Is UV-C light germicidal? Illuminating Engineering Society FAQs give the low-down

The Illuminating Engineering Society (IES) has released a lengthy FAQ on the use of ultraviolet light for germicidal purposes and specifically relative to the COVID-19 virus.

The UV report was authored by the IES Photobiology Committee. The IES notes that companies are offering products such as disinfecting wands that could clearly be dangerous because of UV-C light’s potentially harmful effects on human eyes and skin. The report was created with input from medical and research organizations and government agencies, including the U.S. Centers for Disease Control and Prevention (CDC), the U.S. Food and Drug Administration (FDA), Harvard Medical School, the Icahn School of Medicine at Mount Sinai Medical Center, and others.

On its website, the IES has posted a direct link to the report, an FAQ, and educational videos.

One of the misconceptions the FAQ addresses is the idea that all UV light is germicidal. “Wavelengths in the photobiological
ultraviolet spectral band known as the “UV-C,” from 200 to 280 nm, have been shown to be the most effective for disinfection, although longer, less energetic UV can also disinfect if applied in much greater doses. UV-C wavelengths comprise photons (particles of light) that are the most energetic in the optical spectrum (comprising UV, visible, and infrared) and therefore are the most photochemically active,” the report says.

The report also addresses myths about longer-wave UV. “UV-A and longer (visible) wavelengths do not have germicidally effective emission wavelengths to inactivate viruses,” the report says. “Their relative disinfection capability is very minimal on the order of 1,000 times less effective in terms of fluence rate than the low-pressure mercury germicidal lamp. There have been only very special applications of wavelengths in the UV-A and violet (e.g., 405 nm), which require very high doses not practical in an occupied environment and were not recommended for viral sterilization. The trace amount of UV-B that is emitted from some white-light fluorescent lamps probably has similar efficacy.”

One of the more illuminating parts of the FAQ concerns the dosage of UV-C needed to actually disinfect surfaces. “There are very sophisticated programs to calculate the lamp sizes and in-air dose requirements….but there is a much simpler evidence-based dose that has been developed over many years for TB control, typically specified as about 17 mW of 254-nm lamp-emission radiant power per cubic meter (m³) of space to disinfect air,” the report says. “Although this sounds too simplistic to be true, since air in any room is always moving and mixing, one can correctly assume that all air will be treated—the better the air mixing, the sooner this will happen. Studies at the Harvard School of Public Health and elsewhere show log units of reduction equivalent to 24 ACH to achieve 80% reduction of transmission. Of course, 100% reduction is not possible, because of the multiple modes of transmission. To disinfect surfaces, this depends on the type of surface and its cleanliness; recommended exposures vary from 200 to 1,000 J/m² (20 to 100 mJ/cm²).”