Warm weather has let us keep in touch with family and friends by visiting outdoors. If you are like me, the time spent in parks and on patios has been heaven sent. But with cold weather coming, what do we do now? Maintaining six feet of social distance is good advice for some situations but too simplistic for visiting inside a house or apartment.
Over the last half year, we have learned a lot about how to avoid Covid-19. In this post, I am going to give you some specific advice on how to visit with your friends and relatives indoors while minimizing the chances that anyone will get sick.
Too Many Words!
I know… I know… I spew too many words onto the page. Here is the bottom line:
- Give everyone in the house an N95 or KN95 mask. Assure that everyone wears their mask properly, all the time. Skip the meals because you cannot eat while wearing a mask.
- Limit the size of the gathering to a few people. Skip the parties.
- Install a MERV 13 filter in your furnace. When you have visitors, turn on the furnace fan so that the air circulates continuously.
- If the weather permits it, open some windows so that fresh air flows through your house.
- Wash your hands. Don’t touch your face.
Be Aware
Be aware that I have no magic. My advice is worth every penny that you paid for it. I do not know a sure-fire way to stay healthy.
What I do know is how to distill a broad swath of information into a single, short-ish blog post. I can give you some information, in understandable terms, which will help you make good decisions.
N95 Masks
Buy a box of N95 or KN95 masks. When you have visitors, ask everyone to wear one during the whole visit. These things used to be hard to find but now they are widely available and cost less than $1.50 each. Sometimes you will find masks which are called other things like FLTR95. The important thing is the filtering specification, which should be:
Provides ≥95% Filtration Level with a Particle Size of 0.3 Microns
A mask which meets this specification will trap droplets when someone exhales, preventing at least 95% of them from escaping into the room. When someone inhales, the mask will also trap at least 95% of any droplets which are floating around in the air.
Imagine that you and I are sitting together, talking, and that I am contagious with Covid-19. When I exhale through an N95 mask, just 5% of the badness escapes my mask. When you inhale through an N95 mask, just 5% of the remaining badness makes it through your mask.
5% × 5% = 0.25%
You can see that, if you and I both wear N95 masks, you are only exposed to 0.25% as much badness as if neither of us wears a mask.
Assure that everyone wears their masks properly. The mask should cover the nose and the mouth. It should fit tightly against the face. The idea is that, when someone exhales, their breath goes through the filter material instead of around it. When someone inhales, they draw air through the filter material instead of around it.
Small Groups
By keeping the number of people small, you reduce the chances of infection in several ways. You reduce the likelihood that someone in the group is contagious. You reduce the likelihood the people are talking loudly, to be heard above the din of a large group. And you reduce the number of infectious droplets in the air, on the off chance that more than one person in the group is contagious.
MERV 13 Furnace Filter
When we exhale, speak, sneeze, or cough, we spew tiny droplets of body fluids into the air. If we are infected with novel coronavirus, each of those droplets contain some of the virus. The good news is that, to grab the virus out of the air, the furnace does not need a filter that is so fine that it can trap the virus; it only needs a filter that is good enough to trap the droplets which contain the virus. A MERV 13 filter will do the job.
Here is one example, the Honeywell Elite Allergen Air Filter, which you can buy for under $20. You can get similar filters from many other brands, too.
Install that filter in your furnace and replace it on the recommended schedule. When you have guests, flip the switch on your thermostat so that the furnace fan runs all the time. This will circulate air as quickly as possible, pulling it through the filter and reducing the number of virus particles which may be floating around where anyone can inhale them.
Open the Windows
If the weather permits, open a couple of windows so that you have a breeze flowing through the house. Any breeze is good, as long as it exits the house, carrying potentially infectious droplets outdoors and away from you and your friends. Every little bit of breeze helps, even if it is in a different room than the one you are sitting in.
Since the furnace is running and circulating air, the whole house is essentially one great, big room. It is best if the open windows are in the room you are sitting in but any breeze is better than no breeze.
Physical Sanitation
We know that novel coronavirus can be transmitted on surfaces. We know that washing hands and avoiding touching your face can help prevent disease transmission. Keep doing those things.
The Data
Some folks at MIT have created a calculator which you can use to put actual numbers on actions such as I describe. If you are like me, knowing that something is “better” is not good enough. You want to know how much better.
The MIT COVID-19 Indoor Safety Guideline is an interactive tool which calculates “how much.” With the data from the calculator, you will be better able to decide what protects your health and what is too much effort for the benefit.
The Indoor Safety Guideline takes information about your house/apartment and about what the people in it are doing. It then calculates how long it should be safe to be together. For example, I entered this information:
- Room Specifications
- Suburban house (which pre-selected a bunch of values) and then I changed a few values to more closely match my own home:
- 2600 square feet (total square footage of my home, including the first floor and the basement)
- 8 foot ceiling height
- Hospital & General Surgery filtration system (which is a MERV 14 filter, because that calculates the same results as a MERV 13 filter and it saves me the trouble of using the “advanced mode” of the calculator)
- Human Behavior
- I changed just two values from the defaults:
- Speaking in a normal voice expiratory activity
- N95 mask type
I kept the default risk tolerance of 0.10. That value means that, if someone is contagious during the visit, and we visit for the maximum amount of time, then there is a 0.10 chance (10% chance) of someone else catching Covid-19.
The MIT COVID-19 Indoor Safety Guideline calculated this:
Based on this model, it should be safe* for this room to have:
2 people for 13 days
3 people for 6 days
4 people for 4 days
5 people for 3 days
10 people for 34 hours
For comparison, I changed the mask type from N95 to none. The results changed to this:
Based on this model, it should be safe* for this room to have:
2 people for 17 hours
3 people for 9 hours
4 people for 6 hours
5 people for 4 hours
10 people for 2 hours
I tried another scenario. I changed the mask type back to N95 and reduced the risk tolerance from 0.10 (10%) to 0.01 (just 1% risk of transmission). The results changed to this:
Based on this model, it should be safe* for this room to have:
2 people for 30 hours
3 people for 15 hours
4 people for 10 hours
5 people for 8 hours
10 people for 4 hours
One more scenario, to illustrate how risky it is to return to our pre-pandemic habits. I changed the ventilation system to closed windows and reduced the filtration system to a MERV 6 filter. Then I changed the human behavior to standing, the expiratory activity to loud speech, and the mask type to none. I reset the risk tolerance to 0.10 (10%). The results changed to this:
Based on this model, it should be safe* for this room to have:
2 people for 2 hours
3 people for 1 hour
4 people for 1 hour
5 people for 1 hour
10 people for 1 hour
Youch!
You can play with the MIT COVID-19 Indoor Safety Guideline and use it to find the ways in which you might invite someone into your home this winter, or to decide whether you will visit someone else’s home.
*Should Be Safe
The asterisk on “should be safe” means the calculation is based on airborne transmission only. It is still important that you wash your hands and avoid touching your face.
References
Mounting evidence suggests coronavirus is airborne — but health advice has not caught up. Nature, July 8, 2020.
How to Keep the Coronavirus at Bay Indoors. New York Times, September 27, 2020.
Researchers created a test to determine which masks are the least effective. CNN, August 8, 2020.
