By now we hopefully all know that it is better to be outside than inside when it comes not catching the COVID-19 from other people. You’ve probably also read stories like the famous (infamous) one about how certain patrons in the path of the air conditioner at a restaurant in China got sick, while ones at the other tables didn’t. Incidents like that one have resulted in pretty much shutting down entertaining non-family-members inside, at least in many places. But eventually, that is going to change. Which begs the question of how safe will it be, and how can we make it as safe as possible.
Many medical and commercial establishments are either designed with, or are now being remodeled with, sophisticated HVAC simulators to try and optimize the direction and speed of airflow to maximize safety. In particular, if air is either vented outside or recirculated through HEPA-grade filters, the faster you get it from people and out of the room the better. But that high-end design approach isn’t practical for a typical family and their apartment or home.
So, while it absolutely isn’t a medical-grade or even commercial-grade solution, we took the DIY approach to experiment with a combination of built-in HVAC and portable air filters to see if we could tell how (relatively) “safe” it might be to sit around our dining room table with friends someday, and how best we could prepare for that eventuality. We used an inexpensive fog machine (about $40 online) to create a simulated virus-carrying “cloud” for our tests. It’s not perfect; our setup certainly doesn’t model the extremes of sneezes at one end and potentially aerosol virus transmission at the other. Our approach isn’t really even commercial-grade, because cheap special effects “fog” doesn’t accurately mimic particular droplets or aerosols the way a high-end device costing thousands can. However, we figured that it was a lot better than simply guessing where the air we breathe goes after you exhale in your home or office.
Our Test Scenarios
We did most of our tests around the kitchen table, the most likely place in our home where we’ll be likely to eventually have non-family guests — once that makes sense. As a baseline, we tested with all the doors and windows closed, and no artificial air circulation. Then we turned on our whole-house fan (basically AC without the cooling), to get a sense of how much it would move the air around, and started experimenting with air filters.
Baseline: No Fans or Filters, Windows Closed
As a baseline we started with a closed room, with no AC, fan, or portable air filters:
You can see that the “fog” hangs in the air for several minutes. To reiterate the main caveat with our test methodology, actual droplets would have fallen to the ground much more quickly, even if they weren’t blown away. However, aerosols would have continued wafting around in fairly high concentrations.
Using What You Have: Whole-House Fan
Most homes in the US have some type of HVAC that in addition to heating and cooling can be used simply as a large fan. In many cases, like ours, the air returned to the system is passed through a HEPA filter and cleaned before being sent back into the room, making it a good candidate for an air-moving solution.
So we turned our whole-home AC system onto fan mode to see the effect:
Due to the design of the room, there aren’t any vents or returns in the immediate vicinity of the table. But there are vents in the ceiling a few feet away and a return about 12 feet away. However, the natural air path doesn’t flow across the table. The fan did cut the time the fog hung over the table roughly in half, so it definitely had an effect. Note that some HVAC units run their fans at a higher speed when you turn them to Cool or Heat mode instead of just fan mode, which means they would move the air slightly more quickly in those modes.
Home Air Filter, Placed on Floor
An obvious step for anyone wanting to improve airflow is to add a fan or portable air filter. The advantage of a HEPA-rated air filter is that the air passed through it will have been cleaned, so it doesn’t have to be vented to the outside. With a simple fan, you need to make sure it moves the air away and eventually outdoors. So we took a top-rated Coway 1512HH ($200 on Amazon) home air filter that we bought a few years ago thanks to the California wildfires, and placed in a typical location on the floor next to the table for our next experiment:
Home Air Filter On the Table in Auto Mode
Clearly the floor location doesn’t take full advantage of the air filter’s potential. So we moved it to the table, set to Auto mode. That might be an actual possibility for a home. For a restaurant, it might be similar to the effect of having a built-in air filter somewhere near or above the table:
As you can see this both increases its effectiveness in moving air, and means that much more of the air is actually filtered instead of simply being pushed around. In both cases, though, you can also see that a certain amount of the fog simply gets entrained in the fan exhaust and pushed up (the direction of the exhaust) even though it wasn’t directly affected by the fan.
Home Air Filter On the Table in Full-Power Mode
You can see in the last video that one issue with “Auto” mode is that the air filter doesn’t kick up to high power until it detects particles. Best case, that means a delay until enough particles reach it. Worst case, if everything is only aerosols, it might not trigger its High mode at all. So we did the same experiment again, but this time we set the air filter to full power from the start, to get it moving air more quickly:
Preset to High and set on the table, our room-sized air filter did a quick job of clearing and cleaning the air.
Upgrading to a Larger Air Filter: The Coway Airmega 400S
Next, we upgraded to a larger-capacity Coway Airmega 400S ($652.49 on Amazon), with a theoretical ability to filter the air in a 1,500-square-foot area. (Note: Filter ratings typically mean that the unit can move the volume of air in a room that size with an 8-foot ceiling a few times per hour — which of course isn’t directly useful as a measurement of how effectively it will move air from one particular location.) To start, we placed the Airmega on the floor next to the table:
The Airmega 400S definitely improves on having the smaller unit in the same location, although a lot of its effect is also simply by pushing air up from its exhaust port, rather than pulling it through its side intakes and cleaning it.
Going Whole Hog: Large Air Filter on the Table
For what we figured was the maximum intervention, we moved the Airmega 400S right onto the table, and set it to High. Coway has done a great job with the sound suppression on the unit, so it actually wasn’t that loud (the firm rates the unit at 66 dB, but doesn’t specify in which mode). I could imagine having dinner with it there (although perhaps on a small table next to the big table, instead of at the head of the dining table!) The results were impressive:
You can see that if you’re willing to get a little bit extreme, you can clear the air from a group of people around a table quite quickly. Of course, this arrangement isn’t incredibly practical. But I can imagine some clever designs of overhead lights or tables that incorporate some form of air moving and filtration.
Using Cross-Ventilation to Let Nature Help
Depending on your room layout and the weather outside, simple cross-ventilation can be a big help. In our case, we have sliding glass doors about 10 feet away from the table on either side. We opened those up while there was a decent breeze and re-ran our experiment:
Clearly just opening doors and windows isn’t enough to clear the air quickly unless you have a direct path for the air to travel and a breeze that will take advantage of it. It does clear the air over the course of a few minutes, though. So if you’re using a table or work area after someone else and are worried about what might be hanging around, having some cross-ventilation would be a great idea.
Here’s Why Being Outside Is So Great
Finally, to illustrate how powerful nature is we moved outside, into a breeze. Even aiming the fog generator directly across the table, with the breeze blowing the length of the table, the air clearly extremely quickly:
Note that if someone coughs across the table at you (or at the person next to you and you are downwind of them), there is still some noticeable, if brief, airflow to where you are before the breeze pushes it away. Heavier droplets might also get pushed away more slowly.
First, I’m happy we did these experiments. It gives me a better sense of airflow than I’d had from reading and watching videos — and of course, it is specific to our house. Second, I can easily imagine setting up an air filter near our kitchen table at whatever point we feel safe having people over for dinner inside. As to a choice of air filters, we continue to be impressed by both Conway units. The company also offers a 300 model that is sized between the two, and a non-S version of the 400 that skips the “Smart” bit. Short of adding a portable air filter, turning on our fans makes sense, and obviously keeping doors and windows open to the extent the weather allows.
We still know so little about how COVID-19 actually moves through the air, and how long it persists in a harmful form on surfaces, that there isn’t yet any way to tell how much safer each of these tactics makes us, unfortunately. The experiment has given us some ideas for future tests, where we use different types of “fog” and also some baffling or other controls on the speed, timing, and direction of the generated fog.
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