Some recent virology and aerosol science research^[1][2] might support an ever-so-slightly higher real cost of atmospheric $CO2$ and, more practically, an even stronger case for ventilation indoors with respect to biosecurity and pandemics. 

Basically, ambient $C{O}_2$ concentrations have a direct effect on the duration that aerosolized droplets containing SARS-COV2, and probably some other pH-sensitive viruses, remain infectious. This is due to the presence of bicarbonate in the aerosol, which leaves the droplet as $C{O}_2$. Consider the following equation^[2] and then recall or review Le Chetalier's principle from chemistry.

$\begin{array}{}{H}_{\text{(aq)}}^{+}+HC{O}_3^{-\text{(aq)}}\leftrightarrow H_{2}C{O}_{3\text{(aq)}}\leftrightarrow C{O}_{2\text{(g)}}+H_{2}O& \text{(1)}\end{array}$

More $CO2$ in the surrounding air shifts the equilibrium to reduce the net loss of $C{O}_2$ from the aerosol, slowing the rate at which the pH increases, thereby slowing the rate at which the aerosol loses its infectivity (this virus doesn't do well in a high-pH environment).

For getting an idea of the magnitude of the effect, Figure 2B^[2] and its caption are simple and illustrative: "The effect that an elevated concentration of CO2 has on the decay profile of the Delta VOC and original strain of SARS-CoV-2 at 90% RH. Inset is simply a zoom-in of the first 5 min of the x-axis. Elevating the [$C{O}_{2\left(g\right)}$] results in a significant difference in overall decay assessed using a one-sided, two-sample equal variance, t-test (n = 188 (independent  samples)) of the Delta VOC from 2 min onward, where the significance (p-value) was 0.007, 0.027, 0.020 and 0.005 for 2, 5, 10 and 40 min, respectively." Other figures show differing results for other variants which seem to have different levels of pH-sensitivity.

This acts in addition to - and is not to be confused with - the generally more important (as far as I know) fact that indoor $C{O}_2$ readings serve as a proxy for proportion of rebreathed air and thus aerosol concentrations in the absence of active air filtration. 

An interesting research direction would be to look at likely future pathogens and try to make predictions about how likely they are to be pH-sensitive in a way that would make them extra vulnerable to better indoor ventilation. Apart from that, there's no big call to action here other than the afformentioned small update to your mental model of indoor ventilation. 

1. ^{^}

   The dynamics of SARS-CoV-2 infectivity with changes in aerosol microenvironment

2. ^{^}

   Ambient carbon dioxide concentration correlates with SARS-CoV-2 aerostability and infection risk

Thanks for writing this, I really appreciate your insight. If or whenever it wouldn't cost you too much time, I think the other of the 10 best economics of AI papers from the past few years could be a useful compilation for people.

I think there hasn't been enough research on iota-carageenan nasal sprays for prevention of viral infection for things more infectious than common colds. There was one study aimed at COVID-19 prophylaxis with it in hospital workers which was really promising: "The incidence of COVID-19 differs significantly between subjects receiving the nasal spray with I-C (2 of 196 [1.0%]) and those receiving placebo (10 of 198 [5.0%]). Relative risk reduction: 79.8% (95% CI 5.3 to 95.4; p=0.03). Absolute risk reduction: 4% (95% CI 0.6 to 7.4)."

There was one clinical trial afterwards which set out to test the same thing but I can't tell what's going on with it now, the last update was posted over a year ago. So we have one study which looks great but could be a fluke, and there's no replication in sight.

The good thing about carageenan-based products is that they're likely to be safe, since they're extensively studied due to their use as food additives and in other things. From Wikipedia: "Carrageenans or carrageenins [...] are a family of natural linear sulfated polysaccharides. [...] Carrageenans are widely used in the food industry, for their gelling, thickening, and stabilizing properties." See this section of the article for more.

If it really does work for COVID and is replicated with existing variants, that's already a huge public health win - there's still a large amount of disability, death and suffering coming from it. With respect to influenza, theres's some evidence for efficacy in mice and the authors of that paper say that it "should be tested for prevention and treatment of influenza A in clinical trials in humans." 

If it has broad-spectrum antiviral properties then it's also a potential tool for future pandemics. Finally, it's generic and not patented so you'd expect a lack of research funding for it relative to pharmaceutical drugs.

Do you think this would still occur in a parallel strategy where you simply sell both high-isoflavone and low-isoflavone options without marketing the low-isoflavone option explicitly? Word of mouth could work for exposure and if it did make someone who was otherwise unconcerned about isoflavones become concerned they could simply switch over to the low-isoflavone option?

I'm personally not super concerned about them either but I think the cultural connotations about soy being feminizing might be deep enough that many people won't be swayed and would rather not have to think about it if there was an option not to. Many people are understandably sensitive about physical effects of dietary changes and especially so for anything which has to do with the endocrine system and doubly so for the endocrine system with respect to sex hormones. 

(Full disclosure here: a potential source of personal bias here is having been screwed over by something where the folk wisdom concern about something turned out to be true as opposed to the what many more well-respected health opinions online told me.)

Thank you for writing this - I'm working on a post going over how much cheaper someone could make air purifiers for and it surprises me that it's not a more common topic of discussion. Some food for thought while I finish it up:

1. Indoor air quality affects so many people to at least some extent - consider air pollution, viruses, allergies etc. 
2. Making air purifiers even slightly cheaper vastly increases the number of people globally who can afford one, and directly increases the cost effectiveness of any intervention which involves paying for them. 
3. Noise is a common reason for people under-utilising air purifiers and the affordable end of consumer hardware hasn't solved for this yet. We know this because best-in-class clean air delivery rate (CADR) at a given noise level can be achieved with what is essentially a box with 2-4 air filters and some computer fans on the side (computer fans have become remarkably capable at low noise levels in recent times). These kits can be bought but minimal competition in the space means no one is anywhere close to the reasonable price floor.
4. Competition in the air purifier market has partially been on features which are not necessary when the goal is optimizing CADR/$. Ionization, timers, remote control, app connectivity, odour removal etc. can be done away with for the purpose of achieving "one billion air filters in this decade" or anything of similar scale.

It almost seems too simple: the many things floating around in the air cause a huge amount of death, illness and general discomfort. If you push enough air through a fine enough filter you remove the stuff in the air. If you make the filters cheap and quiet enough, people will be able to buy them and we can send people more of them for the same price.

Of course the air quality problem with respect to pollution is obviously something much more difficult to solve than simply chucking air filters everywhere since people also have to be outside for much of their day. 80,000 hours podcast 170 "Santosh Harish on how air pollution is responsible for ~12% of global deaths - and how to get that number down" is a great introduction. But regardless, people ought to be able to have some refuge somewhere, and indoor filtration and wearing a mask are the only ways is the only way someone can individually guarantee that for themselves. 

Great examples there, thank you for commenting!

And I agree with the other point as well- it's a one-two punch in the sense of the lack of safety net pushing away certain groups of people and thereby also biasing the type of work done away from what would be otherwise optimal.