Installing air filters in classrooms has surprisingly large educational benefits (2020)
From the paper:
To do so, I leverage a unique setting arising from the largest gas leak in United States history, whereby the offending gas company installed air filters in every classroom, office and common area for all schools within five miles of the leak (but not beyond). This variation allows me to compare student achievement in schools receiving air filters relative to those that did not using a spatial regression discontinuity design.
In other words, the paper looked at test scores at different schools in different areas on different years and assumed that the only change was the air filters. Anyone who has worked with school kids knows that the variations between classes from year to year can be extreme, as can differences produced by different teachers or even school policies.
Again, I think air filtration is great indoors, but expecting test scores to improve dramatically like this is not realistic. This feels like another extremely exaggerated health claim, like past claims made about fish oil supplements. Fish oil was briefly thought to have extreme positive health benefits from a number of very small studies like this, but as sample sizes became larger and studies became higher quality, most of the beneficial effects disappeared.
All research is met on HN by people who know better and will tell you why it's flawed. There isn't a greater collection of expertise in the history of the world than on HN.
Edit: I meant to add: What value can we find in this research? It wasn't published as scripture, the perfect answer to all our problems. It's one study of some interesting events and data; what can we get out of it?
Thinking critically includes, most of all, finding value - you need to think critically (and skeptically) to avoid assigning value to things that don't have it, but you must find value. The goal is to build knowledge - just like the study author needs to find knowledge among flawed data, you must find knowledge among flawed studies - and they are all flawed, of course.
Focusing on the flaws and trying to shoot down everything is just craven recreation.
This money and time is taken directly away from funding other, potentially more worthy or more likely to be correct studies.
There is no point of looking at every (flawed) study in the most positive way, unless you have unlimited time and money to pursue every avenue of research.
Often (not always), the studies that are most heavily promoted among the news and in business or politics are really not the best research and other, less visible but more solid research gets ignored in favor of whats popular or what has had good marketing.
This is very frustrating for people doing solid good research, because every so often someone else will come along with wild, exaggerated claims and very little data to back it up, and then gets funding for it.
It takes literal years away from good science just because someone markets and speaks well.
Which is fine in business, but in science this is not something "the market" can or will correct for well, simply because the timespans are so long.
There is no point of looking at every (flawed) study in the most positive way
This line epitomizes the nonsense in the discussion. I didn't say every study, you can't know it's flawed without seriously examining it, and I didn't say in the most positive way at all.
By using these exaggerations, you damage any serious discussion - you give people nothing to respond to except your emotional state.
What I said was, the point is to build knowledge, and so the way to examine research is to find the valuable knowledge - which includes evaluating the accuracy, etc. of that knowledge. There's no other point to it - we're not awarding tenure here, so there's point in keeping some overall score. We just want to learn what we can.
Reading comprehension is important, and especially important in a discussion like this.
I do however really mean that some studies are not worth looking at all in more detail: if the methodology is flawed, the results are meaningless. At most the premise of such a hypothetical (not saying this one necessarily!) study could be used as an idea for further research, but not to build knowledge on or derive knowledge from the results.
My guess is if you raise examples of "good science" the HN peanut gallery will jump in to point out the flaws in that science, too.
you need to think critically (and skeptically) to avoid assigning value to things that don't have it, but you must find value.
This isn’t critical thinking.
This is toxic positivity.
It’s okay to admit that some studies don’t have value to add. If you don’t accept this, you’re going to be tricked by a lot of people trying to get your attention with bad data.
Being able (and willing!) to filter out bad sources, even when they say something you want to hear, is a critically important skill. If you force yourself and others to find something positive about everything then you’re a dream come true to purveyors of low quality or even deliberate misinfo.
some studies
It's almost every study on HN, not some studies, which you'd understand if you read my comment.
This usually happens in usually these cases:
1. when a paper is extremely good and it's results are groundbreaking, or
2. when a study itself claims it has groundbreaking results, or
3. when it's a regular study that's gotten some great marketing/promotion e.g. by their university.
The case of 1. is extremely rare, and even when everyone believed the results and they were peer reviewed by a reputable paper like Science, some of them turned out to be academic fraud that was later retracted.
Most studies that pop up on HN are of types 2. and 3. That's just because otherwise they would not get news attention.
But most studies in general are in category 4: the ones an academic or professional would read going about their daily business / research. These range from terrible, to OK, to really great, but 99% never make the news.
As a (former) academic, I've read lots of papers and like in real life it's usually the people (papers) that get attention who scream the loudest. There are some gems too of course, and it's right to not ignore anything.
But in my personal experience and over time, I've been very right to be very sceptical once a result turns up in the news because of the 3 ways it can get there.
This is amplified even more so with papers that base their results / outcome purely on statistics, such as most experimental studies done. These derive their results from the statistics (sample size, experiment design, etc) so their power and the probability of their result being correct (what the authors say) it directly coupled.
I've made this mistake time and time again, most recently with vitamin D association studies, and I'm grateful to all the people who urged everyone else to take a wait-and-see approach.
Focusing on the flaws and trying to shoot down everything is just craven recreation.
No, its a valuable job to find flaws because its much easier to fix and work on known flaws than to stumble in the dark.
Removing flaws and problems is one of the easiest ways to add value.
The real significance is that things like sample size, to pick a common example here, is easy to understand in a theoretical way and so people apply it to the actual (not theoretical) practice of real research, which they don't understand the practicalities of, and also they overemphasize it because that's pretty much all they understand.
The first thing they look at in a paper is sample size - and hey, now sometimes they have something to 'contribute'! It's just reinforcing the same misunderstandings in others.
It sucks, a little, to have nothing to contribute, but it's a great opportunity to learn from people who do know.
Based on your comment, the effect could be larger as well as smaller.
The reality of any underpowered study could always be “larger as well as smaller”. This statement doesn’t add anything to the conversation.
The mistake is pivoting around poorly structured and underpowered research.
All research is met on HN by people who know better and will tell you why it's flawed.
This is a misunderstanding. People who know how to read studies will always be aware of the limitations.
There’s a difference between saying “everything is flawed” and pointing out the limitations. Most early research comes with significant limitations like small sample sizes or large cofounders. You have to understand these in conjunction with the results to know how to interpret it.
There’s a cynical approach where people see discussion of limitations, don’t understand it, and instead go into a mode where they think it’s smarter to ignore all criticisms equally because every paper attracts criticisms.
This is just lazy cynicism, though. There are different degrees of criticisms and you have to be able to see the difference between something like a slightly underpowered study, and something like this paper where the authors threw a lot of regressions at a lot of numbers and kind of sort of claimed to have found a trend.
For example
https://onlinelibrary.wiley.com/doi/10.1111/ina.12042
https://onlinelibrary.wiley.com/doi/10.1111/j.1600-0668.2010...
Eg if you assume there is a real effect plus a lot of noise, given the study has been published etc the noise will have more likely acted in the favourable direction.
IMHO given the relatively large size of the effect it seems quite likely that the noise part is in fact potentially large (this is much more subjective) which makes is less clear that there is measurable signal at all here. I’d have to see a lot of replication or a very strong explanation of the underlying mechanism to believe the magnitude of the effect, but will very easily believe the sign (with a small magnitude).
All research is met on HN by people who know better and will tell you why it's flawed. There isn't a greater collection of expertise in the history of the world than on HN.
It seemed like a pretty valid criticism. These studies should be taken with a massive pinch of salt because they're fairly uncontrolled.
All research is met on HN by people who know better and will tell you why it's flawed
It is almost certainly flawed, and it is probably wrong: https://journals.plos.org/plosmedicine/article?id=10.1371/jo...
If you are discussing research at all it is important to discuss the flaws too. The alternative I can see would be to take every published paper as proven true even though we know this is not the case.
Based on your comment, the effect could be larger as well as smaller.
Yes, but since we know that there's a huge bias to publish and publicise larger results, you know what way to bet.
1. Why this blog post study is flawed and dumb
2. America is bad and shameful.
I would have said that the point of research is to find the value and build knowledge, while the point of discussing public affairs is to identify problems to fix.
Thinking about it, I'm not sure the latter can't find things that are constructive. But in either field, the exaggerated, dismissive comments/rants are not just a waste but damaging to progress.
You don’t need massive study to find out that kids don’t like suffocating in classrooms.
It’s a bit like mandating reversing cameras on cars. Study says economically they do not make sense, but not squishing your kids trumps that.
but I also think the extreme results from this study aren't going to hold up to further research.
there already is further research. and the results do seem to be holding up.
the study you're quoting from is the one linked in the 2nd paragraph of the article. this is from the 3rd paragraph:
But it’s consistent with a growing literature on the cognitive impact of air pollution, which finds that everyone from chess players to baseball umpires to workers in a pear-packing factory suffer deteriorations in performance when the air is more polluted.
that paragraph links to an earlier Vox article[0] which goes into more detail, and well as linking to all of the various studies:
A wide range of studies about the impact of pollution on cognitive functioning have been published in recent years, showing impacts across a strikingly wide range of endeavors. Stripe CEO Patrick Collison has taken an interest in this subject and compiled much of the key research on his personal blog. Among the findings he’s highlighted include:- Exposure to fine particulates over the long term leads to increased incidences of dementia and Alzheimer’s disease in the elderly (a second study confirms this).
- A study of 20,000 older women found that 10 micrograms of additional long-term particulate exposure is equivalent, across the board, to about two additional years of aging.
- The impacts are not limited to the elderly, however, nor are they exclusively long-term. A range of specialized professionals also seem to suffer short-term impairment due to air pollution. Skilled chess players, for example, make more mistakes on more polluted days. Baseball umpires are also more likely to make erroneous calls on days with poor air quality. Politicians’ statements become less verbally complex on high-pollution days, too.
- Ordinary office workers also exhibit these impacts, showing higher scores on cognitive tests when working in low-pollution ( or “green”) office environments. Individual stock traders become less productive on high-pollution days.
- The same also appears to be true for blue collar work. A study of a pear-packing factory found that higher levels of outdoor particulate pollution “leads to a statistically and economically significant decrease in packing speeds inside the factory, with effects arising at levels well below current air quality standards.”
- Last but by no means least, the cognitive impacts appear to be present in children, with a Georgia study that looked at retrofits of school buses showing large increases in English test scores and smaller ones in math driven by reduced exposure to diesel emissions.
0: https://www.vox.com/future-perfect/2019/12/11/20996968/air-p...
there already is further research. and the results do seem to be holding up.
You’re conflating different results.
The results of the headline study are dramatic in ways that aren’t holding up. The test score increase happened after a single year of putting air filters in class rooms. That’s minimal exposure to purified air for a fraction of the day, 5 days a week, for less than a year of classes.
The other studies are much longe term and look at things like decades of exposure at city scale.
Do you see the difference? The study tried to claim that purifying air immediately improved test scores by dramatic amounts.
There are other serious limitations in the study, like the fact that they can’t even identify which air purifiers were installed or how effective they were. There’s a footnote that says many weren’t even used. There’s a section on air quality monitoring that says they didn’t even detect the VOCs they were trying to filter before they started filtering.
This is the type of study that people implicitly believe because it makes logical sense, but when you read the details you realize that there isn’t much substance in it.
The other studies are much longe term and look at things like decades of exposure at city scale.
Are they? Several towards the end of the list are specifically about short term day-to-day effects across a range of situations.
There’s a footnote that says many weren’t even used.
Quite damning if true.
they didn’t even detect the VOCs they were trying to filter before they started filtering
That is confusing the reasoning that led to the company footing the bill versus what was being looked at here. The lack of VOCs is actually in the author's favor as it eliminates "exposure to atypical VOCs" as an otherwise fairly severe confounding factor.
However, I don't see that it proves pollution is the cause. What about infection? Air filtration can reduce the spread of pathogens. Schools throw together a large pool of people, the bug of the day will go around. Less if there are good filters.
As for painting over asbestos--that's actually considered acceptable. Asbestos sitting there isn't going to hurt you. Asbestos only hurts you when it's disturbed. Removing intact asbestos is likely to increase your exposure, not decrease it.
As for cancer--why are you so sure it's chemicals and not lifestyle?
To me its like looking at schools that buy newer buses and trying to show new buses improve test scores. When in practice the only schools that are buying any significant number of new buses have far more money coming in than in the past and have a lot more to spend on students compared to other schools, which is way more relevant than what year a kid's bus is made. Maybe better buses would improve scores too, but there is no way to tell if 95% of an improvement is due to other unrelated factors based on funding.
A better method would be to just give some schools air filtration for free and see if things help without any other major changes.
This is what the study looked at.
The problem was that it wasn’t randomized within schools or across teachers. They also looked at a very limited time window. They also note that some teachers weren’t using the air filters. They also found that the VOCs they were trying to filter weren’t even detected before the filters were used. They also used some questionable regressions to imply larger trends.
The list of problems goes on and on. It’s fascinating how easily people are tricked into pivoting around this one study, though, simply because it’s the one introduced by the headline.
They also looked at a very limited time window. They also note that some teachers weren’t using the air filters.
To correct and clarify:
They also looked at a very limited time window. They also note that-- after this very limited time window-- some teachers stopped using the air filters. In the words of the authors, this made long-run results "difficult to interpret."
It's the classic German Stoßlüften in action.
However, the air outside also has lots of dusts. Opening the windows does nothing for that, but air filtration can potentially help.
Would you support a small study saying a medicine or a vaccine produces a 20 year life expectancy increase, all that to end up 20 years later with no improvement, everyone on that medicine, and the anti-everything yelling on every platform that the big pharma lobby poisoned our children ?
Even when the studies are on large samples, double-blind, long time range with a clear explanation as to why there's an effect, we have people trying to kill the resulting health campaigns. Don't encourage fake ones !
but expecting test scores to improve dramatically like this is not realistic
That seems like a problem for the reader, not a problem with the text. Why would the reader expect this? Is it the use of present tense in the title rather than past tense?
The smoking gun is really in Table 3 and Table 4, where you can see that the effects that were observed are compatible with a population effect of 0, or alternatively you can look at Figure 2 and note that you could draw a straight line (no effect) within the confidence bands. Doesn't mean the effect is not there, but that there's insufficient evidence that it is, and that we should indeed be very careful about taking the estimates at face value.
Imagine if some schools installed air conditioning in their gym one year. Running times around an indoor track would improve considerably, but mostly because conditions at the point of testing improved. Not necessarily because the air conditioning made the students actually improve their stamina or speed.
Maybe the tests results are better because the children are more rested on the day of the test. Maybe the hum of the machine creates some kind of meditative noise that helps children concentrate. Or maybe none of that is true..
IQ has about a 0.2 correlation with income. The paradox arises when you zoom out for a more macro view. National IQ has about a 0.6-0.8 correlation with GDP per capita.
Performance in class rooms is definitely an IQ thing and different view points will likely generate different sets of data.
I'm interested in seeing controlled trials on individual performance, not just observing real world scenarios.
We seem to know:
- Elevated CO2 in rooms impairs cognitive performance.
- Elevated CO2 in submarines, at levels far higher than you would see in a normal room does not appear to impact cognitive performance.
- Installing carbon filters (what this study actually looked at) might improve classroom performance.
- People don’t like stuffy rooms.
All this is consistent with multiple hypotheses. It could be that we just don’t know anything about it. Or maybe there is some gas or gasses emitted by people that isn’t CO2 that makes people mildly uncomfortable and have worse cognitive performance.
CO2 is certainly a good proxy for ventilation quality in a space where air is exchanged with outdoors but where the gasses in the air are not otherwise changed. Carbon-filtered classrooms and submarines are not examples of this.
Many of those gasses do impact cognitive performance. It's not obvious to me why CO2 would, but if CO2 is going up, so is everything else we breathe out. CO2 where I am is somewhere in the ≈400ppm-1000ppm range -- 0.04% or 0.1% -- and it's pretty inert. I'm not sure what harm it does.
If it does to harm, rising CO2 levels should be much more concerning than "just" climate change.
But I suspect it's other gasses.
1) CO2 levels have risen from under 300PPM in 1860 to over 400PPM right now -- by around 150PPM -- with a rise of about 25PPM per decade for the past four decades.
The difference in CO2 levels in my bedroom with windows open and closed is a couple hundred PPM (500-800ppm range in my bedroom, with windows open and closed, respectively). I can definitely feel a difference in performance if I don't let in fresh air. It's more than climate change (300ppm versus 150ppm range), but not big-O more, and climate change is on-track to get there in another few decades. Conference rooms might be over 1000ppm, but it still big-O similar.
2) CO2 levels are measured in parts-per-MILLION. That argument simply doesn't make sense. The atmosphere is 21% oxygen. Crowding out oxygen is simply not an issue. Critically, from personal experience, if I have some dry ice in a room, I generally don't suffer.
People run into problems when CO2 levels reach a out 5000 ppm over many hours. Even the most dense conference rooms don't hit that.
1. Direct effects on brain physiology: When CO2 levels rise in the blood (hypercapnia), it causes vasodilation of cerebral blood vessels. While this initially increases blood flow, sustained elevation disrupts the brain's normal pH balance, affecting neural function.
2. Acid-base imbalance: Elevated CO2 in the blood forms carbonic acid, decreasing blood pH. This acidosis affects enzyme function, neurotransmitter activity, and neuronal excitability throughout the brain.
3. Oxygen displacement: While not typically reaching dangerous levels in standard indoor environments, higher CO2 concentrations can slightly reduce oxygen availability to brain tissues in enclosed spaces.
4. Inflammatory responses: Research suggests prolonged exposure to elevated CO2 may trigger low-grade neuroinflammatory responses, potentially impairing cognitive processes.
5. Disruption of neurotransmitter systems: CO2-induced acidosis appears to affect several neurotransmitter systems, particularly GABA and glutamate, which are critical for cognitive functions like attention, memory, and decision-making.
Studies have shown measurable cognitive effects at CO2 concentrations as low as 1,000 ppm, with more significant impairment at 2,500+ ppm - levels commonly found in poorly ventilated meeting rooms, classrooms, and offices.
As for indoor vs outdoor:
Rising atmospheric CO2 levels from global warming don't pose the same cognitive risks as elevated indoor CO2. While indoor environments can reach 1,000-5,000+ ppm, causing measurable cognitive decline through mechanisms like acid-base imbalance and neurotransmitter disruption, global atmospheric CO2 is only about 420 ppm. Even with projected increases to 500-1,000 ppm by 2100 in worst-case scenarios, these levels remain below thresholds for significant cognitive impairment. Our bodies can also better adapt to gradual atmospheric changes compared to rapid indoor CO2 accumulation, making climate change impacts the primary concern rather than direct cognitive effects.
If crowding out O2 was relevant why do I feel the same on our local mountain at 10,000'+ vs 0'- in Death Valley? To crowd out that much O2 with CO2 would be lethal. (That's not to say that my performance is the same. There's a big difference in the heart rate I can sustain.) What we feel is the CO2 level in our blood rising because it isn't diffusing into the lungs. Lowering O2 is only detectable with training and that's based on noting the symptoms of oxygen deficiency on brain function. (Such training is relevant in the world of aviation where it might give warning that you need to grab that oxygen mask. The average person will never encounter such conditions, nor have the resources at hand to make use of the knowledge even if they did realize it.)
Elevated CO2 in submarines absolutely impairs performance. One example: there was a guy on my boat who got migraines when CO2 got too high - he was useless. Luckily the fix is simple - just turn on another CO2 scrubber.
There's nothing special about a submarine that makes CO2 somehow different than anywhere else.
I wonder if sorbent quantity correlates with performance
https://pubmed.ncbi.nlm.nih.gov/29789085/
I’m definitely not an expert.
And HEPA filters don’t really scrub CO2 that effectively, even with a carbon membrane, so can we really expect lower CO2 levels?
Considering this, what’s the actual takeaway here? Cleaner air (dust/virus free?) is better for productivity?
I am asking because I want to buy the same filter for work, but I am doubtful that the $700 HEPA filter sounds like the same filter they used, even though the article mentions they used readily accessible 5 stage filters.
Most of the stuff for sale as “carbon” filters has too little carbon to do much. You want quite a lot, and, for some gasses, you can get special impregnated carbon or other media.
Air filter AC is specially prepared, so making your own DIY charcoal (while possible) isn't nearly as effective. You can mostly regenerate/desorb the carbon by heating it to 400F. Never use your regular oven though, not least of which because it reeks. Get a cheap toaster oven and disposable foil pans and do it outside, in a location where it can't possibly catch anything on fire.
Or you can buy from one of the most reliable e-commerce sites in the US:
https://www.mcmaster.com/product/3722T98
https://www.mcmaster.com/product/2169K16
You’re on your own as to buying the correct size.
For what it’s worth, the IKEA carbon filters are inexpensive and appear to be legit.
As a result, the daycare got a grant to get N-95 air filters installed and those UV lights that Chinese restaurants tend to have in the bathrooms. One per room.
What. A. Difference.
The infants and kids coming up are not nearly as sick, and when they do get sick, it's not nearly as terrible. The RSV vaccine has also been a godsend.
I can't really tell/feel what what the silver bullet here, but the combinations have been amazing. So much so that we got them for the house.
Ref: https://en.wikipedia.org/wiki/Respiratory_syncytial_virus_va...
Normally, I use this clinic in Thailand to compare prices for vaccines: https://www.thaitravelclinic.com/cost.html
The RSV vaccine is crazy expensive, even more than the HPV one. Over 200 USD is incredibly expensive in a developing country (like Thailand).
> those UV lights that Chinese restaurants tend to have in the bathrooms
The only disinfecting UV lights I've heard of for use in occupied areas are hanging things that only point up where nobody's going to be exposed. I have heard of some research for safe UV lights that are high up in the UV, they actually still burn but without enough penetration to get through the dead skin layer.
However, if these results were observed in grades 3 or higher, it could suggest a more substantial phenomenon. I randomly picked the third grade, but perhaps there’s a specific age after which the medical community considers a child’s immunity to be significantly enhanced.
I exaggerate a bit, but I found that during covid, where the mask was mandatory in my place, I was never sick. The only few years in my life where I was actually healthy continuously for YEARS, I and my friends could not believe the impact of the mask. But then we were stuck at home, living in constant misery and stress.
Once the masks disappeared, finally we could live again, and got extremely sick the first few years... Maybe a more normal balance would have been better ? Sick a bit continuously ? I think trying to avoid sickness is like trying to swim against the current, nature just works that way.
A good overview of the allergy situation is here: https://en.wikipedia.org/wiki/Hygiene_hypothesis
Though that article goes into more about young human immune systems too. TLDR: Yeah ... it's complicated, but the evidence seems to, just maybe, lean towards more exposure being better. So, you're right, I think?
Imo we're way overdue standards and controls for clean indoor air that are on par with standards for drinking water and food. Like this article shows, we have the tech to provide clean air today. All we're missing is policy to uniformly deploy it.
[1] https://www.sciencedirect.com/science/article/abs/pii/S01966...
[2] SARS-CoV-19: ~75% of sick days, Influenza: ~10% of sick days
[1] https://www.ekathimerini.com/economy/1255154/greek-work-ethi...
In studies of pollution and impacts on health, the confounding factors often have a larger impact on the health outcome than the pollutant, such as particulate level, and therefore significant control of confounders is required to estimate any impact on the health outcome. The strong effect in this study is highly suggestive of a confounder rather than a real effect from particles or other pollutants and therefore would require a much better study design to support tacking action at a policy level with an expectation of a huge impact.
Fine to put filters into improve overall air quality but just not with the benefit rationale suggested in this study.
For a sense of scale, Mathematica Policy Research’s best evidence on the effectiveness of the highly touted KIPP charter school network finds that after three years at KIPP there is significant improvement on three out of four test metrics — up 0.25 standard deviations on one English test, 0.22 standard deviations on another, and 0.28 standard deviations on one of two math tests.
I wasn't sure what .22 std deviations meant, so I looked stuff up a bit. For a normal distribution, going from the average to 1 standard deviation above is going from the 50th percentile to the 84th percentile. Going up .22 standard deviations from the average is going from the 50th percentile to about the 55th percentile.
A review of the effects of installing air filters in classrooms - https://news.ycombinator.com/item?id=22006595 - Jan 2020 (26 comments)
Installing air filters in classrooms has large educational benefits? - https://news.ycombinator.com/item?id=22006033 - Jan 2020 (48 comments)
You could also add potentially get a HEPA with a carbon filter which will get rid of volatile organic compounds which can also be damaging (but also just be the smell of food) but they don't tend to be as effective and depending on the mix of VOCs verses particles one filter may run out before the other and carbon really doesn't capture all that much or well. It is a good way to get rid of smell at least for a while.
There is a whole world of different standards for filtration for industrial and hazard chemicals which the FFP2/3 and N95 standards for Personal Protective Equipment respirators will lead you into if you want to go into that rabbit hole, but for a household typically its mostlt about Particulate matter, Volatile organic compounds and CO2. CO2 is about bringing in fresh air from outside.
Then when outside of the household N95/99 or FFP2/3 respitators do the same job in unclean air environments which is basically everywhere, outside or indoors in public places pretty much never meet the World Health Organisations levels for PM2.5 and often exceed CO2 (a proxy for re-breathing and a high change of viral infection spread) standards too.
I didn't see the model specified. They also said some schools got carbon filters, which is a different type of filter.
There's footnote saying many filters weren't even installed because some teachers thought they made the air "too dry", which is major placebo effect at work (air purifiers don't extract moisture from the air).
The entire paper is really not good quality, to be honest.
You can get a small HEPA purifier for a single room to remove particulates. The size of the filter, noise level, and amount of air moved are things to look for. Stepping up to activated carbon would remove VOCs, but cost significantly more (see IQAir, Austin Air, but ignore the cheap models that don't have 10-20lbs or more of activated carbon).
The whole thing is driven by one data point and a linear trend which makes no theoretical sense in the context of the paper (from the abstract: “Air testing conducted inside schools during the leak (but before air filters were installed) showed no presence of natural gas pollutants, implying that the effectiveness of air filters came from removing common air pollutants”) but does serve to create a background trend to allow a big discontinuity with some statistical significance.
I’m reminded of the walkback of scientific studies showing massive benefits from giving kids in third world countries deworming medications: https://www.theguardian.com/society/2015/jul/23/research-glo....
My beef with Matt Y.’s worldview of “scientifically driven public policy” is that the costs and benefits of public policy interventions are so devilishly difficult to study that you can’t meaningfully use them on realistic time scales to drive policy. This is an exceedingly simple hypothesis—filtering air improves test scores—that can easily be tested while controlling for confounding factors. But even then it’s hard!
A thread on Gelman's article is here: https://news.ycombinator.com/item?id=22006595.