COVID-19 Safety: How The Virus Spreads…

(Last Updated On: August 19, 2020)

Control Risk By Understanding…

COVID-19 coronavirus

COVID-19 safety is a serious concern for the immediate future. However, all around us, states and businesses are beginning to open their doors. How do we escape our prisons without taking premature risk?

This three-part blog analyzes and describes the safest way to pursue the away-from-home activities that we are desperate to do!

Note: Today’s blog and the following two are written for “most of us.” It assumes that you do not have a coronavirus-infected person in your household; and that none of your household members is a healthcare worker with constant exposure to infected patients.

Here’s the plan. Today, our first installment will review the known facts about how COVID-19 spreads. Knowledge is power! In the following blog we’ll apply that knowledge to an essential activity, grocery shopping. In the third installment we’ll offer practical advice for venturing into the world, even when other people may be taking few or no precautions.

COVID-19 Safety: How Does the Virus Spread?

Coronavirus is not a magic substance, where you are instantly cursed if you come close to someone who was close to someone, a hundred links away from someone who was actually infected. And you are not doomed to hell if you use a public toilet. There are degrees of risk, and ways to reduce risk.

Let’s look at the actual mechanisms as they are currently understood.

What Drives Up Our Risk?

CDC’s guide to COVID-19 safety tells us that workers at higher risk include “individuals over age 65 and those with underlying medical conditions. Such underlying conditions include, but are not limited to, chronic lung disease, moderate to severe asthma, hypertension, severe heart conditions, weakened immunity, severe obesity, diabetes, liver disease, and chronic kidney disease that requires dialysis.”

But here CDC is weaving together age and medical conditions. I prefer to think of age-related risk factors in this way:

  • Just being older makes it harder to fight off any infection.
  • Many people have risky medical conditions, but older people tend to have more of them. These conditions reduce their resistance to just about any disease. For example, they may come down with pneumonia and that may be listed as the cause of death. However, underlying comorbidities contributed to their passing.
  • Skilled nursing and assisted living facilities pack many elderly, frail people into close quarters. Low paid, minimally trained workers spread disease among them. Therefore, statistics that show high coronavirus mortality among older people partly reflect the unsafe conditions in their housing environment.

Besides age, another risk factor is close personal contact with many other people. The previous blog estimated the relative risk of various activities, based on number of nearby strangers, duration of exposure and frequency of exposure.

However, some people simply seem more vulnerable, for reasons that we don’t understand. Robust, healthy people in top physical condition contract COVID-19 and days later they are dead. Someday we may understand the virus better and be able to unravel what led to these tragic cases. However, that’s no help to today’s victims and their families.

Droplets Carry Infection


The primary way COVID-19 spreads is via airborne droplets carrying the virus. Big droplets are the principal hazard. Here are common ways that people release those droplets, in increasing order of severity:

  1. Breathing
  2. Talking
  3. Coughing
  4. Singing
  5. Shouting
  6. Exercising (with heavy breathing)
  7. Sneezing

It’s also worth pointing out that saliva harbors bacteria and viruses, including coronavirus. Therefore, for an infected person, kissing and spitting are both unsafe practices.

Protecting Yourself in Public: Mask + Glasses

It has been said that wearing cloth (non-surgical) face masks protects other people more than it protects you. The theory behind that statement is that if you’re wearing a face mask you’re not likely to cough or sneeze on someone else. And folks who say this think that unless the mask is a high quality mask and tightly fitted, it doesn’t keep you from breathing in droplets floating in the air from someone else.

However, this is not true. Wearing a face mask of any kind also gives significant protection to the person wearing it. Big droplets are the principal means of transmission and these are blocked by a mask. Therefore, even a simple cloth cover close to the face provides COVID-19 safety, by intercepting most droplets. Of course, if you are around people who are sneezing or talking loudly, your mask will pick up a lot of their droplets. As soon as you get home you should discard the mask or launder it.

Note added June 27, 2020: The face mask protects the wearer only if they are breathing through it rather than around it. It’s easy to test the fit of your face mask if it’s made of a flexible material such as paper or cloth. When you inhale, you should feel the mask snuggle closer against your face. That sensation confirms that you are protecting yourself.

            Glasses Protect the Wearer

mask & glasses

But a mask is not all you need. The face mask keeps you from breathing in the larger infectious droplets. However, if droplets float into your eyes, that’s a dangerous way for the virus to enter your body and start multiplying. Therefore, in addition to wearing a face mask, you should cover your eyes when you are near other people. For example, you could wear sunglasses when you are near strangers, both when you’re outside and when you are shopping. Choose ones with large lenses that cover your eyes well, but not too dark in tint so you aren’t tempted to remove them indoors.

Not all masks are created alike. Whether it’s paper or cloth or Impervium, it needs to sit close to your face on either side of your nose. Paper masks typically have a bendable metal strip that you can mold to fit your face. If you make your own cloth mask, you can provide this feature by sewing a pipe cleaner into the upper seam. If you shape the upper edge of your mask, you not only filter out more sneeze droplets; you also reduce the fogging of your glasses from your exhaled breath.

What About Aerosols?

Coughing and sneezing generates a large number of droplets. The smallest droplets, generally those below 5 micrometers in size, are known as aerosols.

All droplets, including aerosols, can carry virus particles from an infected person. Several features make aerosols distinctive:

  • The smaller they are, the longer it takes droplets to fall out of the air and settle on a surface. 5 micrometer droplets take over one hour to settle, and smaller droplets essentially do not settle at all. Thus they linger in the air.
  • When a person breathes in larger droplets the droplets tend to settle in the upper respiratory tract. However, aerosols penetrate all the way into the lungs. An infection deep in the lungs is particularly serious.
  • Almost any kind of face mask will block droplets. However, it takes a very high-efficiency face mask, tightly fitted to the face, to block aerosols. For example, an N95 mask blocks 95% of particles 0.3 micrometers in size and larger.

            Where Are Aerosols A Risk? Indoors! 

Throughout the world most COVID-19 appears to be spread by larger droplets, not aerosols. This may be because the larger droplets sneezed by an infected person carry much more virus, which can more easily establish itself in another person. The aerosols, although a potential source of transmission, just don’t add up to much of a risk UNLESS you are indoors.

Therefore, a non-medical mask provides most people with useful protection. However, there are situations where a person will encounter more aerosols, and need either a medical grade mask or greater physical separation:

  • When people talk, they emit more aerosol particles. And the louder they talk, the more the aerosols! If you are indoors the risks can be substantial.
  • Singing also generates many aerosols, as the Skagit Valley Chorale unfortunately learned in March.
  • Medical procedures such as intubation (a procedure used to install a ventilator) generate many aerosols.
  • In general, healthcare workers who spend time around infected patients are likely to encounter many aerosols in addition to droplets; therefore, they need well-fitted hospital-grade masks.

            Note Added July 11, 2020: Attend to Aerosols!

Recent research is giving more attention to the role of aerosols in risking COVID-19 safety. And now, we must give aerosols our serious concern, for a combination of reasons:

  • Aerosols stay aloft for a long time. (already known)
  • Talking loudly produces more aerosols. (known, but not carefully considered)
  • Bars and crowded restaurants are noisy, causing people to speak more loudly. (known, but not thought about)
  • Restaurant and bar heating and air conditioning systems are designed to circulate air everywhere, while controlling temperature and humidity. In general, they do not contain HEPA filters, which could remove many aerosols of the size that carry COVID-19. And if they had them, the filters would have to be well-maintained to remain effective. Thus you are breathing in a share of all the aerosols being emitted by every person in the restaurant!

What does this information tell us?

  • If you’re going to eat indoors at a restaurant, make sure that it’s a quiet one, where people are not talking loudly. And if possible, eat at a restaurant with effective HEPA air filtration. Outdoors is not such a problem.
  • Airline travel is probably much safer than eating at a restaurant, because the air is HEPA-filtered and flows vertically, not horizontally through the aircraft. Thus you are not sharing air with everyone on the plane, and the air you are breathing is probably well filtered.

Note Added August 19, 2020: New Post: Much More About Aerosols

What About Six-Foot Separation?


Social distancing guidelines call for staying six feet away from anyone whom you don’t live with. But some experts, and I, believe that six feet is not necessarily safe.

I will prove this to you. I would bet that you have seen people sneeze so forcefully that droplets landed more than six feet away from them! Moreover, how far droplets travel depends a lot on their size, their velocity and how air currents are moving.

Six feet is simply a rule of thumb. It does not guarantee COVID-19 safety. And as noted above, people shouting or even talking loudly generate more aerosols, which can travel long distances before settling.

What About Infectious Surfaces?

Surfaces are not a big source of infection, so long as you exercise some caution. Here are the facts:

  • Several Hours: Coronavirus can remain infectious on a surface for several hours. If someone who is shedding the virus sneezes, any surface within reach of the sneeze becomes a hazard. That’s why you need to wash your hands after touching any surfaces in public areas, which may have had many passersby. And that’s why when you buy produce, NPR suggests that you rinse it off with clean water and dry it before you put it away.
  • One Day: The virus is gone from cardboard and paper within 24 hours. (When Nola and I receive a non-perishable delivery in a cardboard box, we put the box on our “quarantine chair,” where it will stay until the next day. Then we wash our hands.)
  • Three Days: What about non-porous surfaces such as plastic and metal? Some folks warn us that the virus can live on those for 2 to 3 days. However, that doesn’t necessarily mean that it’s still infectious for that long. That’s why NPR advises that after 72 hours, you can feel confident that there’s no virus on your groceries.
  • Seven Days: After 7 days away from people, CDC advises that surfaces have no residual virus.


  • Outdoor Surfaces: At your own home outdoor surfaces generally require no disinfecting, unlike public facilities such as bars and restaurants with outside seating.
  • Sunlight: Note also that direct sunlight kills the coronavirus on surfaces, often within minutes. Does that make it safe to go to the beach? Not exactly. At the beach, you will probably be close to many other people, most of whom will not wear face masks. The air will be full of aerosols, some of which may be infectious. The breeze will keep them aloft and spread them around. And even if your fellow beachgoers are six feet apart (news photos make that seem unlikely!), that’s not enough distance to protect you from their germs.

Here’s another source for you: an excellent article in Vox, published just a few days ago. They quote examples showing how very risky it is to spend a long time in a confined space with strangers.

This has been a short course in COVID-19 safety, in which we have reviewed what we know about how the coronavirus spreads. In the next two blogs we will apply this knowledge to grocery shopping and to returning to almost-normal outside life.

Image Credits:
– Coronavirus by chiplanay, mask & glasses by Abhi_Jacob, mask distance by iXimus, all via
– Sneeze by j4p4n via


COVID-19 Safety: How The Virus Spreads… — 8 Comments

  1. I’m in New Mexico where the sun is quite strong. In the begining of the lockdown, I recalled that the last step in 18 ohm clean room water is a UV light to kill whatevs. I put my packages and mail in direct sunlight for a few minutes to sterilize and was happy to see this is now an accepted way to kill the virus. Getting out in the sun every day is so important for all of us, and now there is one more added benefit. Thank you for writing about COVID19.

    • Thanks, Lorna, your comment is much appreciated. Your alertness has put you ahead of the curve, and hopefully saved you much grief from the virus. Thanks for sharing your experience, readers will benefit from it! – Art

  2. The May 22 issue of Science has an interesting article about how COVID-19 transmission is revealed by “Case Clustering.” ( Some study results that are quoted:
    (1) Most infected people infect no one else; however, a few people are “superspreaders,” infecting many others.
    (2) What makes a person a “superspreader”? For one thing, talking at a louder volume.
    (3) Superspreading usually happens indoors; a Japanese study found almost 19 times greater risk of COVID-19 transmission indoors compared with outdoors.

  3. Art,
    Excellent summary of the Covid basics and the general rules for minimal protection.

    Be well,

  4. How timely: today there’s a news report about new review article in Lancet that compares the effectiveness of three means to slow the spread of coronavirus: physical distance, face masks and eye protection. See (This article gives a compact summary; it contains a link to the research study.) Here is a summary quote: “…staying 1 meter away from other people appears to reduce your chance of catching COVID-19 by 80 percent. Wearing a mask or face covering appears to reduce your risk by up to 85 percent. And wearing goggles or a face shield seems to reduce it by up to 78 percent.”

  5. Art

    I have frequently enjoyed reading the adventures and trials and tribulations of Art Chester over the years. Your latest series on Covid-19 provides a significant public service which I have copied to some friends and close relatives.