A Microbiologist’s Guide to Reducing the Risk of COVID-19 if You Absolutely Must Gather Indoors

How to use ventilation, air filters, and climate control to reduce coronavirus transmission this holiday season

Adrien Burch, PhD
Nov 19 · 14 min read
Table with winter holiday decorations, a bottle of hand sanitizer, and a mask.
Table with winter holiday decorations, a bottle of hand sanitizer, and a mask.
Image credit: Helin Loik-Tomson.

As we’re driven indoors by chilly winds and shorter days, Covid-19 cases are continuing to rise. Most of us will be spending the holidays alone this year, in group Zoom calls, or with the same family members we already share our daily lives with.

However, some of us have circumstances that require us to risk exposure to the virus. If you fall into this category, you can minimize your risk by thinking about the safety of your air.

You’ve probably already heard that it’s important to increase air circulation when you're indoors. But beyond the suggestions that you upgrade to the best HVAC filter and throw open every door and window, there isn’t much useful advice out there.

Please don’t mistake this article as an encouragement to spend time with others indoors during a pandemic. It isn’t. It is intended to help those of us who must do it despite the danger. And it’s not just about the holidays; these tips can also help people who can’t work remotely, who live with reckless roommates, or whose family members have been exposed to the virus.

I’m a microbiologist, and this is my advice on how to make indoor air safer through ventilation, air filtration, and humidification—without having to renovate your house or live in a hazmat suit until April. It’s based on guidance from the CDC, ASHRAE, EPA, and recent Covid-19 research. You’ll find links to the original sources throughout this article, which I highly recommend checking out.

Why Covid-19 Spreads More Indoors

The CDC and most health authorities recognize that household gatherings are a serious contributor to how people catch this virus. That’s why local health agencies often place limits on indoor gatherings when they’re trying to flatten the curve. This also means that before you invite anyone over, you should always double-check their websites for up-to-date guidance and new legal restrictions related to gatherings — things are changing all the time.

On a microscopic level, Covid and other respiratory diseases spread more indoors than they do outside. This is due to the droplets leaving a person’s nose and mouth, as well as how long the droplets stay in the air and where they eventually settle. When we’re outside, these droplets settle on the grass and sidewalks or float away in the breeze. When we’re inside, those droplets settle on our furniture and food—and build up in the air we breathe.

We also tend to engage in riskier activities when we are indoors. Things like cooking together in a small kitchen, chatting for hours over drinks, and laughing together while watching a movie on the couch. Heartwarming, maybe… risky, definitely. Inside, we’re just far more likely to inhale what others have exhaled.

Treat Indoor Private Spaces Like Public Spaces

When socializing indoors, the most important first step is to continue following the same safety measures you have been following when you go out in public. Since there are already so many other resources that outline these tips and tricks — including the CDC’s guide for holiday celebrations. I won’t list them all, but here’s a highlight of some of the most important ones:

  • Wear a mask.
  • Wash your hands frequently.
  • Skip the hugs.
  • The fewer people in the gathering, the better.
  • Don’t invite people who are currently Covid-19 positive or who may have been recently exposed.
  • Try to avoid eating, drinking, or any other activities that require removing masks.
  • Keep your distance when you need to temporarily remove your mask for eating or drinking. Sitting together at one table probably isn’t a good idea, either.
  • If seated together, maintain a distance of six feet apart by measuring between plates — which correspond to people’s mouths — not chairs.
  • Let the most sanitary person mask up, wash their hands, and serve everyone’s drinks and meals in clean, single-use dishware.
  • Disinfect surfaces before and after the gathering.

When people carefully follow the rules, they can increase their protection from viruses on surfaces — and also get partial protection against Covid-19 in the air.

Notice that I said “partial protection.” As the amount of virus particles builds in the air from multiple people, protective measures become less and less effective. Think of it like sandbags in a flood: They will help, but if the water eventually gets too deep, no amount of sandbags will do any good.

Why Droplets Are Such a Big Threat Indoors

When you spend time around someone who’s infected, there’s always the risk of inhaling airborne coronavirus. That threat grows bigger and bigger indoors if you’re not keeping the air clean.

If everyone wears a mask, a large portion of the droplets will be trapped in the masks before they can accumulate in the air. Research shows that simple surgical masks can reduce the total exhaled viruses in large droplets by 96% and in aerosols by 64%.

Although many masks do a great job at filtering larger droplets, smaller droplets and aerosols — the smallest of droplets — can more easily slip through. In fact, based on the above data, one out of three aerosols will likely slip past a surgical mask.

Unfortunately, the smaller the droplet, the longer it lingers in the air. Once a droplet is in the aerosol size category, it can float in the air for hours or even days before settling to the ground.

Thankfully, 99.99% of the liquid leaving our mouths during a typical cough will be in a droplet larger than an aerosol. But a huge problem in the wintertime — when the heater is on and the humidity is low — is that water evaporates more quickly. That means larger droplets leaving an unmasked mouth can evaporate and shrink before they hit the ground. This leaves them floating in the air for a longer period and increases the likelihood that we will inhale them. To summarize:

  • The smaller the droplet, the more likely it will be able to slip past most masks.
  • The smaller the droplet, the longer it will stay in the air and build up to unsafe concentrations.
  • The smaller the droplet and the longer it stays in the air, the further it can travel — well beyond six feet.

All this means that without a way to constantly clean those smaller droplets and aerosols out of the air — even if you are wearing a mask and staying six feet apart — there might not be enough protection indoors.

How to Keep Airborne Coronavirus Out of the Air

Beyond masks, there are a few different ways you can help make indoor air safer, which I’ll explain in more detail in their own sections below. According to ASHRAE, these are the steps you can easily take:

  • Ventilate with fresh outdoor air.
  • Upgrade the filter on your HVAC system and keep it running.
  • Run a portable HEPA air purifier.
  • Keep indoor temperature and humidity in a healthy range.
  • UV-C irradiation might also help, but the ones that can irradiate a significant volume of air in a room — for example, medical-grade upper-room UVGI devices — are out of most people’s price range.

None of these strategies are perfect, so you will probably want to try a combination of the choices available to you. It might seem wasteful to open a window when the heater is running, but when it comes to cleaning viruses out of the air, the two can work together.

How Much Clean Air Do You Need?

Unfortunately, nobody is going to give you a straight answer to this question because it depends on too many different factors. For instance, when there are more people in a room, or it’s a smaller room, it means that you need to replace the room’s air more often because the exhaled air will build up quickly.

When you ventilate or filter air, it’s often measured in “air changes per hour,” or “ACH.” So if you’re in a room that’s, say, 10 feet high, 12 feet wide, and 8 feet tall — 1,000 cubic feet in volume — then for every 1,000 cubic feet of air that gets filtered or ventilated, that’s equal to one air change. Air changes can be added together, which is why both ventilation and air filtration are often recommended to be used simultaneously during this pandemic.

At the bare minimum, public spaces aim for at least four ACH. Crowded bars and nightclubs aim for 20 to 30. In hospitals, the air change recommendation for patient isolation rooms is 12. On the other hand, a well-insulated house without the HVAC system running can have an ACH as low as 0.35.

You might think that one air change sounds good enough, but because of the way the math works out, one air change only reduces viruses in the air by about two-thirds.

To demonstrate this, let’s use this example: Let’s say someone pours you a cup of whisky, and every time you take a sip, they top your cup off with water. After you’ve consumed a cup’s worth of liquid, you’ll still be left with about a third of a cup of whisky in your cup mixed with two-thirds water. No matter how much you drink, as long as they keep topping it off, there will still be at least a little whisky in your water.

A common household goal is around five ACH. At that rate, you can clear about 99% of whatever’s in the air in about an hour. Give it two hours, and that will be above 99.9%. But that’s only after the sick person has left the room. While they’re still in the room and exhaling, it’s uncertain how high the levels of airborne Covid will be — except that it would be much worse if you weren’t actively clearing the air.

This is why there is no perfect answer to how much air exchange you need, other than more is better. This is also why it’s recommended to start filtering and/or ventilating the air for two hours before the gathering and keep it running at least two hours afterward.

How to Ventilate a Space

You might already know you need to ventilate, but how do you do it and how do you know if it’s working?

Opening up a window is a good place to start. One household study found that even a slightly open window in the bedroom can increase the air change in that room by 400%. However, with only one open window, you’ll probably still be well below the air change levels you’re aiming for. Here are some ways to help further increase your air changes:

  • When opening more windows, cross-ventilation, or opening up windows on opposite sides of the room/house, is more effective than the same side.
  • Opening another window at a different height (either on different floors, or the top/bottom panels of two different windows) can increase air movement.
  • If your HVAC system is set up for it — most houses aren’t — open the outdoor air intake.
  • Keep the bathroom exhaust fan running.
  • You can also run the kitchen exhaust or any other exhaust systems.
  • Air change naturally increases when it’s windy outside.
  • Air change naturally increases when it gets colder outside.
  • Use fans.

Use Fans to Increase Ventilation

If you use fans for ventilation, they work best for speeding up the direction that air naturally wants to flow. Before placing a fan, use a feather, smoke from a match, or just general common sense to figure out which windows air naturally blows in and out from.

Personally, I prefer to place fans blowing out of windows to enhance ventilation. That way, a more gentle flow of air can come in through the other open windows, and it won’t create as much chaotic air movement in the room, which could be potentially dangerous.

Whenever you have air moving in from one direction and out the other, the flow of air could either carry droplets away from you — or it might, unfortunately, bring them straight at you. When you use a fan to speed up the movement of air, it can also speed up the positive — or negative — benefits of the airflow.

It’s best to keep that in mind when choosing where to seat your guests. Don’t place people near fans or air ducts so that air blows directly between them, or at least make sure that vulnerable guests are in a safer position. Also, if you’re running an overhead fan, make sure it draws the air upwards instead of pushing it downwards, which will just blow air between people.

Another thing to keep in mind is that it’s not always recommended to ventilate. For instance, if someone in another part of your household is sick, or you’re in an apartment building with a shared hallway, then exhausting air out of an open window might start drawing in contaminated air from outside your safety bubble. If someone in your household or building has Covid-19, make sure that the airflow isn’t moving from their space into yours.

Use a Carbon Dioxide Meter to Gauge Your Ventilation

In an ideal pandemic, anybody who invites guests over would keep all their windows open. But it’s cold outside and heating is expensive, so most of us would prefer to leave as many windows closed as possible. It’s not a perfect solution, but you can check how well your ventilation is working with an inexpensive carbon dioxide meter.

Earth’s air currently contains around 0.04% (400 ppm) carbon dioxide. This gas is naturally exhaled by every human and pet in your household and will start to build up in an enclosed space. Although NIOSH defines any room that reaches above 1,000 ppm as “inadequately ventilated,” classrooms, closed bedrooms, and many other indoor environments regularly exceed 2,000 ppm.

Here’s a different way of interpreting these numbers, based on average exhales containing 38,000 ppm:

  • 400 ppm carbon dioxide = 100% fresh air
  • 780 ppm = 1% recently exhaled air
  • 1,000 ppm = 1.6% recently exhaled air
  • 2,000 ppm = 4.2% recently exhaled air

Personally, I prefer to keep my inhales at less than 1% of someone else’s exhale, but it’s up to you to choose your comfort level. In previous studies of tuberculosis, another disease that can easily spread through the air, keeping carbon dioxide levels in classrooms below 1,000 ppm was recommended for lowering the risk of contagion.

If you decide to use a CO2 meter, keep in mind that indoor combustion using a gas stove can raise indoor carbon dioxide levels, while filtration devices can help clean the air without lowering carbon dioxide levels. And in case you were wondering, houseplants won’t make much of a dent in either direction.

Filtering Air With an HVAC System

Your HVAC system is probably equipped with a filter that you’re supposed to change out regularly. Normally, these filters just screen out large particles, but if you buy a more expensive filter designed to trap smoke and viruses, your HVAC system can do some of the work filtering your indoor air.

Ideally, you would use a filter rated MERV 13 or higher. However, the fancier the filter, the harder your HVAC system has to work to pull air through it. Before changing your filter, double-check your HVAC system’s specs, and only use the highest rated filter it can handle.

If you’re able to upgrade to a good HVAC filter, set it to circulate constantly. If you’re the math-y type and really want to dive deep by figuring out how many air changes your HVAC system can add to your coronavirus protection, here’s how to do it:

  1. Multiply your house’s livable square footage by the height of your ceilings (typically around eight feet) to calculate its cubic volume.
  2. Look up your HVAC system, and check what the “maximum airflow” is, which will be written as CFM (cubic feet per minute).
  3. Multiply your HVAC’s CFM by 60 to figure out how many cubic feet it can filter per hour.
  4. Then divide that number by your house’s cubic volume — which will tell you how many ACH your HVAC system can contribute.

I don’t blame you if you’re totally uninterested in doing this calculation. My guess is that it will be somewhere between two and ten.

Filter Air With Portable Air Filters

Another decent option, especially for those of us without central air, is to use a portable HEPA filter to clean the indoor air. HEPA filters are rated to capture minuscule particles similar to the MERV 13+ filters, and they often list how many air changes they can perform for specific room sizes. Alternatively, they’ll list the CADR (clean air delivery rate) — which is how many cubic feet or cubic meters per minute it can clean. If comparing two options, be sure you’re double-checking that the units you’re using are the same: cubic feet or cubic meters.

Here’s how to interpret a filter’s CADR: If the box says “CADR of 140, covering a 300-square-foot room in 17 minutes,” it means it can complete one air change in a medium-sized room in 17 minutes. The manufacturer assumes an average room height of 8 feet, so 300 X 8 = 2,400 cubic feet. Divide that by how many cubic feet per minute it can filter (140, the listed CADR), and you get approximately 17 minutes. And, as we already covered, this means that after 17 minutes there will still be around a third of the viruses or other contaminants in the air.

I know this math is a pain, but on the plus side, if you made it this far in the article, you’re better able to appreciate the work of certified HVAC technicians!

Try keeping portable air purifiers near the center of the action, not tucked away in a corner. However, something to keep in mind with air purifiers is that the air cleaning rating is only true if the air purifier is operating at full blast. Most of us are sensitive to loud noises, especially when we are trying to chat with guests — but if you turn your filter down to a quieter setting, you won’t be getting nearly as much coverage.

Air purifiers that include extra perks like UV lights haven’t been demonstrated to be any more effective, so they’re not generally recommended over a standard HEPA filter.

Control Airborne Covid Through Temperature and Humidity

Although it’s not difficult to convince most of us to turn up the heat when it’s cold outside, many of us ignore the importance of humidity… and suffer through static cling and dry noses all winter long.

However, there are a number of reasons why you probably want to keep your indoor environment at a cozy temperature and the room humidity between 40% to 60%. These are some of the top theories that scientists think makes viruses like the flu more contagious in the wintertime.

The colder it gets outside, the harder it can be to keep it warm and humid inside. Many houses regularly hit 20% relative humidity or lower in the wintertime. However, buying a humidifier and blasting it isn’t recommended for every household because, depending on your home’s insulation, humidifiers might encourage mold growth in your home — which comes with a whole different set of health problems.

What You Should Do

Ultimately, how each one of us handles our environment in this pandemic will be an independent decision.

I personally combine a mixture of all of these strategies — not because I’m inviting guests over, but because I live in a household that shares its air with another independent household.

I won’t lie: Trying to stay warm, adjusting which windows are open to keep carbon dioxide levels below 800, and the struggle to keep a humidifier full of water and running 24/7 is a lot of work! But once you get used to it, you also get to reap all the other added benefits that come with breathing air that’s comfortable, clean, and fresh.

Oh, and maybe you won’t catch and spread an incredibly dangerous disease. There’s that, too.

Better Humans

Better Humans is a collection of the world's most…

Adrien Burch, PhD

Written by

Happily sifting through academic research so you don’t have to. Microbiologist, educator, entrepreneur, writer. (Yale, UC Berkeley)

Better Humans

Better Humans is a collection of the world's most trustworthy writing on human potential and self improvement by coaches, academics, and aggressive self-experimenters. Articles are based on deep personal experience, science, and research. No fluff, book reports, or listicles.

Adrien Burch, PhD

Written by

Happily sifting through academic research so you don’t have to. Microbiologist, educator, entrepreneur, writer. (Yale, UC Berkeley)

Better Humans

Better Humans is a collection of the world's most trustworthy writing on human potential and self improvement by coaches, academics, and aggressive self-experimenters. Articles are based on deep personal experience, science, and research. No fluff, book reports, or listicles.

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