The lighting industry began to take an interest in UV lighting about four years ago, when UV LEDs started to appear on the scene. I wrote an Explainer piece for Lux Review in July 2017; its still out there – Lux Explainer: UV LEDs. Things have moved on since then and our current focus on viral infection has seen a step change towards UV disinfection. This is an overview of the ultra-violet technology as it currently appears.
I’ll take you on a journey down the radiation spectrum, out from the visible spectrum, down into the far reaches of ultra-violet, ending where it continues into the X-ray band and out into the Gamma (Stop that: you’re starting to sound like The Moody Blues!).
The things that have changed in the past four years have occurred at each end of the UV spectrum, and, conveniently, we start with ‘visible ultra-violet’.
UV-V (405nm):
Also known as near-UV, this is being promoted in some quarters as providing ‘safe’ UV disinfection. Blue light is intrinsically antimicrobial and, because its outside of the UV band proper (which starts at 400nm) it falls outside of that controlled jurisdiction. UV-V is described as ‘replicating the sterilization effect of sunlight’. For it to be effective, the pathogens that are being sought need to be exposed under this radiation for a period of some time.
A few lighting manufacturers have dabbled with the idea of combining UV-V within their commercial product, the principle being that UV-V is ‘safe’ if office workers or hospital staff, say, are exposed to this ‘sunlight radiation’ all day long.
Unfortunately, it doesn’t look as if its that simple. In the same way that it’s not necessarily healthy to stay out in the sun all day, evidence is coming through that suggests that its not a good idea to spend your day under UV-V radiation.
Nevertheless, UV-V has an important role to play in the laboratory as a popular source for analytical spectroscopy.
UV-A (315 – 400nm):
UV-A occurs in nature as it is a component of the sunlight that is not absorbed by the earth’s atmosphere.
In the world of the everyday, we come across ‘artificial’ UV-A light as the ‘black light’ used in entertainment venues – those weird purple-ish lamps up in the ceiling above dance floors, for example.
UV-A lighting is used in specialist applications such as material curing, such as setting dental fillings. It is also being developed for use with indoor horticulture, especially among cannabis growers, who report a bigger and more potent yield.
It is the typical source for tanning beds. It is not necessarily a healthy thing to do, as the Skin Cancer Foundation tells us: UV Radiation & Your Skin from 2019.
UV-B (280 – 315nm):
Most UV-B from the sun is absorbed by the earth’s atmosphere. It is known as the ‘biological spectrum’ and is used in industrial and medical processes: the National Psoriasis Foundation reports on UV-B phototherapy
UV-B light is always used in special enclosures to protect against exposure.
The active part of UV-B radiation in sunlight was used historically to bleach and disinfect woven fabrics hung in the ‘tenter-fields’. It also causes our bodies to produce Vitamin B, so it is vital to us – but overexposure to UV-B is likely to outweigh any benefit that Vitamin B might bring.
UV-C – 100-280nm: aka UVGI (ultra violet germicidal irradiation):
It was the introduction of UV-C LEDs that generated the interest in UV lighting four years ago.
Typically, UV-C radiation is locked inside sealed containers because of the very real risks to organic matter if exposed to it. UV-C is a DNA disruptor and needs very little time to wreak havoc in living systems.
It has only ever been considered as a source within sealed system and is extremely valuable in water disinfection and air scrubbing systems. It is also being used as the means for destroying pathogens on surfaces, by having UV-C projection systems within physical spaces- all the way from aircraft cabins to operating theatres – though not so far, to our knowledge, in hotel rooms.
It is this last potential feature that is causing so much interest at the moment. With coronavirus invading our physical spaces and settling on unprotected surfaces, UV-C is being seen by some as an obvious answer to disinfection. Its not so simple, inevitably.
Like any kind of light, UV-C travels in straight lines and objects will cast shadows that UV-C will not reach. One answer is to make the light mobile and recent work in Ireland has demonstrated a robotic UV-C unit that can find its way around a room. Nevertheless, it is difficult to see how a room can be guaranteed to be pathogen-free if it is well-populated with furniture.
If robotic travel can be resolved, that still leaves the question of safety. Ultra-violet light is, by its nature, invisible. Its not good enough to hang a sign on a door saying ‘No entry: cleansing in progress’ – so what do we need?
In the States, where UV-C lighting has been used to assist in surface cleansing for some years, glass doors have been used so that the equipment can be seen from outside (warning visible light on the units, obviously); automatic shut-off is needed for when the unit detects movement within its ambit; and it might be worthwhile to consider some way of detecting non-moving humans and animals, just in case no one checked in the cat basket.
FAR-UV (222nm – the ‘sweetspot):
FAR-UV has been under discussion for the past few years and – if true – may be the magic bullet. If its not true, its as dangerous as everything else.
Research is suggesting that 222nm has a very special characteristic – it does no harm to mammalian skin while continuing to be a DNA disruptor on bacteria and viruses.
FAR-UV products are already available on-line. Based on the principle of providing a continuous low dose-rate, FAR-UV is claiming to reduce the spread of airborne microbial diseases. Note that: ‘Airborne microbes’. The jury is still out as whether Covid-19 is airborne at all. The evidence seems to leaning towards that being the case, but ‘official’ opinion has yet to accept the idea (at the time of writing (07.apr.20)
The thing that is emphasised with FAR-UV is the low dose rate, at only 20J/m2, or 2mJ/cm2. It has been tested on ‘normal’ influenza virus.
The expected claims are all there; that it increases the safety of occupants by reducing contact with pathogens, providing a safe germicidal effect in the air and on room surfaces.
Ultra-violet light and the Precautionary Principle:
Put simply, the Precautionary Principle emphasises caution, pause and review before leaping into new innovations that may prove disastrous. Critics argue that this approach is an obstacle to progress. I don’t often quote Donald Trump, but I’ll make an exception here: “We can’t have the cure being worse than the problem”. If the arch anti-scientific voice of our time can come out with that, I’m happy to join in the applause.
Final words:
UV disinfection is a great tool when used appropriately. A time of immediate concern and occasional panic is, perhaps, not the best time to leap into the unknown – but it is certainly a time for outlier theories to be put to the test. Be precautionary by all means, but please do not become an obstacle has to be the message.
As I’m sure you’re well aware, I’m just a lighting designer and I’m not an expert on this topic. I get my information from talking to those who do know and reading what I can find. Having said that, if this helps to make you more aware of the current situation, I’ll take that as a win; and the experts are always out there to help.