Shedding light on how much UV-C you need for disinfection

The idea of using UV-C light to kill the COVID-19 virus by disrupting its RNA has captured the imagination of many in the technical community. Unfortunately, important application details about factors such as necessary exposure time and intensity levels may be hard to discern, particularly for those who are new to the technology. A complicating factor is that most light meters aren’t sensitive to UV-C wavelengths, making it tough to gauge the output of specific UV-C sources. And meters that work at the UV-C range are specialized instruments that can be pricey.

One problem in evaluating UV-C sources is that they usually carry power ratings based on their power consumption rather than their power output. For example, one rule of thumb for mercury lamp-type UV-C sources is that their actual 254-nm output is between 10 and 30% of their input rating because of heat losses and the inefficiency of the lamp ballast. Better quality, larger, high-power lamps typically reside at the higher end of the scale.

Quality is important because it is possible to find inexpensive UV-C lamps from China sold online whose quality is questionable. Many of these are advertised as being in the 10-W range. Experts say that for a 10-W lamp, a good ballpark output would be 1 W.

Form factor is another consideration. A typical 10-W lamp is about 8-in long, but the light-emitting part of the tube is only about 5-in long. And lamps typically must warm-up before reaching full output. As in other kinds of lamps, the process of turning the lamps on and off can diminish their life. Some lamp makers specify that cycling more than four times daily can diminish bulb life.

An additional factor figuring into disinfection is that while a UV-C lamp puts out 254 nm light for disinfection, the wavelength for peak disinfection efficiency is 265 nm. For most microbes, 265 nm provides 20 to 30% better disinfection. In that regard, some makers of UV-C LEDs use this wavelength disparity as a selling point because LED chemistry in the semiconductor material can be adjusted to get light output at a specific wavelength.

However, there are still cost disparities between UV-C mercury vapor lamps and UV-C LEDs. Though the costs of UV-C LEDs have dropped, they are still more expensive than ordinary LEDs having the same output level. For example, consider the LTPL-G35UVC275GM UV-C LED from Liteon Optoelectronics. This 2-W device has a peak output at 280 nm and was recently listed on the Digi-Key website at $9.45 each. Even considering differences in efficiency, a UV-C source comprised of these LEDs will cost more than a UV-C lamp with a similar output.

There are guidelines for UV-C disinfection sources issued by standards bodies such as the Illumination Engineering Society. But without a means of actually measuring UV-C levels, developers are still flying blind. So it is helpful to review test results of representative lamps. For example, the UV-C LED maker Klaran cites experiments conducted at EMSL (a third-party microbiological testing facility) which measured the log reduction of common microbes exposed to light from a Klaran UV-C LED device at several time intervals and at various distances. Exposure testing for the MRSA virus revealed that it takes approximately 120 sec to get a 6 log reduction of MRSA at a distance of 2 in from the light source. (Six log reduction means the number of germs is 1,000,000 times smaller.)

Klaran doesn’t specify which of its UV-C lights were used in the tests, but indications are a Klaran light engine may have been used which contains nine Klaran UV-C LEDs said to provide >500 mW of UV-C radiant flux at a 265-nm wavelength.

Another data point comes from researchers at the Georgia Tech Research Institute (GTRI) who built two prototype chambers to evaluate PPE disinfection using both mercury vapor lamps and LEDs. They used the prototypes to evaluate different power levels and disinfection times with a variety of face shields and face masks used to protect workers from the coronavirus.

The goal was to provide disinfection chambers that were as small as possible in the interest of portability. The chambers can accommodate face masks and at least one face shield. The idea is to have healthcare workers put their face masks and face shields in the box, close the lid and set a timer. Then they can swap out one set of PPE while the other set gets disinfected

Researchers say disinfection takes about eight minutes, depending on the lighting source which have varying intensity levels. (Unfortunately, GTRI hasn’t spelled out the intensity of its UV-C sources or the vendors supplying them.) The research team designed the chambers to provide the level of UV exposure that earlier studies had shown would destroy the RNA of the closely related SARs-CoV virus. The researchers say they did not attempt to evaluate the ability of the UV light to inactivate the SARS-Cov-2 virus that causes Covid-19.

Other engineering considerations for the UV-C boxes included the need for cooling the UV sources, providing consistent exposure of the PPE to UV light using reflective walls in the chambers, and protecting the mercury vapor lamps from damage during use.