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This Is How Many Respiratory Particles You're Rebreathing Indoors

Computer generated image of blue droplets on a light blue background with white specks.

Jian Fan/Getty

Key Takeaways

  • Researchers at Colorado State University have calculated the chances of respiratory disease transmission risk in different situations. 
  • Over a long time, whether you are indoors or outdoors is a bigger predictor of risk than your distance from another person. Being outside for interactions is safer than being inside because the wind is present to scatter respiratory particles. 
  • You should try to reduce how much time you spend indoors. If that’s not possible, avoid crowding and try to increase airflow by opening windows. 

For the first time, researchers have measured the chances of transmitting an infectious disease such as SARS-CoV-2 to another person at a range of distances and in various settings.

The study's findings, which were published in the journal Environmental Science & Technology, could help people make safe choices for themselves and their loved ones as they await a COVID-19 vaccination.

Providing Evidence for Social Distancing

Throughout the pandemic, the general advice has been to avoid crowded areas and stay six feet apart from people outside of your home or "bubble."

However, the researchers behind the latest study have pointed out that there has not been much data to back up these recommendations. With their research, the authors have endeavored to provide numerical evidence for social distancing and other public health directives.

How Researchers Assessed Risk

The researchers’ study hinges on a metric that they dubbed “effective rebreathed volume” (ERBV for short). In essence, ERBV is the amount of particulate air that is exhaled by one person and inhaled by another.

The metric primarily depends on two factors: proximity (or how close the people are to each other) and confinement (whether they are inside or outside). Simply put, ERBV (and, by extension, infection risk) increases with proximity and confinement.

The closer you are to another person, the greater your likelihood of potentially contracting or transmitting SARS-CoV-2 in the form of contaminated respiratory droplets. 

Tami Bond, PhD, a professor of mechanical engineering at Colorado State University and lead author of the study, tells Verywell that "different spaces can be compared with ERBV numbers" and many people don't realize "that you can use simple principles to compare spaces.” 

How Does Confinement Affect Transmission Risk?

Bond and her co-authors calculated ERBV for three different droplet sizes in several scenarios. The size range was necessary because “particle size affects transport, removal onto surfaces, and elimination by mitigation measures.”

The team found that 10 to 15 minutes into a sufficiently distanced interaction, confinement—or being indoors—is the best predictor of ERBV for the two smallest sizes of droplets: those one micron in diameter (about the size of a SAR-CoV-2 droplet) and those 10 microns in diameter (about the size of pollen or dust particles). 

The results of modeling experiments supported the conclusion by demonstrating that interacting outdoors at a distance of six feet was safer than interacting indoors at a distance equal to or greater than six feet.

If you plan on catching up with a friend for more than a quarter of an hour, you should take your conversation outside. 

Given these findings, would increasing social distance indoors from six feet to 12 feet make a difference?

"Within six feet, you are still in a person’s cloud,'" says Bond. "But even when the cloud dissipates, the particles are still there and remain confined in the room. That is true whether the other person is six feet, 12 feet, or 20 feet away from you."

Why Is Being Outdoors Safer Than Being Indoors?

The reason that rebreathing is less of a concern when you're in a backyard instead of a kitchen has to do with the natural elements.

When you're outdoors, forces such as the wind carry away some of the respiratory particles, which reduces the number available to be inhaled. When you're inside, the lack of an outlet causes the particles to collect in ambient air. 

The researchers’ data reveals the degree of the ERBV disparity.

  • Talking outdoors for 4 hours at 6 feet apart: You will have rebreathed between 0.14 and 0.24 liters of one-micron particles.
  • Talking indoors for 4 hours at six feet apart: You will have rebreathed between 65 and 100 times the amount of particles you would have if you were outdoors.

In the simplest terms, Bond says that indoor spaces "allow particles to build up, outdoor spaces don’t."

Bond says that if you do have to dash indoors for a few minutes, it's not necessarily dooming. "What really matters is confinement with other people for a long time," she says. "Once you have gotten out of the cloud that comes from being in close proximity, you have to address the confinement provided by the walls.” 

Can You Minimize Indoor Risk?

The researchers hope to build on their study by pinpointing the ERBV threshold for COVID-19 transmission. 

“What I personally would like to get to is the combination of contact tracing and built-environment characteristics so that one could identify ERBV at which transmission had never occurred,” Bond says. “If you did that, you could tell people which environments were safe, instead of having people be afraid of everything, as they were early in this pandemic.”

Individuals and institutions have attempted to replicate environmental dispersion with mixed results. Even though ventilation and filtration tactics and systems reduce transmission of one-micron droplets (ERBV1) by 13 to 85%, they are less effective at reducing larger sizes. These measures only reduce transmission of 10-micron droplets by three to 50% and are completely ineffective against transmission of 100-micron droplets.

In the study, the researchers suggest mounting a defense against the final category of droplets by “offsetting occupancy, wearing face coverings, and separating occupants between rooms—even with doors open.”

What You Can Do In Your Personal Spaces

Start by trying to increase airflow into your house or apartment. For example, bathroom fans can reduce ERBV1 by 15 to 30%, kitchen range hoods by 30 to 40%, continuously running central air conditioning by 15 to 55%, and open windows by 25 to 65%, depending on their width and location.

If you plan to filter, Bond says that “you should make sure you have a high-efficiency filter that removes one-micron particles—that is what the filter raters call ‘MERV 13’ or higher numbers.” 

Follow These Basic Guidelines

As to whether you should avoid public venues such as bars, restaurants, and department stores, Bond says that it depends.

“Crowding, like more people per square foot, is more risky," she says. "Staying for longer times is more risky. Ventilation and open windows are less risky. High ceilings are less risky.”

If you adhere to these basic principles, it will significantly minimize rebreathing. 

Since particles are only propelled forward by momentum within 1.5 meters (about five feet) of the exhalation site, the researchers wrote that “the two-meter distancing guideline addresses much of the proximity effect.”

When possible, avoid indoor settings. If that's not possible, make sure that you’re wearing a mask, following social distancing protocols, and situated in a low-traffic, well-aerated location.

What This Means For You

A new study has shown that when it comes to minimizing the spread of potentially infectious respiratory droplets, you're safest being outdoors. However, if you have to be inside, there are things that you can do to make the space less hospitable to these droplets. Turning on fans, opening windows, and running the AC can all reduce transmission risk.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

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  1. Bond T, Bosco-Lauth A, Farmer D, et al. Quantifying proximity, confinement, and interventions in disease outbreaks: a decision support framework for air-transported pathogens. Environ Sci Technol. 2021:55(5):2890–2898. doi:10.1021/acs.est.0c07721

  2. Manning A. Indoors, outdoors, 6 feet apart? Transmission risk of airborne viruses can be quantified. Colorado State University News. February 26, 2021.