Open Source: Free As The Air You Breathe

[Carolyn Barber] recently interviewed a 15-year-old who has been making Corsi-Rosenthal boxes for people in his community that are at risk for COVID. Not only is it great that a teenager has such community spirit, but it is also encouraging that [Richard Corsi] and [Jim Rosenthal] made an open-source design that can help people at a greatly reduced cost.

If you haven’t seen one of these boxes, it is essentially a box fan inside a cardboard box with MERV-13 filters on all sides. While these high-quality filters aren’t as efficient as HEPA filters, the box makes up for it by moving a prodigious amount of air and by being much less expensive. The article says you can build a unit for $60 to $100, which is considerably cheaper than other filters with similar performance.

There’s been at least one research paper on the efficacy of the filters and the results were generally quite positive. Schools are taking a great interest in these boxes because they are inexpensive and effective. Of course, the filters don’t last forever, but one of the creators estimates in a classroom with 25 students, a three-year run of the box would run about $4.46 per student per year. Not a lot to pay for clean air.

We love hearing about tech helping people and especially open source that makes big impacts. Usually, when we think of air filtering, we are thinking about laser cutters or 3D printers. However, we have seen inexpensive HEPA filters, too.

18 thoughts on “Open Source: Free As The Air You Breathe

  1. Has anyone done any research to see if you can run one of these boxes in filter-upstream-of-fan mode for months, then briefly reverse the airflow and blow all the junk out of the filters and get more use out of them?
    I’ve used hand pumped water filters that did this and vastly increased the filter lifetime.

    1. We have an air filtration system in our manufacturing facility that works kind of like that. About once a minute, the fans will slam into reverse and knock the debris off the filters and into a funnel that leads to a set of collection barrels.

      1. Those style fans are some of my favorite. I sometimes wonder how they would work with a closer fitting duct or tunnel to prevent the air from in front of the fan from getting sucked back in when using with a filter.

    2. These filters aren’t purely mechanical filters (particles too big to go through get caught) but also electrostatic filters (particles otherwise small enough to pass through get drawn into the filter and caught by a static charge). You could reverse the flow and possibly dislodge a lot of particles, allowing for better airflow for mechanical filtering, but you can’t restore the electrostatic charge lost when filtering the finer particles (like viruses).

  2. Reversing flow doesn’t work with fine particle air filters we need for catching viruses, as at the fine particle size they don’t work as mechanical strainers, but by attraction et al. 3M has some good explanations on this.

    With air filters, what usually works well is having a “coarse” filter up stream, that gets replaced frequently, so the fine particle filters aren’t prematurely clogged by large particles. Even having cheap glass batting upstream against the typical home HVAC air filter works well. Of course, having two or even three filter grades upstream would optimize that, unless you make a stupid stack.

    For the “filter-box” in the article: having the fan blow air into the box and the filters installed with their support grid towards the outside, would allow for adding whatever coarse filter stack one wants on top of the fan. Easy access for replacement too.

    Fan pulling air out of the “filter-box”, would require course filters on the outside of all of the fine particle filters.

    1. For a design like this, a simple cloth or mesh pre-filter might be enough, and, if it’s cloth, you could just toss it in the laundry every so often.

      As for having the fan push air through the filters instead of pulling the air through, it depends on the fan, as some have much worse performance when pushing against resistance than pulling against resistance.

  3. You’d get better protection per unit of effort by maintaining a healthier respiratory tract microbiome, give it as much attention as you give your teeth. See: Rapid initiation of nasal saline irrigation to reduce severity in high-risk COVID+ outpatients. doi/10.1177/01455613221123737

  4. Just spitballing on possible price reduction but what if you used a 24″x30″ filter in a triangle, and got the fans and filters at bulk price instead of retail.

    Around here 20×20 or 20×24 merv13 filters are $15 each and fan is about $30 US. The price quoted is likely Canadian.

    1. TL:DR one filter taped against a box fan is plenty good enough.

      I didn’t know about the Corsi-Rosenthal boxes, but for a couple years I’ve been running a box fan with a single MERV-13 flat against the “suction” side of the fan. I just tape all the way around the edges for a “pretty good” seal. (If you think about it, a good seal doesn’t actually matter when the input and output are the same room- the filter catches whatever passes through it, and side slip just reduces efficiency a tiny bit.) I replace the filter every few months. Only requires one filter at a time.

      I just recently bought a couple PMSA003I air quality breakout boards from Adafruit, and I can vouch for the effectiveness of just the single fan + filter. My apartment is approximately ~600 sq. ft. Without the fan running, the PM2.5 level in my apartment is ~5-8 ug/m^3, or more depending on outside air conditions. (Note: EPA guideline for “healthy” outdoor air is ~12 ug/m^3 and under). I have the fan running 24/7 at full power, and it’s very unusual for the PM2.5 level to read anything other than 0 or 1 ug/m^3 anywhere in the apartment, unless I’m cooking or running the dryer.

  5. Does anyone have any idea how many CFM these are moving? I’m wondering if instead of a box fan, a standard bathroom ventilation fan would provide more air flow. (mostly because I have one kicking around)

    1. My local school district keeps the thermostat at 80 in the summer to ‘save money’. Ya gotta make it as cheap as possible to convince the boomers that it could possibly be worth considering…

      Now, having all the cool stuff in the base design, and making it more expensive the bigger the filters are? That would be a good strategy.

    1. There are two reasons I can think of. One is that it is easier to construct that way. Another is to slow down the airflow. Those types of filters have high resistance to air flow therefore they are most efficient when the flow is slow. Fans however have a high airflow. The big open space and the large surface area of the filters makes the air move slower through the filters.

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