Wednesday, March 18, 2020

Quantifying risk minimization by moving events outside

While hanging lights in the grade-school, I got to thinking about the possibility of having Mass (what Catholics call their main, religious service) outside.

Our Bishop cancelled all public Masses for a few weeks while events sorted themselves out. For Catholics, that is a very, very big deal. We get great spiritual strength from attending Mass and it is deeply ingrained within us.

Specific Covid-19 transmission risks
I could only think of three pathways for Covid-19 transmission.
  • Transmission by touching
  • Transmission by inhaling/touching suspended aerosol
  • Transmission by direct impingement of sneeze ejectia
Since it is not possible to quantify exactly how much risk is from each path, it is prudent to consider them equal and then to estimate the degree of risk mitigation as the path that was least improved.

Transmission by touching

It is an item of forensics faith that Object A cannot touch Object B without the surface of A being contaminated by material from the surface of B and vice versa.

Transmission by touching could result from people shaking hands. It could be from touching the top of the pew in front of you that had been contaminated by an earlier service.

If hand-shaking was prohibited and if the worshipers brought their own Bible (BYOB!) and their own folding chairs (soccer chairs) or stood during the service then the only risk is from the Communion distribution and from touching the outside door handle of your vehicle.

I estimate that the risk of transmission-by-touching in an outdoor service is reduced to less than 1/1000th the risk of a traditional, indoor service if Communion is not shared.

If Communion is shared, the Host (what we call the wafer) can be placed in disposable, single-use cups by a masked, gloved person and they can be consecrated in that condition. I submit that would reduce the transmission-by-touch to almost zero as the Priest handed out the single-use cups.

Under these conditions, holding the religious service outside reduces the risk of transmission by touch by 99.9%

Transmission by suspended aerosol

The average wind-speed in Lansing, Michigan in Marce is approximately 12 miles an hour. It is somewhat lower in the morning and lower yet in suburban areas, four feet from the ground. Therefore, I will use an average wind-speed of 4mph as a defensible estimate.

The suspended aerosol from a sneeze in the center of a 100 foot congregation would clear the outer edge of the congregation in 8.5 seconds.

In an enclosed room with an air-turnover every four minutes, only 2.5% of the aerosol would have been cleared.

One of the quirks of HVAC is that the incoming stream mixes with the room in the air. Think of putting a full gallon of milk beneath the faucet in your kitchen sink. There is still milk in the jug even after you have run a gallon of water into the jug.

Arbitrarily using 1% as a threshold for zero, it takes about 28 minutes for the air to "clear" even when the air is changed over every four minutes.

Dividing 28 minutes by 8.5 seconds (0.005) suggest that holding a religious service outside reduces the risk due to suspended aerosol by 99.5% (one part in two-hundred).

Transmission by direct impingement

Transmission by direct impingement of dominated by the behavior of the person sneezing and the spacing of the people.

Changing the behaviors of the people sneezing is outside the scope of this essay.

For the sake of simplicity, let's assume the ejectia flies forward. Consequently, the person sitting to either side of the sneezer is at less risk than the people sitting in a cone shaped region ahead of the sneezer.

In a typical church, approximately 15 square feet allocated for every person at max capacity. Much of that is used for aisles, a bit for the altar. If you just look at the pews, they might be on a 54" pitch and at a packed service you will have a body every 24" of pew length which works out to one person for every 9 square feet.

In my hypothetical, outside worship space I envision lines painted on the ground similar to lines on athletic playing fields. Those lines would be spaced at some recommended interval to minimize ejectia transmission. Congregants would stand or place their soccer chairs on the lines.

Suppose the lines were placed ten feet apart, the number of people within the forward cone of ejectia would be diminished...first because of the greater pitch between "pews" but also because most people space soccer chairs about 24 inches apart so they can egress between them.

Using 10' by 4' spacing there is one person every 40 square feet which reduces the risk of transmission by direct impingement by approximately 75%.

This comes with a caveat: All of the larger ejectia eventually falls out due to gravity while the smaller droplets are "suspended" and were discussed in the section immediately above this one. At some distance, all of the larger ejectia will have fallen on the ground and the risk is not reduced by 75% but by 100%. I don't have the expertise to say whether that is ten feet or twenty feet or forty feet.

All comments will be appreciated, especially if the writer has some insight into how far the larger sneeze droplets will travel.

Bottom-Line: At this point, holding religious services outside would reduce risk of Covid-19 transmission by a minimum of 75% when compared to the risk of indoor services.


  1. Really like the analysis, Joe. Our Sunday Mass last week was "optional" and all song books and mistelettes, and the Holy Water had been removed before we arrived. Attendance was roughly 50% of normal. As usual, the sixty-and-over crowd was the most heavily represented.

    I hope that some of your other readers can flesh out this sneeze droplet carry distance, to tighten up that part of your analysis, so we can put it to use.

    Just imagine the effect of a billion people who are restrained from worship, on the worldwide spiritual battle. Saint Michael, defend us in battle...

  2. This applies to a lot more than churches; assuming hospitable temperatures, a large number of functions could be moved outdoors. With a roof, however, even a high one, one is not really "outside" in the open are as the roff itself will affect air flow and air currents.

    I wonder if an "open shingle" style roof might be the answer; a row of, say, a 20" wide roof-length "shingle" spaced 8-10" above the next lower roof-length "shingle" and overlapping it by 10-12" would keep falling rain out (wind-driven rain would get in), allow free air movement and not be a "lid on the oven" in hot weather.

    There are shooting ranges constructed with such a roof to prevent any rounds exiting the firing area and provide rain coverage and shade, producing a "semi-indoor" environment that's actually outdoors for ventilation purposes.

    1. I think that is a great point about pavilions. It sort of splits the difference and does provide protection from torrential rains and gives shade.

  3. Check out Drop Nuclei for airborne particulate information

  4. We are dealing with presumably a virus. If the virus is present you have a ~13.8%* (Worst stat I could find) chance of contacting it and having severe symptoms. To reduce that chance of contracting the virus we are told to wash our hands. This is true do that, it doesn’t kill the virus, soap makes it so the virus cannot stick to you and gets washed down the drain, remember that . A sneeze puts microscopic droplets (aerosols) in the air, when the moisture evaporates it can leave a charged particle behind call drop nuclei which float around in air, then settle on surfaces. Cover your mouth to limit this. We are told to limit close contact, also good. Cleanliness is incredibly important, maybe next to godliness. Your shoes bring in an incredible amount of foreign material into your household, leave them in the garage, outside, or in a specific spot, don’t wear them around the house, pretty simple. Work clothes, if they are dirty, take them off outside your living area. Dirty clothes, clean them, don’t let them pile up in your bedroom. Counters and horizontal surfaces, keep them clean, put dirty dishes in the dishwasher or clean them, don’t let them hang around in the sink. These are some of the things you should consider, here’s why.
    Have you ever sat near a window and a ray of sunshine beams through and you see all the tiny particulate in the air floating around? Assuming you have, my specialty is analyzing and telling you what you are seeing and what it is composed of. Generally, this stuff flying around is the general wear and tear of your house, clothes, you , others and life in your house, outside stuff coming in your house and so on.
    Generally we find cellulose and synthetic fibers (clothes ) lots of skin fragments (you) Mold, danders from pets, combustion products, pollens and so on. We have been breathing this in since the day we were born, it’s normal and our bodies can handle it. If one of these categories becomes overwhelmingly large or a foreign particulate becomes added to the mix, odds increase of a possible reaction and we are all about decreasing them. For example, mold counts go up because a leaky waterline to your fridge and it soaks your drywall in an area you don’t normally look at. You feel different, something is not right, you begin researching on the internet and you conclude you may have a mold problem (sometimes at this point people think you’re crazy) along with a dose of fear many websites thrive on.
    Some people panic and are told to move out of their homes and they do. A legal mess usually ensues. A reasonable investigation will find the mold, remove the stuff, clean up, and things go back to normal.
    I tell you this because the one thing you cannot do is remove all possibility of contact with particulates but with reasoned thought and actions you can minimize risk. Minimizing airborne dusts can help minimize possible contact. Change your furnace filters, if you’re especially concerned about indoor air quality get a HEPA air-cleaner at a home project store. You will notice the clarity of your air increase dramatically. These are simple solutions, known to be true, and will help minimize risk and alleviate the fear of the unknowns.
    Here in Michigan there are Ad Council billboards telling people to prepare for an emergency and they have been up for years. Folks I have seen in the stores are not panicking, they are prepping rather quickly. I think they look a bit sheepish knowing they should have listened and done this sooner. Good for them. My hope is that this is the beginning of a great awakening. Eyes open and ears hearing.
    Jesus said fear not, he also said beware of the scribes. He also said be confident in his word, that is truly the only thing we can be certain is true.

    1. My I post your entire comment as a Quest Post? I will attribute it to whoever you want.

      It is too good to sit buried in comments.


  5. Yes please do, below is the beginning I cut off for brevity if you want to add that. Sorry for the delay, I left my computer on at lab and was unable to post so I drove in this morning to reply. Clad is short for Cladosporium, one of the most common fungi in the world, lol, so how about just another quality ERJ reader.

    The Corona/China/Covid19 virus situation is a reality. What the concern here is the correct response to the actual threat the virus deserves. Some consider this ridiculous, some consider this the end of the world and some just don’t know. Our responses are based on what information we decide to enter our mix to formulate our opinion. Watching Fox News this morning the opening of the show reminded me of a B movie that I decided was not worth watching. Fear is leading everything and that is a bad leader. Should this be an overblown reaction I hope people promoting this are held responsible and our fear is unfounded. If this is truly a major threat we do have to do our duty and do whatever it is we can. Remember this entire crisis is about other people and our reactions toward them. Can we take care of ourselves and others? Yes we can.
    I’m a laboratory director who has worked for over 20 years to decrease fear and panic of environmental concerns unless they should be truly a significant risk. I will begin by saying that most of the fear and panic is media generated with manipulation of fear to drive behavior. A response to fear can be real enough to cause greater damage than what you are fearful of. When the news agencies raise concerns there is a need to process what they have said and I hope to instill reason and understanding on how to properly respond.
    Since most of what you have heard is hearsay or second hand knowledge at best coming from lawyers or news folks, you must hold that against what you know is true. I do not believe in conspiracy theories, I know people are doing things, so I call them speculation theories; we are all trying to figure out what they are doing and why. Approaching this from a scientific frame of mind, and knowing only a few things to be true, how do we come up with a solid plan to put each one of us in a strong position to weather through all the nonsense while taking care of what absolutely is necessary should something really be happening.


Readers who are willing to comment make this a better blog. Civil dialog is a valuable thing.