higher UV absorption than conventional 254 nm. The possibility of photo-reactivation is also
decreased due to the high energy of 222 nm.
Spectral-selective excimer lamps are the artificial light source of choice when generating far-UV
222 nm light for sanitation. The term “excimer” is short for excited dimer, which refers to the
presence of dimeric molecules with nanosecond-lifespans in an excited state. The specific
working excimer molecule is closely associated with the UV wavelength that the lamp emits, as
well as the light’s use and application. In the case of 222 nm excimer lights, the working
exciplex molecule is KrCl. The compound consists of one atom of krypton (Kr) and one atom of
chlorine (Cl). This granular approach to leveraging a single UV wavelength, e.g. 222 nm, 172 nm,
308, nm, etc., suggests that excimer lamps release (and can be classified as) quasi-
monochromatic light. A bandpass filter can be applied to remove lower, higher and stray UV
bands to ensure safe and effective sanitation.
Excimer germicidal lamps typically take the form of conventional, compact linear lights. These
lights are non-toxic, mercury free and do not contain fluid. Excimer lights feature instant on and
off capabilities, reaching 100 percent output in less than a second. This artificial light source can
be toggled and deployed in locations within a temperature range of 0°C to 100°C. Compared to
conventional UV 254 nm light sources, excimer lamps offer high power density. Reflectors
around the lamp can be added to improve light beam distribution and intensity. With a
compact footprint, smaller disinfection lights can be used to eliminate microbes. The units
require support from a driver that protects the device from abnormal power conditions and
ensures power stability. Unlike traditional 254 nm UV sanitation lights, excimer lights do not
require extensive safety gear during use. Furthermore, the lamps can be secured close to the
target surface without generating intense heat in the surrounding area.
4.2 Safety Considerations
Far-UV 222 nm light contains effective germicidal properties that are significantly safer to
humans, compared to other penetrative UV wavelengths, as the bands are not cytotoxic or
mutagenic to mammalian cells. A 2017 study
6
highlights the level of safety 222 nm light
imposes on mammalian skin, which directly resembles human skin, during exposure. Authors of
the study cited that far-UV light cannot penetrate the stratum corneum of the skin, thus
protecting foundational basal cells from damage. By comparison, UV rays in the 254 nm
spectrum can penetrate the stratum corneum of the skin, capable of reaching as far as the
stratum spinosum, and damage basal cells. Far-UV 222 nm is considered to be safe on human
eyes, as suggested by the researchers of the study. This specific UV band cannot penetrate the
thick barrier protecting the cornea.
Researchers in the 2017 study tested the safe characteristics of far-UV 222 nm by exposing skin
samples to 222 nm UV rays. In the tests, a 222 nm UV-C excimer light was positioned at a
distance of 9 cm with a power density of 0.036 mJ/cm
2
. Researchers also tested chronic
exposure of far-UV 222 nm light by administering a far-UV dose of 157 mJ/cm
2
in a 7-hour
