UVC air purifier design and testing strategy

By DirectedEvolution @ 2022-06-01T05:35 (+27)

At EAGxBoston 2022, I had the privilege of meeting other EA-linked physical engineers interested in working on systems for pandemic prevention. Will Bradshaw and Kevin Esvelt of the MIT Sculpting Evolution Group organized much of the community-building and information-sharing efforts within our cohort.

They work on a range of physical defenses that would offer barriers to a broad array of pathogens, while minimizing the risk of introducing new hazards as the underlying technology develops. An example is the nucleic acid observatory, which seeks to develop a global network of wastewater and environmental sampling stations to better track and anticipate the spread of disease outbreaks.

I'm a biomedical engineering graduate student at the University of Michigan, and I've been considering another one of their ideas: far-UVC air purifiers or upper-room ultraviolet germicidal irradiation (UVGI). The basic idea here is to expose the air in a room to a particular wavelength of light that happens to destroy the DNA and RNA of pathogens, while being safe ^[1] for humans. This air is then trapped in a filter that physically removes the particles from the air.

With a UVGI, the air near the ceiling is constantly irradiated with UVC. Whatever fans, HVAC, and natural air currents exist in the room mix the air, causing the air from below to rise into the pathogen kill zone.

Cost of UVGI at scale

According to the CDC, the cost to install a UVGI system, which broadcasts UVC light across the ceiling of an 8 foot/2.4 meter or taller room is $1,500-$2,500 in a 500 square foot space, which is perhaps the size of a school classroom. Is that economical at scale?

Let's get a rough estimate of the square footage in San Francisco, and then see what it might cost to put in UVGI in the most-trafficked 10% of its interior spaces.

The city of San Francisco is 46.87 square miles, or 1,306,000,000 square feet, including outdoor space. Let's assume the city has no high-rises (a roughly accurate assumption, from what I hear).  We'll assume that 50% of the square footage is indoor space. Taking the most highly-trafficked 10% of that indoor space would leave 65,300,000 square feet to cover with UVGI. If it costs $1,500 for every 500 square feet, that's a cost of almost $200 million for San Francisco, or about $230 per person.

Let's say we scaled this up to the entire country. $230 per capita would be about 4% of annual US tax revenue, comparable to the entirety of spending on education, training, employment and social services.

Does UVGI have to be so costly?

You can buy two HEPA air purifiers with UVC light from BestBuy, advertised as working for 246 square feet each, for $300. That's more costly than the CDC's UVGI system, but we're not even trying yet to bring the cost down.

I've been spending time sourcing components, and it looks like you can buy the parts for a UVC filter - a bulb, socket, wall plug, fan, filter, casing, and wiring - for about $100. I expect it could be assembled by an experienced person in about 10 minutes. But if you scale things up, it looks like the component costs could come down to closer to $15-$20 per unit.

A 6" desk fan might be rated around 191 cubic feet per minute of airflow, which could turn over 4,000 cubic feet of air (the volume of an 8' high, 500 square foot room) in 20 minutes. That might be adequate to circulate the air.

If a system like this could be made for $20 in materials and would be adequate for a school classroom, then that makes it possible to gather more data on effectiveness.

Testing a cheap UVC air purifier in elementary schools

The bad approach

How would we test such an air purifier to see if it really helps with reducing rates of illness?

If there's a flu going around, most people have lots of opportunities to get exposed. Even a very effective air purifier would only eliminate one of the several public places many people might visit on a given day as a potential exposure site. Showing that such a purifier was effective would require blanketing a large area in such devices to get adequate coverage. Even then, you would have to do something like comparing rates of illness in that location to see if there was a sudden drop. Would that be convincing to you?

The better approach

If instead you deployed these air purifiers specifically in elementary schools, it might be much easier to see an effect. Many elementary-school-age children are required to spend half their day in a single room with a fixed group of their peers. This is probably the most important location for contributing to their disease risk, and that of their families.

Producing about 100 $20 air purifiers would allow you to perform a randomized controlled experiment, by supplying an elementary school with air purifiers and tracking the effect on attendance over the course of a school year. Half of the air purifiers would have a UVC bulb with a MERV 13 filter, and the other half would have a placebo UVA bulb with perhaps a MERV 8 filter. This would make it possible for the study to be double-blind. The cafeteria, library, halls, and other shared spaces would all have real purifiers, so that, in the case that the filters work to reduce the spread of disease, that is not obscured by diseases spreading in the lunch room.

Since teachers already track attendance, and much student sickness shows up as absence, it would be possible to simply compare attendance rates between the UVA and UVC filter rooms.

UVC air purifiers are already available for retail purchase in the USA, so as far as I know, such a study would not need to be run under the auspices of the FDA. It mainly would need buy-in from parents and from a school.

There are several ways to make such an experiment appealing to these groups. One is the hope of having fewer diseases both among the students, teachers, and families participating. Another is the prospect of obviating the need for masks in the classroom. And a third is contributing to meaningful science, perhaps facilitated by workshops or presentations by the scientists and engineers organizing the experiment.

With better data on the effectiveness of these lights, it might be possible to motivate increased investment in UVC purifiers. For example, my father, who retired after a career as the executive director of a nonprofit health clinic, told me that with adequate data supporting the effectiveness of such devices, he would have been willing to spend a great deal of money installing them in the clinic's waiting room.

Requested feedback

1. What would be your key questions about the design?
2. Do you think the smart thing to do is look for a grant to fund the early work, or to spend one's own money designing the prototype before looking for funding for larger-scale production?
3. How hard do you think it would be to find a school willing to host the proposed experiment?

1. ^{^}

   We actually need more safety data on UVC light. While it seems that the radiation itself doesn't harm people, these lights may turn oxygen into ozone, which can create health problems, particularly for those with asthma. This may not be a real issue, and it also seems that some bulbs avoid this problem by shielding off the particular wavelength that turns O2 into ozone. But figuring out how to source these bulbs at wholesale prices would be a challenge!