So you STILL want to go out? Watch out for the aerosols!

You've looked at the graph from my last blog post (click here) and decided that maybe taking a 10 or 20% chance of being in a crowd with one or more infectious people is OK with you.

You decide to go out with your buddies.

At this point you might ask, "What else can I do to reduce the risk of getting the virus?"

It will help to have a little background on how viruses are transmitted. If you can keep this in mind, you can probably pretty much totally avoid getting sick. 

How Viruses are Transmitted

There are six different ways that viruses can be transmitted between humans, according to epidemiologists (see the appendix for a list).

For the Coronavirus, there are three that we have to worry about:

• Direct human to human contact
• Contaminated surfaces (fomites)
• Transmission through the air:  Droplets and aerosols

You already know how to control the first two.  

Don't kiss, shake hands or hug, avoid touching your face, wash your hands frequently with soap and water for at least 20 seconds, and use plenty of hand sanitizer.  

These strategies are nearly 100% effective and, if you abide by them, then I can virtually guarantee you won't get sick with Covid, even in the presence of infected people.  

Except for one thing:  Transmission through the air.

Droplets and Aerosols Cause Infections

Viruses do not fly through the air by themselves.  

As we learned in the May 19th blog post on masks (click here), coronavirus particles must be surrounded by water in order to remain viable.  They always come in the form of droplets and aerosols when transferred through the air.  

In fact, several very recent studies have found that droplets and aerosols are perhaps the primary way that people get Coronavirus (Reference 5), (Reference 6).  

And, intuitively, this makes sense: the virus can only infect cells that have something called "ACE2 receptors" on their surface.  Guess where these cells are located?  Not on your skin, on your tongue or in your throat - the virus can't infect those cells.  They are deep in your nasal passages and in your lungs.  The only way for these cells to become infected is for you to deeply inhale aerosols and droplets containing viruses.

(Cells with ACE2 receptors are also found in your blood vessels, intestines and several organs - but the virus can't get to these areas without passing through the acid in your stomach or via infection of the lungs for example.  And as the disease progresses, these areas have, indeed, been shown to be a later target of infection. Very recent research has also found ACE2 receptors in the human eye. It is unclear whether this is a primary path for infection though. (Reference 20))

OK, so the way to not get sick is to avoid droplets and aerosols.  How can I do that?  After all, we have to continue to breathe. 

Protecting Yourself from Droplets and Aerosols

Droplets are defined by research scientists as small drop of water greater than 5um (micrometers) in diameter.  Droplets will quickly fall to the floor and are probably a factor within 3 feet or so of the person expelling them.

Aerosols, on the other hand are far more pernicious.  They are tiny drops of water, smaller than 5um in diameter, and can remain suspended in the air with viable viruses for 3 hours or more (Reference 14).

A droplet can hold thousands of virus particles.  A mist of aerosols can too.

Droplets and aerosols are expelled when people breathe, talk, sing, cough or sneeze, as many as 300,000 of them at a time (Reference 15).  They can travel 60 mph when you cough. Even just speaking can generate 10,000 particles per second (Reference 16).

An asymptomatic but contagious person can expel 100x the minimum dose of a virus needed to cause infection, just while talking and performing a light activity (Reference 17).  

Let's see how we can stop ourselves from inhaling them when we are out and about.

1) Masks

Masks work.

There is no longer any doubt.

Since the publication of my blog post on masks (May 19th, click here), there has been an overwhelming amount of additional evidence uncovered for the effectiveness of masks (for example, Reference 7, Reference 8, Reference 9, Reference 10).

We've learned that masks definitely lower the reproduction rate of the virus (reference 7, 8).  Further, we've learned that they can prevent droplets from turning into aerosols.

If a mask is 90% effective on egress and 50% on ingress, and everyone is wearing a mask, then the transfer rate of droplets and aerosols can be reduced by 95%. 

I would recommend against entering any room unless everyone is wearing masks.

2) Stay outside

You are much safer from infection with a group of people outside than inside.  There are two reasons for this: 1) dilution and 2) UV light.

When you are outside, you are surrounded by an infinite amount of fresh air that will dilute the concentration of any virus-containing droplets or aerosols.  Physical processes of diffusion and forced air convection (if there is even a small breeze) are very effective at diluting the effects of the virus.

There is good evidence that viruses can be inactivated by UV light that is contained in sunlight ( Reference 13)(Reference 12).  In one experiment, 90% of the virus was inactivated in as little as 6.8 minutes when hit with simulated sunshine.  And on a cloudy day, as much as 80% of the UV light from the sun can get through, so even when the sun is not brightly shining, you will have some amount of protection.

3) Avoid crowded rooms and watch ventilation patterns

Inside, things are much better for the virus (and worse for you).  There is no UV light, and not much of a chance for droplets and aerosols to disburse.  So try to stay outside when you are with others.

When you absolutely, positively have to be inside with other people, try to limit your time there. Especially if the room is crowded and poorly ventilated. Your chance of getting sick is directly proportional to how much time you spend in a potentially contaminated area.  

If you do find yourself spending time indoors with a bunch of strangers, say, at a restaurant - watch the ventilation!  A now infamous study in China uncovered a pattern of Covid-19 infection, traced back to a combination of table placement and air flow from an AC unit (Reference 18).

4) Be conscience of aerosol-generating activities

It turns out that going to choir practice isn't such a good idea anymore.  Back on March 10, in Washington State, one superspreader passed on infections to 52 other singers at a practice (Reference 19).  Two people died.

Singing (and yelling) generates droplets and aerosols at a high rate (Reference 4).  Singing at bluegrass jams.  Singing at church.  Yelling at a rally or a protest.  (Reference 21)

If you are going to sing with other people, do it outside.

A couple more things to remember

If you are going to be going out, or be with others in this new Covid world, it is much better to limit your crowd to people you know.  People who have good "Covid Hygiene."  People who you can contact (and they can contact you) in case someone gets sick with the virus. (This is what contract tracing is all about.)

Furthermore, think about contact tracing in advance.  This is what (should) happen if one of the people in any of the crowds you were with (including you) gets the virus.  Can you retrace your steps?  Do you have a way of figuring out who was in the crowd that day?  It might be a good idea to keep your calendar updated with all the details of your comings and goings just in case.

And don't forget to wash your hands.

Appendix

The six ways of viral transmission are (Reference 1):  

• Direct human to human contact
• Contaminated surfaces (fomites)
• Transmission through the air:  Droplets and aerosols
• Vector (e.g. mosquito)
• Oral
• Zoonotic (from infected animals) 

We can pretty much eliminate zoonotic transmission of the coronavirus, unless you intend to visit a wet market in Wuhan sometime soon.  Vector and oral transmission are also highly unlikely for this virus, which leaves us with three that we really need to worry about:

• Direct human to human contact
• Contaminated surfaces (fomites)
• Transmission through the air:  Droplets and aerosols