How to ensure you irradiate using the correct UVGI dose
By Robert Yeo, April 2020
Introduction
The use of germicidal ultraviolet (UV) radiation has been established as a means of disinfecting objects and drinking water for well over a century, but this technique has received renewed interest as a result of the COVID-19 global pandemic. The use of UVC radiation for ultraviolet germicidal irradiation is abbreviated to UVGI. In parallel, there has been an explosion in the number of opportunistic vendors marketing UV light “wands” which are being touted as some kind of miracle cure for expunging viruses and bacteria from domestic environments. This article will review the science behind the use of germicidal UV radiation, describe how it can deactivate harmful bacteria and viruses, and explain how to measure and quantify the output of a UV light source in terms of its effectiveness for the purpose of disinfection of air and surfaces. The effectiveness and the photobiological safety of using UV light wands in a domestic environment will also be considered.
Ultraviolet Radiation
The ultraviolet is defined as that part of the electromagnetic spectrum having wavelengths between 10 and 400nm, sandwiched between X-rays (<10nm) and visible light that covers the range 380 to 780nm. Depending upon its biological effect, the UV is divided into a number of sub-bands. The UVC band extends from 100 to 280nm, however that below 200nm is termed “vacuum ultraviolet” as the radiation is absorbed strongly by oxygen in the atmosphere. Between 280 and 315nm, the UV radiation is termed UVB, and is responsible for causing erythema (“sunburn”). Between 315 and 400nm, this is called the UVA band.
UV radiation has both beneficial as well as harmful effects on the human body. For example, UVB radiation promotes the production of vitamin D in the human body, which is linked to cell metabolism. On the other hand, exposure to too much UVB will lead to sun burn and can lead to some forms of skin cancer. In general, human eyes and skin are most at risk with exposure to UVC radiation, specifically in the 265 to 275nm band.
It is worth noting that normal silicate glass absorbs both UVC and UVB radiation, although longer UVA wavelengths are transmitted. More specialised quartz glass on the other hand does exhibit good transmittance with most UV wavelengths. In terms of plastics, while polycarbonate exhibits high absorbance below 390nm, acrylic (PMMA or polymethyl methacrylate) provides good transmittance in both the UVA and UVB bands. In practical terms, you cannot get sunburnt through normal glass, while many of the plastics employed as diffusers in luminaires will not transmit shortwave UV radiation (UVC or UVB).
In summary, the UV wavelength range is defined as follows:
- Vacuum UVC: 100-200nm
- UVC: 200-280nm
- UVB: 280-315nm
- UVA: 315-400nm
Germicidal Ultraviolet Radiation
Germicidal UV refers to short wavelength ultraviolet radiation that renders bacteria and viruses inactive. Specifically, ultraviolet radiation in the UVC band (200-280nm) is considered germicidal, with peak effectiveness in the 265-270nm range. The acronym UVGI refers to ultraviolet germicidal irradiation.
The Earth’s upper atmosphere absorbs all of the UVC radiation emitted by the sun, passing only longer UV wavelengths (UVB and UVA). So, there are no naturally occurring sources of UVC radiation on the surface of the Earth. Instead, UVC radiation is created artificially, either from specialist UV sources such as low-pressure mercury discharge lamps (peak emission at 254nm), as an emission from an arc welding process, or from specialist UV LEDs, although the irradiance that LEDs produce in the UVC is of questionable efficacy.
Scientists believe that UVC radiation may “deactivate” the SARS-CoV-2 virus that is responsible for the COVID-19 respiratory disease. Although this thesis has yet to be proven, tests performed on related coronaviruses, including the SARS coronavirus, have revealed that UVC germicidal radiation is effective as a means of deactivating these viruses. In general, germicidal UV radiation is understood to damage (mutate) the RNA and DNA nucleic acids in a virus, which prevents replication, leading to its deactivation.
A key metric in the effectiveness of the UVC radiation is the “dose” delivered. Dose is the product of a lamp’s irradiance and the cumulative exposure time. Irradiance is the total light (radiant flux) received at a surface per unit area of the surface exposed. It is expressed in units of Watts per square meter (W/m2). Dose is the product of irradiance and time (in seconds) and is expressed in units of Joules per square meter (J/m2).
COVID-19 is understood to be spread by coming into direct contact with respiratory droplets, or by transfer from an infected surface via a person’s hands to their mouth, nose or eyes. The extent to which the COVID-19 virus is spread via airborne transmission is not fully understood, but scientists are extremely concerned that airborne transmission is a real danger. Germicidal UV lamps are therefore being proposed both to irradiate contaminated surfaces and to irradiate the air in a room.
While UVC radiation is of proven utility in terms of its effect on viruses and bacteria, exposure to the human skin and eyes must be avoided. Notwithstanding the recent ill-informed speculation by certain politicians on the use of UV light on (or indeed within) the human body as a possible treatment for COVID-19, UVC is known to have a very harmful photobiological effect. Specifically, the use of UVC germicidal radiation for hand sanitisation is strongly discouraged. Also note that neither UVB (280-315nm) nor UVA radiation (315-400nm) provides for any significant germicidal disinfection. UVC lamps should be operated in strictly controlled environments, often within light-tight enclosures, to avoid the possibility of exposure to our skin or eyes. As with all sources of artificial optical radiation, UVC germicidal lamps must comply with the risk group labelling and safe use requirements given in EN 62471:2008, “Photobiological Safety of Lamps and Lamp Systems”.
How Much UVC Light is Required for Effective Sterilisation?
The “action spectrum” of the COVID-19 virus to UVC radiation is currently unknown. An action spectrum is the relative response as a function of wavelength for a given effect. However, a standardised germicidal action spectrum could be assumed to apply, and that published by the IESNA (the Illuminating Engineering Society of North America, www.ies.org) is based on the deactivation of the E. coli bacteria which extends from 235 to 313nm, with a peak response at 265nm (Figure 1, shown below).
Anecdotally, a target irradiance of approximately 2mW/cm2 (equivalent to 20W/m2) at 254nm would require that the UVC germicidal light source remain stationary over the surface to be disinfected for tens of seconds. For a more complete description of the UVC dose required to achieve a 90% deactivation (the D90 value) with a variety of Coronaviruses, please refer to this paper published on ResearchGate.net. The range of D90 values for the stated viruses ranges from 7 to 241 J/m2, with an average of 67 J/m2. This would be equivalent to exposing the virus to an irradiance of 67 W/m2 (or 6.7mW/cm2) for just over 1 minute.
However, this throws up a major concern. As with any light source, the irradiance delivered onto a surface varies with the separation between lamp and the irradiated surface. The irradiance follows an inverse squared relationship which means that if the source to sample separation is doubled, the irradiance will drop to 25% of the original level. So it is incumbent upon manufacturers of UVC sterilisation products to specify the distance at which their lamp achieves its specified irradiance. Either that or the user must purchase a suitable UVC radiometer or an appropriately configured UV spectrometer system to measure the irradiance in-situ.
UVC Light “Wands”: Caveat Emptor
The COVID-19 pandemic is being seen by a number of opportunistic business people as an excuse to market low cost, UV light “wands” as a kind of miracle cure for disinfecting domestic surfaces of the virus. Some of these devices are of very questionable efficacy and it is clear that uninformed consumers are being taken advantage of.
An informal market surveillance conducted by the author revealed that most manufacturers offering low cost, consumer-orientated, UV light “wands” failed to specify a working distance for their product, assuming even that they quoted an actual irradiance specification at all (most did not). This is extremely worrying, as the general public are not light metrology experts and cannot be expected to know that the effectiveness of these devices varies enormously both with the distance they are held over a surface and with the time that the surface is left exposed to the UV radiation.
Moreover, many of the UV light “wands” on offer seem to suffer from a number of fundamental flaws. Some seem to be equipped with violet LEDs that do not emit in the required UVC band. Others seem to be fitted with plastic diffusers that would not transmit the UVC light anyway. Still more are being manufactured using UVC LEDs, but produce an intensity that is an order of magnitude lower than can currently be obtained from an equivalent discharge lamp, requiring that the wand be held over a surface for an excessively long time to achieve the required UVC dose. Also, few if any vendors take the trouble to point out that skin and eye exposure to the UVC radiation should be avoided, due to photobiological safety concerns.
Measuring UVC Radiation
There are two optical radiation measurement methods used to quantify the amount of light emitted from a germicidal UV lamp – radiometry and spectroradiometry.
The simpler and lower cost method is to use a photodetector filtered to receive the 254nm peak emission from the low-pressure mercury lamp. This type of instrument is referred to as a radiometer and will be calibrated to record dose or irradiance in units of Joules per square meter or Watts per square meter (or equivalent) at 254nm. In their favour, such radiometers are affordable, portable, and simple to operate. Their main drawback is that they only reveal how much light is present at 254nm; they yield no information on the amount of UVC radiation as a function of wavelength over the entire UV or UVC waveband. Pro-Lite supplies UVC radiometers made by Solar Light Corporation, for details see: radiometers.
The more complete approach is to configure a spectroradiometer that measures the complete UVC (or entire 200-400nm UV) waveband and reports the irradiance as a function of wavelength (spectral irradiance). Often equipped with a flexible optical fibre with cosine receiver as the collection optics, the spectroradiometer will generally require the use of a laptop computer, so will not be quite as simple to use or as portable as a handheld radiometer; however, the recorded spectrum of the light source allows for the computation of its effectiveness, taking into account the actual (or presumed) action spectrum of the germicidal effect. Pro-Lite supplies modular spectrometer systems made by Avantes (see: AvaSpec-ULS2048) as well as the dedicated Specbos spectroradiometer made by JETI (see: Specbos 1211 UV).
In Summary
UVC germicidal lamps are a proven means of disinfecting surfaces infected by all manner of bacteria and viruses, and the SAR-CoV-9 virus responsible for the COVID-19 respiratory illness is expected to be no exception. Key to the effectiveness of the UVC lamp is the irradiance delivered at a surface, which in turn is a function of the separation between lamp and the object being irradiated. To ensure that a sufficient UVC dose is delivered, the use of a purpose-designed UVC radiometer or spectrometer configured for UVC measurements is highly recommended. Caution should be exercised when considering the use of low cost, consumer-orientated UV light “wands” that are currently flooding the market, as these have a dubious efficacy and questionable photobiological safety. UVC germicidal lamps are emphatically unsafe for exposure to unprotected human eyes and skin, and must not be used for hand sanitisation under any circumstances.
Further Reading
- Illuminating Engineering Society of North America (IESNA) committee report IES CR-2-20-V1, “Germicidal Ultraviolet (GUV) – Frequently Asked Questions”, April 2020. https://bit.ly/2xNzKUy
- European Commission Scientific Committee on Health, Environmental and Emerging Risks (SCHEER), “Preliminary Opinion on Biological Effects of UVC Radiation Relevant to Health with Particular Reference to UVC lamps”, July 2016. https://bit.ly/2wXySw5
