> Both filters are on the intake side of the box fan. This means you don't need a bungee cord because the intake has negative pressure, the filters just "stick"

I agree with putting them on the intake side, but I still want them to stay put when the fan is turned off (with the switch or to move it to another outlet or in a power outage).

There are large rubber bands you can buy specifically to hold the filter in place on the box fan. I've seen them on Amazon.

What air quality sensor are you using?

You seem to have relatively clean air already. I probably wouldn't even bother with air purifiers in such a situation.

Where did you buy air filters? I live in a third-world country and couldn't find them anywhere, so I had to buy a factory-built purifier. It really struggles to keep up when it's 1000+ µg/m³ outside. I'd like to add a second one (preferably DIY).

> You seem to have relatively clean air already. I probably wouldn't even bother with air purifiers in such a situation.

By my estimates, a lifelong exposure to 33 µg/m³ rather than 0 µg/m³ would on average result in a loss of around 1 disability adjusted life year.

https://dynomight.net/air/#a-heuristic-to-quantify-harms

Obviously this is very hard to estimate and you shouldn't trust this as anything more than at most a rough order of magnitude estimate. But it does suggest that the ambient levels they are experiencing are worth reducing if you can do it easily.

Might those numbers might be exaggerated a bit due to confounders? Specifically, the high density and traffic causing stress.

It's entirely possible! But my perspective is that there's a very one-sided risk. It's extremely unlikely that particles are beneficial. Thus, given the uncertainty I think it's good decision theory to run a purifier, especially given how easy it is relative to other lifestyle changes that could plausibly have a benefit of the same magnitude.

Running a purifier does cause noise however, which isn't healthy; assuming your purifier is close by. (No idea at what point those two effects are of similar impact.)

Do you live in a place where forced-air heating is common? If so, you should be able to buy filters wherever people go to buy miscellaneous hardware for houses. In the US, they are rated on a scale called MERV. In other countries, other rating systems are used. You want something at or near the high end of the scale — getting near 100% removal with, for example, a real HEPA filter is unnecessary and potentially counterproductive, but getting to at least, say, 30% reduction in fine particles per pass is important. In the US, this is around MERV 13.

Sure, air quality is relative, but 100 AQI is the point where I start getting headaches. Obviously it can get much worse.

I got the air filters on Amazon, which can be US centric, but usually you would get them at a home/hardware store. They are made for slotting into a furnace/HVAC set up. For places where that is less common, they may be harder to find.

> It really struggles to keep up when it's 1000+ µg/m³ outside

Also worth noting that 1000ug/m³ is not accurate, maybe you are thinking of some other measurement? In the literal middle of a forest fire, you may see readings of ~300 ug/m³. No where on earth sees readings of 1000+ µg/m³.

It is very much accurate. This is a pretty typical PM2.5 level at winter time where I live. Put a few thousand houses heated with coal close together, add −30-40 °C winters, a particularly bad location where wind almost never blows (maybe ~30 days per year total), and voila.

In California, there is the Caldor Fire raging right now in South Lake Tahoe. Heres what it looks like on the ground: https://www.reddit.com/r/weather/comments/pfczpl/caldor_fire...

The highest purple air sensor, near/on the fire front, in raw PM2.5 ug/m³ is at 368 when I checked: https://www.purpleair.com/map?opt=1/mPM25/a10/cC0&select=735...

Well, if you find it hard to believe, I welcome you to visit us and live here for a year or two. Don't forget to bring a good supply of spare lungs.

These measurements are supported by the official (state-sponsored) air quality monitoring.

I am not saying we have 1000+ micrograms 24/7. The daily average in winter time is closer to 300-500, depending on your location. However, I see such extreme measurements pretty much every single evening, with some going above 1500 µg/m³.

Back in the days of the great London smog particulate concentrations were going up to 5 _milli_grams per cubic meter:

https://sci-hub.se/https://pubmed.ncbi.nlm.nih.gov/29596258/...

It's not impossible or even particularly unusual.

I don't think the purple air sensors (laser scattering plantower sensors) are very accurate at such extreme measurements. There are numerous laboratory permanent sensors deployed by various state, federal and local authorities that you can compare. I've seen purple air at much more extreme values. Still terrible.

I expect there are a lot of particles bigger than 2.5 um?

I think you have us all curious now - where do you live? Sounds like Siberia, except I'd expect there to be more wind.

Relatively close, but no cigar. I'll quickly deanon myself if I describe the exact place, there's not many HNers here.

> You seem to have relatively clean air already. I probably wouldn't even bother with air purifiers in such a situation.

Everyone should have filters. Filters are for more than dust, they have also been shown to greatly reduce the amount of COVID in the room air. (i would assume flu and other viruses as well). Cheap and easy, with no stupid controversy like masks or a vaccine (even though both are better than an air filter, every little bit helps)

I live in an area that had over about 6 weeks with us pm2.5 aqi above 300 almost every day and many days exceeding 400 and it shocks me how many people still don't have filters of any kind and see this kind of DIY fan as excessive and silly. Ok, more filters for me I suppose.

Not sure how this helps. As soon as your kid sneezes near you, you're gonna catch covid. Air quality seems like a solid usage but in your house vs covid/flu? Your fan is gonna lose. Covid from outside your home? It's a non-issue and doesn't happen.

> As soon as your kid sneezes near you, you're gonna catch covid

That isn't for sure. Last I checked (over a year ago, so different variants) 50% of spouses sleeping together didn't catch COVID.

We are reasonably sure that amount of virus particles you are exposed to is a factor on if you will get COVID. So the more you can eliminate the better your odds are.

Delta is at least twice as virulent as the old strains.

'Masks', as worn by most, are assuredly not.

Edit: I'm not anti-mask. I would love it if you explained how ill-fitting cloth in a single pass is better than multi-pass HEPA or equivalent filtration rather than (or even as well as) down-voting, though.

Sure, yeah.

A simple cloth mask doesn't really protect the wearer; it protects others from the wearer. It slows droplets down as you exhale them. So they travel X feet, instead of X+Y feet. Depending on air currents and other variables the level of protection this offers others will range from "nothing at all" to "some".

Obviously, a HEPA filter is orders of magnitude better. But, is there a HEPA filter directly between you and the person next to you in line at the supermarket?

> A simple cloth mask doesn't really protect the wearer; it protects others from the wearer.

Sure. I'd call that 'different' rather than 'better', but I suppose it's subjective.

> Obviously, a HEPA filter is orders of magnitude better.

You would think, and yet my comment saying so, objecting to 'masks are better than an air filter' (when qualified to ill-fitting cloth as worn by most) is apparently highly objectionable.

> But, is there a HEPA filter directly between you and the person next to you in line at the supermarket?

Not a comparison I made, but yeah, if I went to a supermarket (I'm not, because I almost entirely had groceries delivered pre-pandemic, so now easily enough entirely) I would wear an FFP3 dust mask rather than ill-fitting cloth, which is two nines to HEPA's 3.5.

(More than that is probably not justified since fit won't be perfect, some leakage. Industrially etc. where there's a need for greater filtration I expect that's when you have to step up to fully enclosed hoods. I digress..)

   > Not a comparison I made

Sure, but it's the relevant one.

   > if I went to a supermarket [...] I would wear an FFP3 dust mask 
   rather than ill-fitting cloth, which is two nines to HEPA's 3.5.

Good. That would be very safe. Ideally everybody would do that. I'll let you think about how feasible that is.

I mean, your entire argument seems to be that cloth masks aren't perfect, I guess?

They sure aren't. But that's not the claim anybody is making. The evidence-backed claim is that they are better than nothing -- and "doing absolutely nothing" is apparently the hill that hundreds of millions of Americans have chosen to (literally) die upon We are trying to take a step forward from that, via a simple harm-reduction method that almost anybody can afford.

Let me ask you something. How effective would a public health measure need to be before you'd endorse it?

1%? 10%? 99%? 100%?

There have been 40 million confirmed cases in America, and likely several times that amount in reality. Over half a million deaths. Each single percentage point improvement in prevention represents > 400,000K cases and > 6,000 deaths.

Anything is better than nothing. Even if half the air isn't filtered because of bad fit, that is still half the air that is filtered.

During fire season this year, living in one of the cities with the worst air quality in Canada this year, I had to double up my fans to keep up when it reaches 400+ US pm2.5 aqi in my home office that is about 300 sq ft. I am using filtrete 1900 20x20x1 from amazon (Canada)

I also have a Dyson air cleaner and it works for moderate to light smoke but it's just not enough for heavy smoke.

I haven't been able to prove this well yet but I got the dyson air purifier and it seems to have done really well at reducing my hey fever. Even when leaving home it seems to be not much of an issue. Possibly just reducing the allergens in the air for most of the time makes it not an issue when I experience them outside for short periods.

Hay fever that’s caused by particulates in your house sounds weird. Maybe get a check for mold?

The pollen comes in through the windows and gets me hard. While I'm at home I have no issues, when I go outside I start to get hit with them but by the time it's really kicking in I'm usually on my way home again anyway.

It's hard to actually prove the air filter did anything since a lot of factors for me have changed since last season but this year I am affected far less than I was last and not feeling any effects right now while some coworkers are.

Thanks for sharing, that's neat how simple it is. What air quality sensor setup do you have?

HM3301: https://wiki.seeedstudio.com/Grove-Laser_PM2.5_Sensor-HM3301...

Attached over I2C to an ESP32 running ESPHome: https://esphome.io/#air-quality Which in turn reports to a raspberry pi running Home Assistant: https://www.home-assistant.io/installation/ Took about 20 mins to set everything up.

> Took about 20 mins to set everything up.

Given you have Home Assistant installed and have previously used ESPHome. And attached wires.

Otherwise, you may be up to a journey that takes many hours/days. That's the difference experience makes :)

The only thing set up was the raspberry pi having an OS installed. Had never used HA or ESPHome before. I was shocked how easy it was if you're comfortable with a terminal. I'd never even used docker before.

My steps were:

1. Install docker

2. Install/run HA through docker

3. Install/run ESPHome through docker (there's a config wizard)

4. Edit the ESPHome .yaml from (3) to my preferences

5. Connect 4 wires from the ESP32 to the air quality sensor

6. Plug ESP32 USB to RPi

7. Run docker esphome on the above yaml. This compiles, flashes, and boots your ESP32, auto-connecting to your network

8. Add the ESP air quality sensor to HA through the web GUI

That's (almost) exactly what I did and without prior docker experience. I must admin it was less hassle with docker, but still docs for particular containers had to be read to understand configuration options.

+ setting up for external access, letsencrypt and tying that everything within docker-compose. I didn't manage to do it in 20minutes, but more like full working day maybe?

Anyway, HA is recommended. Haven't tried ESPHome, but Tasmota is an alternative that also does the job, if you someone ever wants to research the options.

Turnkey microcontroller projects can be really quick. Adafruit and Sparkfun sell many of their breakout boards with special cables for I2C now, so you might not even have to solder anything. From there, you can drag a Python script to the microcontroller (which appears on your PC as a flash drive).

Building enclosures for these projects used to be an annoyance, but everyone provides CAD models of their stuff now, so you just kind of lay it out in your CAD program, extrude a box around it, and print it out.

Never been a better time than now to be building sensors and that sort of thing. (Even with the STM32 shortage!)

As for the Raspberry Pi -- my rule is to treat embedded systems like hardware. Download a Linux image with the software you want on it, copy it to an SD card, plug that in, and go. I use this model for Octopi and while I might have changed the password and uploaded a profile for my printer file, that's it. If the RPI blows itself up, $35 and 5 minute fix. What version of systemd is running? Don't care. How close is the clock to Coordinated Universal Time? Don't care. It's an appliance and the time on the front is blinking :P

(I spent most of my free time this week unbreaking my Beaglebone that I ostensibly don't treat that way -- I log in and fuck with shit. Though when debugging failed I just blew everything away and copied over what I saved in the project's Git repo. It was enough to be back up and running with no loss of functionality. I guess the key is: don't let it run away from you -- it can be an endless time and energy sink.)

A 3D printer alone is a many-weeks learning experience, not to mention learning CAD if you've never done it before.

BTW, a 3D printer running is a potent source of VOC pollution and particulates, meriting an activated-carbon filtration system for wherever it is.

I've been doing this wrong all along then? I've got my filter on the outtake side. What is the difference I wonder.

The difference is your "fan/blower" will not function as well as a "compressor". You want to vent towards the largest, least constrained volume available, from higher constraints towards lower constraints.

From my understanding the biggest difference is the blades not getting covered in grime, because you're pre-filtering the air.

I've also heard airflow dynamics arguments, but I feel like I've heard them argued in both directions.

You can use two fans, one on intake, one on exhaust. Or three, if you have a particularly fine filter. But building another whole unit to run in another room might be better.

With a #9, one-inch filter, a single fan does fine.

Outtake muffles agitation from the airflow.

Intake still allows the air coming out to be more turbulent, mixing with the existing air better and giving it more "throw" in larger spaces.

You should be fine, here is the comparison

https://www.youtube.com/watch?v=zE4qmN_EOXw

edit1: exact time of the result https://youtu.be/zE4qmN_EOXw?t=250

Nice setup! Could you also share the names (brands/model, e.g. 3M Filtrete 20x20x4) of the three components? I see a few options out there but rather go with something that is vouched for here.

How strong is the air flow with the filter attached? Is it a slow trickle or can you really feel a decent amount coming out?

Also, 4 inches is quite a lot of depth for a MERV filter, could you elaborate on this point?

4 inches is deep for a standard dimensioned MERV-rated filter. It’s normal for a high quality but sadly usually proprietary fancy HVAC filter (Aprilaire, Lennox, etc).

That's true, but I want to know what they meant when they said the 4-inch pleating is designed to reduce air resistance. Since it's an unusually large amount of depth for a MERV filter, I would expect the opposite to happen?

The 4-inch pleating translates into a larger surface area of filter material, which in turn means the flow rate per area is reduced. So you have the same amount of air going through 4x the area of filter material compared to a 1" filter. Its analogous to using a thicker wire with the same current.

Air resistance goes down with thicker filters because filter area goes up.

Note, the actual filter material is not "thicker", but is just folded or "pleated".

Ok, yes. Do we know how the air resistance/air flow varies as a function of surface area? Is it almost linear? I'm wondering how important it is.

I've used both. I don't have hard numbers since they are different filters, but I replaced a 1" filter with a 4" filter and I can run my fan at a lower speed.

Feel the air coming out, vs without a filter. The slower the air is, the fewer opportunities you get to trap each dust grain. Overall miss rate improves exponentially with airflow speed.

That doesn’t make sense, the higher the flow rate the higher the energy each particle has so more chance of it making it through the filter.

You might imagine so, but that turns out not to be how it works. Each time through there is a chance the particle will be blocked. It doesn't matter how fast in went in, because the mv^2 of a dust mote is tiny at any plausible speed. What matters is that N trips through gives you (1-p)^N probability of not getting caught, which falls off very fast... in fact, exponentially.

A deeper-pleated filter is the way to reduce velocity in traversal.

Is such device useful to minimise dusting of objects in the room? Like plants, cupboards etc. ? Do you still have to clean dust in the room?

After using a purifier for two years pretty much non-stop (close to 24/7) I am pretty certain they don't help with dust at all. That's not why we install them, though.

Re: home dust filtration

How can you even search for this? Google results are not relevant to dust at all:

https://www.google.com/search?q=diy+air+dust+filter

This is a really strong example of how bad goog sucks for long-tail queries.

SEO spammed: https://www.google.com/search?q=home+dust+filtration+old+hou...

I've found that tacking 'pdf' onto search queries gets through some of the SEO garbage. For instance I did 'home dust filtration pdf' and found this pdf:

https://www.epa.gov/sites/default/files/2018-07/documents/re...

which cites this paper about deposition rates of different particle sizes:

https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=86094...

> How can you even search for this? Google results are not relevant to dust at all:

Try this: https://www.google.com/search?q=diy+air+%22dust+filter%22

I have a couple of blu air purifiers. They really cut down on the amount of dust in the room. We used to have to dust weekly, in the rooms where we have the purifiers we dust maybe every other month now and you can't tell a difference.

Since this describes such a different experience from my sibling comment, I'll have to add that my purifier cleans particulates just fine (close to 0 µg/m³ in summer and 20-40 µg/m³ in winter), which is supported by three separate PMS5003 and a couple of years of data piped into Grafana.

I don't know why it's so useless against dust. Maybe we have more coarse-grained dust which quickly falls down on the ground.

The blu air filter has a prefilter specifically for dust and it gets filthy. That has to get washed in the laundry regularly. Maybe some combination of that, it's cfm's and placement might make a difference?

I have 2 coway units in a ~800sqft apartment and still have tons of dust but I open the windows a lot when its not smoky and I have a dog so who knows. The pre-filter does trap a lot of dust so there is obviously some (maybe minor) reduction.

There is still apparent dust after years (have cats) but OMG the filters get gross.

How do you measure your air quality? What sensor do you use? I have a similar setup and I'd like to measure the results.

I know its working because I don't smell smoke, but I'm curious to see a measurement

thanks, do you have a website to put updates ? I'd love to know how it goes long term wise (filter changes, average air quality, your health)

Thank you for sharing !

What sensor do you use?