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Musings of an Energy Nerd

Ventilation Requirements in the Covid Era

Should we be increasing our ventilation rates?

A well-designed ventilation system introduces fresh outdoor air into a building at a known rate. The three major types of ventilation systems—exhaust-only systems, supply-only systems, and balanced systems—all involve the introduction of fresh outdoor air, although not all systems do a good job of distributing the fresh air to the various rooms in a building. Most researchers agree that adequate ventilation can help limit the spread of Covid-19 in indoor spaces. [Image credit: Fine Homebuilding]

So far, building scientists have not achieved consensus on the issue of the best ventilation rate for buildings. As readers of Green Building Advisor probably know, the issue of how much fresh air to introduce into office buildings, schools, and homes has been controversial for years. (For more on the controversy, see “How Much Fresh Air Does Your Home Need?” and “Ventilation Rates and Human Health.”)

Now that public health authorities and building managers are coping with a global Covid pandemic, the ventilation rate controversy has become even more complicated. In this article, I’ll do my best to outline the latest controversies and to provide some advice on ventilation rates in the age of Covid.

Why do we ventilate buildings?

The ventilation rates specified in current building standards (for example, the rates specified in ASHRAE 62.2, the most common residential ventilation standard referenced in the U.S.) are not based on reducing transmission rates of viral diseases; rather, current recommended rates are based on reducing indoor levels of carbon dioxide (and a few other known air pollutants) to acceptable levels, and to a lesser extent on reducing levels of human body odor.

Dr. Jeffrey Siegel is an ASHRAE Fellow and a professor of civil engineering at the University of Toronto. (ASHRAE is the American Society of Heating, Refrigeration, and Air-conditioning Engineers.) When I asked Dr. Siegel about the origins of the ventilation rates in current standards, he said, “I don’t think infectious disease entered into any discussions of ventilation systems, except in some medical environments.”

According to Dr. William Bahnfleth, a professor of architectural engineering at Pennsylvania State University and chair of ASHRAE’s epidemic task force, “The ventilation rates in part are intended to lower the levels of bioeffluent, or in lay terms, body odor, to the point that most people will think the air quality is acceptable. Epidemics were not really incorporated into the thinking.”

That said, there are good reasons to believe that ventilating indoor spaces with outdoor air reduces the transmission risk for viral diseases. According to Dr. Max Sherman, a senior scientist at Lawrence Berkeley National Laboratory and a recipient of ASHRAE’s Holladay Distinguished Fellow Award, “We know that if you increase ventilation, you will reduce the concentration of aqueous aerosols, and reducing the concentration reduces the risk.”

Transmission of Covid-19 happens through the air

During the early months of the Covid pandemic, public health officials weren’t sure of the most common ways that the virus was being transmitted. Was it mostly spread by touching surfaces? Or by inhalation of large respiratory droplets that were catapulted a short distance by an infected cougher? Or was it mostly spread by smaller airborne particles—particles small enough to float around and travel for a significant distance—via so-called aerosol or airborne transmission?

At first, the World Health Organization (WHO) downplayed the importance of airborne transmission. However, as evidence accumulated that airborne transmission is significant, WHO eventually changed its tune.

For more information on this contentious issue, see these articles:

At the risk of oversimplifying:

  • If transmission occurs by touching surfaces, the best prevention is handwashing.
  • If transmission occurs by inhalation of large droplets, the best prevention is social distancing.
  • If transmission occurs by inhalation of small aerosol particles, the best prevention is improved indoor ventilation.

The last two transmission methods can also be reduced by wearing a mask. Most experts believe that all three mechanisms play a roll in Covid-19 transmission; however, there is an ongoing debate about which mechanisms are most important.

Outdoors is safer than indoors

By now, most of us know that anyone worried about Covid-19 transmission should prefer outdoor spaces to indoor spaces. If you are thinking about holding a social event, you should therefore plan, weather permitting, to have the event outdoors. While it’s true that sunshine helps kill viruses, the main benefit of an outdoor location is all that fresh air—especially when there is a breeze that quickly moves virus particles away from infected people. These facts are relevant to anyone pondering the ventilation rate question.

Catching Covid-19 has much in common with dying in an earthquake: It mainly happens to people who are indoors, not people who are outdoors. Virtually all superspreading events occur indoors. As building scientist Joseph Lstiburek explains, “The safest place to be is on the beach.”

Why higher ventilation rates might be a good idea

A rising chorus of voices is now calling for higher ventilation rates. The main reason why ventilating indoor spaces may reduce viral transmission is because the introduction of fresh air dilutes the concentration of virus aerosols in indoor air. If you want the interior of your house to be safer, therefore, one approach is to leave one or more windows open—especially if the weather is mild.

Another approach is to make sure that your house has a ventilation system that introduces fresh outdoor air to the building. All well-designed ventilation systems—including supply ventilation systems, exhaust ventilation systems, and balanced ventilation systems—introduce fresh air into a home, although some systems do a better job of fresh air distribution than others.

If a house has a forced-air heating and cooling system that recirculates air and also includes an effective filter, then the filter can help reduce the viral load—especially if the filter is rated at MERV 13 or better.

So, what ventilation rate are we aiming for?

Many publications and web sites advise building owners to increase the building’s ventilation rate to reduce the risk of Covid transmission. For example, an ASHRAE website with Covid-19 advice for homeowners has a section with the heading, “Increase ventilation rate.”

Another example can be found in a resource published by the ASHRAE epidemic task force,  which notes, “Increased Ventilation: The Building Guidance clearly encourages building operators to increase their systems’ outdoor air ventilation to reduce the recirculation air back to the space.”

In spite of this chorus of voices calling for increased ventilation, building scientists and public health authorities aren’t yet sure what ventilation rate makes sense for facilities aiming to minimize Covid transmission.

When I spoke to Dr. Siegel, he noted, “Our evidence concerning how rates of Covid transmission are affected by ventilation rates is to my knowledge nonexistent. But we know that places where ventilation is poor is where we have seen transmission. Transmission rates of lots of other respiratory viruses have been affected by ventilation rates, so that is well demonstrated.”

Some authorities therefore advise that we should be increasing ventilation rates beyond the recommended rates outlined in today’s standards. But other authorities note that superspreading events aren’t being reported in buildings with well-designed and properly functioning ventilation systems, so existing ventilation standards may be just fine. Superspreading events happen in crowded poorly ventilated buildings. Had these buildings been equipped with a ventilation system that met minimum standards, viral transmission may not have occurred.

I asked Dr. Bahnfleth—who, like Dr. Sherman, is a recipient of ASHRAE’s Holladay Distinguished Fellow Award—whether we should increase the ventilation rate in our buildings beyond the levels recommended by existing standards. He responded, “That’s an unanswered question right now. If you have more outdoor air ventilation, you will reduce the indoor [virus] concentration. But do we know what the target ventilation should be? At this stage, no. We know that ventilation is important, so we need to have a minimal amount of ventilation. But the reason I say I’m not sure how far we should go is that we haven’t yet seen studies showing that there have been any superspreading events in well-ventilated spaces.”

When I asked Dr. Siegel about his advice to facilities managers at schools, he told me, “Increasing the ventilation rate should not just be done blindly. For schools, I would say increase ventilation for spaces that are poorly ventilated.”

Homes are different from schools

We know that adequate ventilation is especially important in buildings (like schools) where many people gather together. But what about homes? If you’re like the typical family, you’re not socializing indoors these days, and the people who live in your home are members of your “social bubble.” If you live in a single-family home, you’re probably not worried about Covid transmission from your spouse or children (although there are exceptions to this rule), which is why most of us don’t wear a mask when we’re home.

That said, if you live in a multifamily building, you may have to worry about encounters with neighbors in the lobby or elevator. That’s why those who live in multifamily buildings need to be more concerned about indoor ventilation than those living in a single-family home.

“If we’re talking about a detached single-family house, the only reason to increase ventilation is in a house where someone is sick,” Dr. Siegel told me. “In a multifamily building, it becomes more complicated. The kind of transmission we are seeing in multifamily buildings is when there is close contact in common areas. … It’s unconscionable to me that multifamily buildings don’t require masks in all common areas.”

When I suggested to Dr. Sherman that there weren’t many reasons to worry about viral transmission in a single-family home, he responded, “But what if the cleaning lady comes in to clean?” I must admit that I hadn’t considered the fact that some people hire professional cleaners—presumably, these cleaners might be either male or female—to work in their homes. If your family falls into that category, you should consider methods to reduce viral transmission on cleaning days.

The role of indoor relative humidity

Researchers note that those concerned with indoor viral transmission should strive to keep indoor relative humidity between 40% and 60%. Avoiding indoor relative humidity above 60% makes intuitive sense; after all, several studies have shown that damp buildings are associated with indoor mold and high rates of childhood asthma.

On the other end of the spectrum, it’s not clear at first why an indoor relative humidity level of 30% or 35% would be worse than 40%. Here’s the explanation: Airborne viruses are surrounded by a droplet of water. If the droplet is large, it soon sinks to the floor (in other words, it falls ballistically), ceasing to be airborne (and therefore ceasing to be inhalable). If the droplet is small, however, it stays suspended in the air for a long time—meaning that it can travel a long ways, and that it is more likely to be inhaled. When the indoor relative humidity (RH) is below 40%, the moisture in each droplet evaporates, turning large droplets into small droplets. Small droplets are more likely to lead to viral transmission than large droplets, so we don’t want the droplets to evaporate quickly.

[Author’s note: For more information on droplet evaporation, see Comment #2 at the bottom of this article.]

Maintaining indoor RH above 40% isn’t always easy, though, especially in cold climates during the winter, where ordinary building operation often results in an indoor RH of 30% or less. If a building owner decided to increase the ventilation rate above normal levels during the winter, in hopes of reducing the indoor viral level, it’s likely that indoor RH will drop further still. Moreover, trying to address low indoor RH with a humidifier can be risky, since humidifier operation is associated with condensation on windows or on cold surfaces in hidden sections of the thermal envelope. So what’s a building owner to do?

The consensus seems to be that trying to manipulate the indoor relative humidity should take a back seat to more important considerations like introducing outdoor air and installing MERV-13 filters. Dr. Bahnfleth told me, “There are certainly a lot of practical problems when trying to humidify a building to 40%. … It’s important to do things in a particular order, based on what has the most impact on your risk of being infected. I don’t think I would put humidification high on the list. It’s more important to lower the concentration of viruses in the air.”

Filtration is a good idea

The novel coronavirus is incredibly small—about 100 nanometers in diameter. So how can a furnace filter remove viruses from the air?

It turns out that we’re not really worried about so-called “naked viruses,” since naked viruses aren’t responsible for airborne transmission. We’re worried about viruses contained in small water droplets. These droplets are relatively easy to filter out of the air with a MERV-13 filter—so air filtration is an effective way to reduce airborne viral loads.

Dr. Sherman told me, “T­he coronaviruses in general don’t survive naked. They need a droplet around them. We don’t believe the naked virus is a big threat. The stuff wants to be in a small particle of sputum or water, an aerosol droplet, of at least 0.3 micron.”

Both Dr. Sherman and Dr. Bahnfleth believe that it makes sense to install a MERV-13 filter in your air handler, as long as the filter doesn’t introduce static pressure problems. Dr. Sherman noted, “If you look at the HVI database, where they list the initial pressure drop of filters, you can find lots of MERV-13 filters that have a lower pressure drop than some MERV-8 filters.”

A few fresh-air fanatics have wondered whether it makes sense to convert their forced-air systems to deliver 100% fresh outdoor air instead of mostly recirculated air. However, since you probably want your indoor air to be filtered, this is a bad idea. If you want your air handler filter to remove viruses from the indoor air, then you need to have recirculation.

Dr. Bahnfleth said, “In terms of HVAC, I think that one idea that concerns me is that ‘no recirculation is a good thing in all cases.’ In other words, going to 100% outside air mode. If you do that, you defeat the purpose of having filters in your air handler. High efficiency filtration with recirculation is actually a good thing.”

Which measures matter most?

Dr. Siegel knows what situations we need to avoid. “The number one way to describe spaces with high transmission rate of Covid is ‘crowded poorly ventilated spaces,’” he said. “So we should address crowding and ventilation.”

When it comes to schools, Dr. Siegel advised, “If money were no object, I would increase ventilation everywhere. If a school is nowhere near the requirements of ASHRAE 62.1, then meeting 62.1 is really more important than anything. My advice is to find the most poorly ventilated environment in the school and spend the dollars there.”

Dr. Siegel noted, however, that ventilation is a secondary measure. “For primary measures, the number one measure is using masks. Keeping people as far apart as possible is number two. Then come handwashing and surface cleaning to round out the primary measures. Ventilation is a secondary measure—it comes after all of the primary measures. … I put filtration firmly in the secondary category.”

Dr. Bahnfleth also had advice for facilities managers at schools. “The first thing to do [for schools] is to make sure that your systems work,” he said. “According to a GAO report on schools, 41% of U.S. school districts include schools with HVAC systems that are inadequate. So first, we need to get things working as they are supposed to work. After that, I would recommend filtration—MERV 13 when you can do it.”

If you’re looking at measures involving residential HVAC systems, here’s Dr. Sherman’s advice: “First, meet minimum ventilation requirements. The next step is filtration. If you’re having a party, you can always open a window.”

Fresh air is good

Many people, including my wife and I, like to leave their bedroom windows open wide when the weather is mild. Outdoor air has a refreshing smell and is probably beneficial to human health. The only downside to introducing large quantities of outdoor air into a modern building is the energy penalty associated with conditioning the outdoor air by heating it during the winter, cooling it during the summer, and dehumidifying it when necessary.

No building owner has an unlimited budget for energy, but many owners are willing to pay higher energy bills to reduce the chance that building occupants will catch a potentially fatal disease. As we await more data from researchers on Covid transmission mechanisms, many building owners will have to do their best at balancing their need to keep energy bills low with their desire to keep occupants safe.


Martin Holladay is a retired editor who lives in Vermont.


  1. vivian_girard | | #1

    Another very informative, rational and well edited column from this “retired” editor. Please come back on these pages as often as you can, Martin; it will keep your mind sharp and make the rest of us less ignorant. In the world of building science education and on GBA in particular, you are irreplaceable!

  2. GBA Editor
    Martin Holladay | | #2

    I just received an email from Dr. Max Sherman, one of the experts I interviewed for the article. In his email, Dr. Sherman provided further details on the way virus-containing droplets evaporate in low-humidity environments.

    He wrote, "I have learned a little more about the droplet evaporation issue, which is more complicated and more interesting than I thought. The virus-containing droplets are of course mostly water, but they have a few percent of proteins in them. While the water evaporates on the order of minutes, the hydrated proteins do not and form a nice nest to protect the virus. The aqueous droplets essentially turn into teeny snot balls something like a factor of 3 smaller in diameter. I am guessing those little balls may eventually dry out further, but this likely to be much slower and by then they would probably be stuck to something or the virus will have become inactive."

  3. Deleted | | #3


  4. burke1 | | #4

    We have a small office with carbon dioxide controlled ventilation through a cross flow air-to-air heat exchanger (hydronic heating, mini-split cooling). We normally maintain CO2 between 900 and 1000 ppm. We are now maintaining it at 600 to 800 ppm, given a lack of any specific guidance. In conjunction with office dividers, masks required, and having in-person meetings outside, I hope that improves the odds for not spreading Covid 19.

  5. 808Dave | | #5

    Are there more details available concerning the air-filtration in at least one recent study, which found infectious Covid-19 virus well outside the mindlessly-enshrined 6' separation distance, in a hospital room housing a Covid patient? "The room had six air changes per hour and was fitted with efficient filters, ultraviolet irradiation and other safety measures to inactivate the virus before the air was reintroduced into the room." See

  6. GBA Editor
    Martin Holladay | | #6

    Here is the link to the actual paper: "Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients."

    Q. "Are there more details available concerning the air-filtration in at least one recent study?"

    A. Yes, the paper has a few more details than the New York Times article. Here's what the authors of the paper wrote: "The room 133 had six air changes per hour and the exhaust air underwent triple filter treatment (minimum efficiency 134 reporting value [MERV] 14, 75%-85% efficiency for 0.3 μm particles), coil condensation (to remove 135 moisture), and UV-C irradiation prior to recycling 90% of the treated air back to the room."

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