizers also tend to be marketed as quiet, which typically means they aren’t purifying much air volume. 

So, What Should I Do About IAQ? 

When it comes to reducing airborne transmission of pathogens like coronavirus, we have to take a practical approach that looks at the costs and benefits of various actions: 

Cut off germs at the source (masks, social distancing) – biggest effect/lowest cost 

Ventilation (more outside air through windows and HVAC) – big effect/cost varies 

Remove germs through filtration: Upgrade HVAC (room-to-room transmission) – small effect/cost varies 

Remove germs through filtration: Use portable HEPA (within-room transmission) – big effect/moderate cost 

UV & air ionizing – effectiveness varies by product and application/moderate cost 

An organization needs to consider their budget and the particulars of their space, seek expert advice when needed, and be proactive and practical. Air testing can even validate that implemented measures are working. It’s a great idea to invest in improving indoor air quality. It makes people feel safe, and when it’s done wellthey can actually be safe, too.



1) CDC Guidelines for Environmental Infection Control, Background C. Air (2003) 

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