Yes, ultraviolet light in the "C" range, also known as UVC, has been shown to kill SARS-CoV-2, the virus that causes COVID-19, although these devices may take a long time and be less effective than other disinfectants. Also, more research is needed to test UVC on various household surfaces, and to determine how effective consumer products may be when used in the home, rather than under laboratory conditions.
The big challenge with using UV-C light is being sure your UV lamp provides a large enough dose of UVC light to all the surfaces you need to disinfect, such as a mask, phone, or an entire room, and that you are not exposed to the UVC light, as it is dangerous. UVC works fastest and most reliably on non-porous surfaces, but it may be easier and faster, as well as safer, to clean such surfaces with liquid disinfectants. Think of using UV-C light to disinfect an object like using a hair blower to dry an object -- it can take a lot of time, but with UV-C you often won't know when you're done.
Many UV products marketed as "killing 99.9% of germs" may be so weak that you would need to hold them for an hour at different angles just to disinfect a mask. Masks can be more easily disinfected other ways, such as in a washing machine (for cloth masks) or at low temperature in an oven (for N95 masks).
If you are still interested in using UV light to help protect yourself from coronavirus, here are the things you'll need to know. Further below are our reviews and comparisons of UVC products marketed to consumers.
How UV Light Kills Germs
UV radiation kills viruses and bacteria by damaging their genetic material (DNA and RNA). Of the three main types of UV light, UVC (which has a wavelength range of 200 to 280 nm) is the most effective for inactivating viruses, with the most effect wavelength being about 260 nm (Lytle, J Virol 2005).
In order to be effective, the right "dose" of UVC must be applied. The dose is a function of the UVC intensity or "irradiance" from a specific distance from the object times the number of seconds the object is exposed. Irradiance is measured in milliwatts (mW) per square centimeter (cm2), and the dose of UVC is measured in millijoules (mJ) per square centimeter (cm2) of the object being irradiated. (In scientific terms, 1 mWs/cm2 =1 mJ/cm2).
So if your UVC lamp has an irradiance of 5 mW/cm2 at a specified distance from an object, then holding the lamp at that distance from the object for 8 seconds will deliver a dose of 40 mJ/cm2, because 5 mW/cm2 multiplied by 8 seconds = 40 mWs/cm2 or 40 mJ/cm2.
A dose of 40 mJ/ cm2 is generally considered sufficient to disinfect (99.9% reduction in infectivity) a wide range of bacteria and viruses, including certain coronaviruses that infect animals (Malayeri, IUVA News 2016).
Although sunlight does not contain UVC light (because UVC light is filtered by the earth's atmosphere), it still appears to be effective in reducing the infectivity of SARS-CoV-2. According to preliminary tests funded by the U.S. Department of Homeland Security (DHS), simulated summer sunlight reduced SARS-CoV-2 in aerosolized saliva by 90% within 6 minutes versus 19 minutes in simulated winter/fall sunlight. Compared to no sunlight, the rate of decay of infectivity was 32 times faster with the summer light and 14 times faster with the winter/fall light (Schuit, J Inf Dis 2020). Note, however, that these reductions are not instantaneous: If you immediately inhale droplets from a cough or sneeze of an infected person, it won't matter if you are outside on sunny day or indoors. In addition, researchers in China have reported that high temperature and UV radiation do not appear to reduce the transmission of COVID-19 (Yao, Eur Respir J 2020).
What Dose of UVC Light Kills SARS-CoV-2?
In its published guidance for consumers, the FDA states that UVC radiation has been shown to destroy the outer protein coating of other coronaviruses and "may also be effective in inactivating the SARS-CoV-2 virus," but notes that "there is limited published data about the wavelength, dose, and duration of UVC radiation required to inactivate the SARS-CoV-2 virus."
A laboratory study (not yet peer-reviewed) showed that when pieces of fabric from N95 masks and stainless steel were contaminated with a high concentration of SARS-CoV-2 and then exposed to a large hospital-type UVC lamp (containing a large array of small, LED UVC lamps), it took about one hour for the virus to become undetectable on the mask but just about 12 minutes on steel. The distance from the UVC lamp to the objects was approximately 20 inches (50 cm), at which distance the lamp had an irradiance of just 0.005 mW/cm2 (or 5 microWatts/cm2 or 5 µW/cm2) (Fischer, medRxiv, 2020 — preprint). This means that the effective dose needed to kill the virus to the point of being undetectable on the N95 fabric was 18 mJ/cm2 and, on steel, it was just 3.6 mJ/cm2.
Bear in mind that extra time might be needed to disinfect surfaces of larger objects and those with curved surfaces which would require different lamp angles. Futhermore, as the FDA cautions, "It is important to recognize that, generally, UVC cannot inactivate a virus or bacterium if it is not directly exposed to UVC. In other words, the virus or bacterium will not be inactivated if it is covered by dust or soil, embedded in porous surface or on the underside of a surface..."
Risks of UVC light
UVC light can damage cells in your skin and eyes just as it damages microorganisms, so you need to use it without exposing yourself. The FDA warns you should "never look directly at a UVC lamp source, even briefly." Seven cases of injury to the surface of the eye were reported during the initial months of the COVID-19 pandemic in Miami. Symptoms have included eye irritation, burning, blurred vision, and pain and/or sensitivity to light several hours after exposure to UV light lamps without proper eye protection at home, work, and, in a dental office where a UV device was used for decontamination. The average duration of exposure to UV before injury ranged from ten minutes to four hours, and symptoms resolved within 2 to 6 days of treatment with standard lubricating drops and/or antibacterial or steroid eye drops. The physicians noted that while UV-C protective goggles would likely prevent this type of injury, conventional sunglasses (with exposed sides) would still allow for harmful UV rays to reach the eye surface (Sengillo, Ocul Immunol Inflamm 2020).
UVC light devices should never be used to attempt to disinfect hands or skin.
Repeated UVC treatment can gradually decrease filtration efficiency of N95 masks and the rate at which it does so can vary by brand/product; some research suggests that this no more than 9 treatment cycles should be used (Peltier, Infect Control Hosp Epidemiol 2020).
In addition, repeated use of UVC may weaken some components of objects, such as rubber straps on masks.
The FDA also advises that some UVC lamps contain mercury (which is toxic even in small amounts), so extreme caution is needed in cleaning up and disposing of a UV light that has broken. Some products may also give off unsafe levels of radiation, which may cause injury to the skin, eyes, or both after only a few seconds of use. Sign in to the full answer for details.
Be aware that the FDA has expressed risk with using UV light, as well as ozone gas, to clean, disinfect, or sanitize continuous positive airway pressure (CPAP) devices and accessories.
Do UV products sold to consumers work?
As noted above, a UVC sanitizing or disinfecting device should have a stated wavelength between 200 to 280 nm, preferably closer to 260 nm. But just as important, it needs to disclose its irradiance in mW/cm2 (or microW/cm2, written as µW/cm2; 1,000 microW/cm2 = 1 mW/cm2) and the distance at which this can be achieved.
Many products state their wavelength, but few products provide their irradiance, which is needed to determine if they can deliver an effective dose to kill coronavirus in a reasonable amount of time. For example, there is probably no point in buying a UV wand with low irradiance if it means that you'll have to slowly move it over an object for an hour. On the other hand, the higher the irradiance, the greater the risk to your skin and eyes when a light wand or lamp is used.
For home use, UVC "boxes" seem to make more sense than wands and open lamps. These enclosed boxes are designed to disinfect phones or other small solid and generally flat objects, like credit cards, keys, or watches. Porous or non-flat objects may cause shadowing that prevents the light from reaching parts of the object. Because the boxes are enclosed, they are safe to use, and because you can place your phone in the box and walk away, they don't take extraordinary effort. The time needed in these boxes tends to range from about 1 to 10 minutes.
If you are considering purchasing a UVC light device, the FDA suggests the following:
- Ask the manufacturer about the product's health and safety risks and about the availability of instructions for use/training information.
- Ask whether the product generates ozone.
- Ask what kind of material is compatible with UVC disinfection.
- Ask whether the lamp contains mercury. This information may be helpful if the lamp is damaged and you need to know how to clean up and/or dispose of the lamp.
How do UV products compare?
We have reviewed and profiled several widely sold UVC products, including handheld wands (GermAway, Max-Lux Safe-T-Lite UV WAND and Verilux Clean Wave) and ultraviolet light boxes (PhoneSoap, HoMedics, and 59S).
Sign in or join now to read our reviews and get our Bottom Line opinion about ultraviolet devices (including how they compare to other disinfectants such as bleach wipes) and other methods such as ozone generators or air purifiers for disinfecting coronavirus.