Protecting ourselves from aerosol transmission | COVID-19 Aerosol Transmission FAQs (3/9)

3.1. At what distance from an infected person can I get infected?

There are three relevant scales. Note that these terms are not used consistently across scientific disciplines.

  1. “Close proximity,” < 1-2 m

    • There is a lot of COVID-19 transmission in close proximity situations. But it is not 100% efficient. As we understand from contact tracing, some close proximity encounters do not lead to infection.
  2. “Shared room,” sharing the air of the same room at the same time

  3. “Long-range,” when not in the same room, e.g. different rooms in the same building with some air through HVAC, or entering a room 2 hrs after someone infectious has been there etc.

    • There are no documented cases that we are aware of. This is consistent with the expectation for a generally less contagious virus.
    • Some cases of long-range transmission of COVID-19 are being documented, but they appear to involve fecal aerosols transmitted through the sewer lines of high-rise buildings in Hong Kong and China, and not respiratory aerosols produced when breathing or speaking.

3.2. Do the 1-2 m or 6 feet guidelines guarantee lack of infection indoors?

Like other recommendations, these social distance rules will reduce risk, but not eliminate it. The 6 feet rule is based on the idea that large ballistic droplets fall to the ground within 6 feet, although they can travel farther in a cough or sneeze, up to 28 ft (8.5 m). The 6 feet rule also helps with aerosols that do not settle to the ground because they are most concentrated close to the person who released them, like cigarette smoke is most concentrated close to the smoker. Dilution over distance is the main reason why social distance reduces transmission of COVID-19.

Where will the concentrated aerosols from a person go? It is almost impossible to say. Indoor air is a constantly changing, turbulent environment. Even though you can’t feel it, indoor air is constantly moving (typically between 0.05 to 0.1 m/sec, or 0.15 to 0.3 ft/sec) due to heat from your body (adults typically give off as much heat as an old incandescent light bulb, 75 to 100 W), sunlight warming surfaces, people moving and breathing, and mechanical heating and cooling induced airflows. You likely have seen this when looking at visible aerosols floating in a sunbeam, or when extinguishing a candle indoors. The aerosols rarely are all going the same direction.

This constantly changing environment will dilute plumes of aerosols. The dilution will be greater with more time and space. Hence, the farther away you can be from other people indoors, the better.

3.3. How long does one need to inhale infectious aerosol to be infected?

Catching a whiff of exhaled breath here or there is very unlikely to lead to infection. The amount of time you spend in close proximity or in a shared room with an infected person affects how much virus you actually inhale, which will dictate your risk of becoming infected. There is no clear amount of time as far as we know, but it would seem to be in the order of minutes.

The CDC says that 15 minutes of talking with an infectious person in close proximity is typically needed to get infected. However, that seems arbitrary to us and is not supported by evidence as far as we know. It can also give a false sense of security that a 5 or 10 minute interaction is safe because it is under the 15 min. threshold.

3.4. How long can the virus stay in the air indoors?

How long the virus stays in the air with the capability to infect depends on three processes.

Deposition depends on the size of the droplet/aerosol that’s carrying it, as well as on the amount of clutter and air motion in the room. Virus has been found in tiny aerosols, smaller than 1 micron, and these can stay floating in the air for more than 12 hours, BUT these small aerosols will typically leave a building in the air faster than they settle on indoor surfaces.

How fast does air leave a room? It is a little complicated. Think about a cup of black coffee. How much milk do we have to add to the cup before we only taste milk? If we add one cup of milk to our cup of black coffee (allowing it to overflow) the result will still be a tan mixture. In fact, due to mixing it will be just two thirds milk. We would need to add three cups of milk to get our original black coffee cup to be 95% milk.

Indoor air behaves the same way. As outdoor air enters an indoor space it mixes with the air already indoors. So how long does it take to replace aerosol laden air from indoor spaces with outdoor air? In residences, 95% of the indoor air will likely be replaced with outdoor air in a time frame that ranges from 30 minutes to 10 hours. In public buildings, 95% replacement may take between 12 minutes to 2 hours. In a hospital, 95% replacement might take 5 minutes.

So how long a virus can stay in the air indoors is highly dependent upon the indoor environment.

3.5. How can I protect myself from aerosol transmission indoors?

We can never be perfectly safe, only safer. Hence, we need to take as many steps as possible to reduce the risk of our activities. You should try to avoid or reduce as much as possible situations that facilitate inhaling the “smoke” (exhaled air) from others. To reduce risk avoid:

  • Crowded spaces
  • Close proximity to others
  • Low ventilation environments
  • Long durations
  • Places where people are not wearing masks
  • Talking, and especially loud talking / shouting / singing
  • High breathing rates (e.g., indoor aerobic exercise)

Each one of these features potentially increases the aerosol concentration you might inhale indoors. So if you must enter one of the above situations, complete your tasks as quickly as possible to reduce your exposure duration and risk.

3.6. Is there a way to remember all the things I need to reduce or avoid?

We have proposed an acronym / mnemonic: Avoid Crowding, Indoors, low Ventilation, Close proximity, long Duration, Unmasked, Talking/singing/Yelling/breathing hard (“A CIViC DUTY”). A volunteer made the posters below. There is a lot of room for improvement by creative people on both the acronym and the graphics. A version in Spanish can be found here. To adapt the posters into other languages (or improve the English version), make a copy by clicking on this link and edit away.


Another series of posters with the same message can be downloaded from here. Examples below for the first couple:


3.7 Aren’t your recommendations the same as Japan’s 3 Cs?

They are similar, but our recommendations encompass several additional items. The 3 Cs recommend avoiding indoors, low ventilation, and crowded situations. But it does not mention avoiding unmasked situations, of long duration, and where talking / singing / shouting / breathing hard.

And if the mechanism of infection (aerosols, like smoke) is not mentioned, people don’t understand such recommendations, and have a hard time following them or adapting them to new situations. In our experience, once people understand “it’s like smoke, and you want to breathe as little smoke as possible”, they very quickly figure out what they need to do in their specific situations.

3.8 Is there a way to understand the relative risk of different environments?

The microCOVID project provides risk estimates for different locations, activities, and personal risk levels.

National Geographic has adapted the aerosol transmission estimator into a simplified online calculator to allow visual estimation of risk under different situations.


Figure: screenshot from the National Geographic aerosol COVID-19 risk estimator.

The table below has been proposed by Jones et al. (2020). It provides the relative risk of different environments, using almost the same parameters of the “A CIViC DUTY” acronym and the mathematical models.

Risk of coronavirus transmission in different settings: A multitude of factors beyond social distancing can affect transmission.


Figure: estimated relative risk of COVID-19 transmission for different activities. Table from Jones et al. (2020), as redrawn by the Washington Post. (We understand that the table is qualitative, and there is some debate about some details. We plan to work in an improved version of this table using the aerosol transmission estimator).

3.9 Is there a more quantitative way to investigate ways to reduce aerosol transmission?

Yes, several mathematical models have been developed that allow you to estimate the risk of infection (with higher uncertainty) and the reduction of risk when doing one or several things (with much better accuracy). One of them is here: See the Readme and FAQ pages for extensive information on methods, assumptions, limitations, links to other similar tools etc. It requires some familiarity with math and spreadsheets. If you find it too confusing, find someone who can help (e.g. the science teachers in a school).

Another useful tool is the NIST FaTIMA model. Although it does not estimate the risk of infection, it has more detail to estimate exposure.

Several similar tools have been developed by other researchers, and the publicly available ones we know of are linked in the FAQ page of

Next: 4. Outdoors

Previous: 2. General questions about aerosol transmission

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