It's becoming clear that the SARS-CoV-2 virus will be around for a long time. We're probably much closer to the beginning than the end. Hotspots will shift and new variants arise so it makes sense to be ready to mitigate risks as much as possible. Vaccinating the world should be a high priority along with next generation vaccines, better treatments, testing, high quality masks when the risk is high, etc. etc.
Improving local air quality is low hanging fruit. To first order that means high quality masks in hotspots along with fresh air. Room air filtration and circulation can be a powerful tool. As living animals we exhale carbon dioxide. In pristine location, say a mountain top in Hawaii, you can measure the baseline concentration. Air is about 420 ppm (parts per million) carbon dioxide - a bit over 0.04%. Enclosed spaces with people have higher concentrations. You're breathing some of the air from other people. If people are the only source, you can calculate the percent of the total air in the room that's exhaled:
CO2 concentration (ppm) % of total air that is exhaled
420 0%
600 0.5%
800 1.0%
1000 1.5%
2400 5.0%
With these numbers it was believed the risk is low for up to an hour if everyone was wearing a simple cloth mask and one person was infected with the alpha variant with carbon dioxide concentrations as high as 800 ppm. The delta variant changes the threshold. Now the one hour number is about 600 ppm with everyone wearing a surgical mask.1 Of course the probability of an infected person being in the room is low if you're in an area where the infection rate is low.
It would be good to know the concentrations where ever you go: offices, classrooms, stores, theaters, taxis, airplanes etc.2 Consumer grade carbon dioxide meters can be purchased for $100 to $300. If you're really interested I can make a recommendation, but I've only used a few. We'll probably see a rapid improvement as these are going to be essential tools. Japan is requiring them in many public places and school rooms.3 It's important to have an NDIR sensor (basically it's doing spectroscopy with an infrared laser to measure concentration - there are many other techniques, but this is the cheapest with enough accuracy and repeatability). These are accurate to about 50 ppm, so ignore the more exact looking reading and just round up to the next multiple of 50 - if it reads 537 just call it 550. It takes about two minutes to get a stable reading and you'll want one that stores measurements. The model I like talks to an iPhone app which gives you a nice chart of the day. For businesses and schools it's a good way to check if the rooms are getting good airflow. I saw one schoolroom that exceeded 3000 ppm when students were present. It turned out an air return duct was blocked - a thirty minute fix gave the students clean air again.
High carbon dioxide concentrations make people drowsy and sap cognitive abilities. It hasn't been carefully studied, but readings over 1000 ppm can make you drowsy and perhaps a bit feeble minded. One Army study suggests 800 ppm should be a warning level. Half-jokingly I told a superintendent that he could justify the cost of measurement though improved test scores. It turned out that's what sold him. Maybe it's a way to upgrade the collective intellectual capacity of an office. Another benefit is a possible reduction in airborne diseases like the flu.
You can't always open windows and doors to get a solid draft, but HVAC systems can often have their fan settings increased and room vents adjusted. High quality filters can capture viruses along with other particulate like some from a fire. MERV-13 is a sweet spot for price, the ability to snare virus sized particles and not requiring more powerful fans. And you can buy or build portable air cleaners.
The hot ticket for rolling your own is a very simple design known as the Corsi-Rosenthal cube. Here's the public domain diagram we used to build 84 of them for about $5,000 (the cost shown in the drawing doesn't involve much shopping around). We used the cardboard from the shipping boxes the fans came in. The shroud is very important. Square corners disturb the airflow and can drop the unit's efficiency by about a third. A normal sized classroom for 30 to 40 students probably needs two units. Keep them away from walls, doors, and openable windows. The filters will probably last the school year if you turn them off when people aren't in the room. Most of the classrooms are showing readings under 600 ppm with these and the school's HVAC system fans all on high. The boxes make a big difference. Commercial air cleaners with good filters probably make more sense in offices and schools if they want to go long term, but 600 cfpm MERV-13 filters are about $500 and in short supply these days. Like carbon dioxide meters I suspect we'll see some movement on air filter and fan design.
Some learnings:
Duct tape is frustrating!
Schools have mountains of duct tape.
Eighth graders are the perfect people to enlist if you want to build a lot of them. Eighth grade girls if you want them to be semi-presentable.
_________
1 These are rule of thumb numbers based on empirical evidence. But it's complex with many confounding factors.
2 A closed car can be spectacularly high. I sat in my car for fifteen minutes with one other person and recorded 3800 ppm! You can drop this by opening the windows. Opening them diagonally makes a lot of sense for creating airflow. Airliners have good general circulation, but a given row can be bad. A flight is probably much safer than a half hour Uber ride if the driver has the windows up.
3 People are already taking measurements in gyms, restaurants, etc and putting them online with locations and business names. It's ad hoc and not very standardized, but there are over 100 locations in NYC the last time I checked. It makes much more sense for a business to do their air handling homework and post numbers in their establishment as well as their website. Hopefully at some point a respectable aggregator will step in.
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