TL;DR: Some plastics degrade under exposure to UV light and I am concerned this could hamper widespread adoption of far-UVC. This post outlines the rationale for these concerns and seeks feedback from the far-UVC community on their importance.

My simplified line of reasoning is:

Epistemic status: this post is the culmination of spending ~5-10 hours thinking, researching, and writing up this post. I feel pretty certain that UV stability is worth at least thinking about with respect to far-UVC adoption but very uncertain about it being something that blocks far-UVC adoption. I have spent some time learning about far-UVC through discussions, reading, and preparing to interview someone in the space for a podcast but don’t feel I have a very deep understanding of the space.

Summary

UV light damages plastics that are not UV-stable. Many of us may have encountered this in cheap outdoor furniture whose plastic components change colour or become brittle and break easily after being left out in the sun too long. My concern is that if most of the plastic materials used in indoor materials are not UV-stable — meaning they undergo irreversible physical changes when exposed to UV light — then placing far-UVC lights indoors could cause unwanted damage to plastics and limit the demand for far-UVC.

In this post, I focus on two ways by which this damage could hamper the uptake of far-UVC: consumer preferences on aesthetic effects and building regulations on physical degradation. These may not be the only ways and I’m uncertain about how concerned to be about each of them. However, I think they illustrate why the UV stability of plastics is concerning to me and why I’d like to see more research into it.

Both of these concerns could result in damping the market for early adoption of far-UVC. My impression is that demand for far-UVC will be required to bring down the cost of the technology. If the price of the technology remains high, this could inhibit adoption and make far-UVC an intractable defence mechanism for pandemics and global catastrophic bio risks (GCBRs).

My aim with this post is to present my rationale behind this concern and get feedback from the far-UVC community on the magnitude of this concern relative to other bottlenecks in the space.

Effects of UV light on plastics

The aesthetic and mechanical effects of UV light on plastics are two examples of why I think this could be worth spending more time thinking about the UV stability of plastics. Of the two, I’m more worried about the mechanical effects as if mechanical degradation results in blocking the installation of far-UVC then this could be a significant issue for adoption.

Aesthetic effects

TL;DR: Aesthetic changes to plastics may result in consumers being unwilling to adopt far-UVC lighting.

The aesthetic effects of UV radiation on non-UV-stable plastics appear to primarily be colour change — notably a yellowing of plastics. Other aesthetic effects that I’m less certain about and would ideally like to research more are cracking, stickiness, chalkiness that rubs off on contact, and texture change (eg. increased roughness). The aesthetic effects seem significant from a personal preference point of view — I wouldn’t want all the plastic surfaces in my home or office to turn yellow over time.

A recent study on the use of far-UVC on public transport buses simulated 6.2 years of exposure to far-UVC light^[1] and found “...that far-UVC radiation at 222 nm causes significant colour degradation in all the polymeric materials tested. The degree of color degradation varies depending on the type of polymeric material and the duration of exposure to far-UVC radiation. An obvious color difference was observed on FRC and PVC materials, where ∆E00 values of 6.431 and 7.194, respectively, were obtained after 290 J/cm2 radiant exposure”. For context, “the value of ∆E00 can be interpreted to range from normally invisible difference (<1) to very obvious difference (>5).” So it seems colour change is significantly noticeable after enough exposure.

For me, questions still remain around how much people actually care about this but my intuition is that any perceptible colour change would be seen unfavourably by consumers. If the selling point of far-UVC is something like “you can install special light globes that kill germs in the air without you even knowing” then “these lights will turn your plastic stuff yellow” may undermine the “without you even knowing” part of the sell.

Mechanical effects

TL;DR: Mechanical degradation of plastics could create safety hazards which cause regulatory bodies to block the installation of far-UVC indoors.

I think the effect of far-UVC on mechanical properties warrants greater concern than the aesthetic effects from an adoption perspective. Mechanical properties include, among others, the strength of the material — how much force can the plastic take before breaking — and brittleness — how well can it absorb stress before suddenly fracturing without significant deformation. An example of a brittle material would be glass, which doesn’t deform much before breaking and this is in contrast to an elastic material like a silicone phone case which deforms a lot before breaking.

If materials are degrading to the point of failure then this could create hazards in the indoor environment. An example of this could be increasing the brittleness of electrical coverings such as power points or light switches. If they were to become weak and/or brittle to the degree that switching on the light or plugging in an appliance causes the plastic housing to break and expose live wires then this would be a significant hazard. Another example is permanent damage to medical equipment or other ‘mission critical’ or expensive devices that are used in indoor settings.

Consumers — both the general population and business consumers such as hospitals, offices, government, etc. — wouldn’t want to risk this happening which is a concern in the same way the aesthetic effects are. However, I think a more significant effect could be intervention by building code authorities who ensure safety in the built environment. If a significant hazard is created by the mechanical degradation of plastics exposed to far-UVC light then installing the lights may become restricted by building codes. I see this as a more significant effect as it would prevent consumers from installing far-UVC even if they wanted to.

Why limits on early adoption could be bad

Far-UVC emitters are currently prohibitively expensive for widespread adoption, however, others have reason to think the cost could come down over time as more R&D is done on the technology. For this to happen there needs to be a market for the technology to create financial incentives for R&D. Some of the current bottlenecks to creating this market are a lack of safety and efficacy studies and a lack of standards. UV-stability of plastics could be another hurdle to creating a market for far-UVC by reducing the rate of early adoption. Without this market to incentivise R&D it seems unlikely that costs will come down and far-UVC will not be financially viable to defend against GCBRs.

What have others said?

• Convergent mentions the degradation of plastics in their Far UVC Executive Summary but the emphasis is on the release of volatile organic compounds (VOC) and their health effects (they suggest air filtration and careful design for high-power far-UVC systems will likely mitigate health concerns from VOCs)
• The “Air Safety to Combat Global Catastrophic Biorisk” report from 1Day Sooner and Rethink Priorities mentions plastic degradation. They note that “much of the issue could be avoided through careful materials choice” and that “Generally, the rate of degradation may overall be negligible compared with the standard lifetime of consumer products” (page 27).
  • The former claim seems weak in the context of indoor environments as using only UV-stable plastics would require a total rework of how indoor products are manufactured and would likely increase costs significantly. Additionally, the amount of coordination across companies, industries, and countries required to make this happens makes this seem pretty intractable to me.
  • The latter claim does not provide a reference and the bus study suggests that lifetime changes are non-negligible.
  • They do reference a Boeing study that found no significant mechanical degradation in plastics from far-UVC exposure (the link to the study doesn’t work though). The report notes that the exposure time was low for the Boeing study. The results figures (figures 8 - 12) in the bus study suggest that degradation increases with increasing exposure so more research here seems required.

Conclusions

Overall, I feel pretty certain that the UV stability of plastics is worth more attention than it is currently getting in the far-UVC publications in the EA space that I have read. I’m uncertain about the relative magnitude of concern/effort that should be placed on the UV stability of plastics relative to other bottlenecks to far-UVC adoption such as comprehensive safety testing, efficacy trials, developing clear standards, and technology development.

I think there’s a good chance that someone has already looked into this and found it to not be an issue and I am very open to input on this concern from the far-UVC community.

Open questions

Some questions that I would want to look into to refine my understanding of how concerned I should be about the UV stability of plastics (in no particular order):

• To what degree do consumers care about the aesthetic degradation of plastics in the indoor environment?
  • What is the limit that consumers will tolerate?
• What level of degradation would be allowable under building codes?
• What are the magnitudes of aesthetic and mechanical degradation under prolonged exposure to the intensity of radiation proposed for effective far-UVC?
• What plastics in the indoor environment are/aren’t UV-stable?
  • For those that aren’t UV-stable, which are most concerning and why?
• What insights do these papers add:
  • Evaluation of the degradation of materials by exposure to germicide UV-C light through colorimetry, tensile strength and surface microstructure analyses
  • Damage to Common Healthcare Polymer Surfaces from UV Exposure

To clarify: I don’t expect that I will have the time/bandwidth to actually look into these anytime soon.

Acknowledgements: thanks to Jessica Wen and Dan Epstein for reviewing drafts of this post.

Disclaimers: the views and mistakes in this post are mine alone and don’t necessarily reflect those of High Impact Engineers.

1. ^{^}

   From the paper: “According to Welch et al. [34], it is sufficient to expose surfaces for 2.0 mJ/cm2 under far-UVC 222 nm irradiation to deactivate the COVID-19 virus; for other pathogens, such as influenza, it should be almost 4.0 mJ/cm2, and to deactivate most widespread biological pathogens, up to 8.0 mJ/cm2 exposure is necessary. It takes approximately 1 h for a regular city bus to travel from end to end, after which the bus driver takes a break for approximately 10–15 min. Assuming that during each such break a disinfection is performed with an exposure of approximately 10 mJ/cm2 and the duty cycle of the city bus is approximately 16 h a day, 7 days a week, the annual irradiation exposure would be approximately 47 J/cm2. In our case, 150 h of irradiation in a 222 nm far-UVC irradiation chamber was approximately equivalent to a radiation exposure of 290 J/cm2, which corresponds to approximately 6.2 years of city bus disinfection.”