Sanitization Science Content Section 5


Germicidal Efficacy and Mammalian Skin Safety of 222-nm UV Light

Authors: Manuela Buonannoa, Brian Ponnaiyaa, David Welcha, Milda Stanislauskasb, Gerhard Randers-Pehrsona, Lubomir Smilenova, Franklin D. Lowyc, David M. Owens, and David J. Brennera

This study discusses the germicidal effects of 222nm (Far-UV) light, which is significantly harmful to bacteria, but without damaging cells in tissues.


Understanding the Metrics

Our current standard of measuring light is focused on illuminance (lux), which intends to estimate the visual (photopic) effects of light. However, non-visual (melanopic) effects can in fact be measured. As light is a stimulant, and certain properties provide more stimulating effects than others, it is those properties that circadian lighting technology seeks to manipulate for desired effects. These effects are measured in what we call an m/p ratio (melanopic/photopic ratio), and lighting manufacturers present this information in a decimal form, for instance, 0.3 or 0.9. The lower the decimal, the better that light is for evening hours before sleep. The higher the decimal, the greater that light’s effect is on your biological system, meaning you’ll want a higher value for alerting effects during the day.


Corelated Color Temperature

This is measured in Kelvin and tells how warm or cool the light is. A higher temperature means a cooler, more bluish light. A lower temperature will be a warmer light, and will look more yellow. Color-tuning products use CCT to determine if a light is good for day or night, but it is not an accurate means to determine biological impact.


The Brightness of Light

The intensity of light for LEDs is sometimes measured through a comparison unit: equivalent watts. In traditional lighting, the wattage was directly related to light output, so LED lighting uses ‘equivalent watts’ for familiarity, but wattage is no longer reliable for determining how bright a light can be, because LEDs are much more energy efficient. The intensity of light is relevant to the m/p ratio, in that a brighter light will have a higher biological impact, offering more alerting effects.


Color Rending Index

This is how well a light source renders colors. A high CRI is helpful to see colors and contrast, leading to higher visual acuity. Depending on the types of wavelengths that make up a light, colors will look different, and certain ones will ‘pop’ under different light sources. For crisp, clean light, and true color visibility, a high CRI of 90 or above should be sought.

M/P Ratio

Melanopic/photopic ratio

This is a measurement that demonstrates a lamp’s biological impact. The lower the decimal (i.e. .34 or less) the better the lamp is for light before sleep. The higher the decimal (over .90) the better that lamp is for alertness and daytime use. While color tuning products use CCT, for true biological lighting, look for the m/p ratio.