Alternative Room Disinfection Modalities –Pros and Cons - Use of Biocidal Surfaces for Reduction of Healthcare Acquired Infections - page 194

One laboratory study of surface disinfection used 40-W UVC lamps at
approximately 2.4 m above nutrient agar plates cultured with a variety of bacterial
and fungal species [ 84 ]. The duration of exposure was 30 min resulting in a four
log reduction in all organisms tested except Candida, Bacillus subtilis spores,
Aspergillus spores and Mycobacterium fortuitum. Aspergillus spores exhibited the
highest resistance with only 2.5 log reduction. Penicillium and Stachybotrys have
shown similar resistance in other studies. Prions and viruses were not evaluated.
Another study used four 8-W UVC lamps at a distance of 30.5 cm, for durations
between 3 s and 6 min at 55 % RH and again at 85 % RH against four different
bacteriophages [ 87 ]. Tseng and Li found that 90 % reductions were similar to those
achieved with non-spore forming bacteria and that viruses are more susceptible in
air than on surfaces. They also found that survival was inversely related to dose
(as expected). Double stranded DNA viruses were found to be most resistant, and
viral inactivation required higher doses of UVC at 85 % RH compared to 55 %.
These results are consistent with Riley and Kaufman's earlier work with Serratia
marcescens . These authors found the organism to have exceedingly low death
rates at humidities above 80 % and that photo-reactivation of the organism was
likely. RH level is a critical factor influencing the effectiveness of UVC as a
germicide [ 88 ]. In their research on the effects of UVC irradiation on Deinococcus
sp. , Bauermeister et al. found that it was more detrimental to bacterial cells at a
higher RH than at low RH (33 %) and that desiccated cells were highly resistant to
the effects of UVC. The above studies suggest that RH levels between 50 and 60 %
are essential for UVC irradiation to be effective.
Researchers also explored the potential for using UVC irradiation to disinfect
medical supplies and equipment. For example, Fisher and Shaffer believed that
UVC could be used to disinfect N95 disposable respirators [ 85 ]. The reason to do
this is that during an epidemic of influenza or other airborne pathogen, there may be
a shortage of single use N95 and that UVC germicidal treatment and respirator
reuse may extend the supply. Nerandzic et al. used a handheld, UVC wand to
disinfect hospital surfaces such as keyboards and portable medical equipment [ 89 ].
There were three settings, and the highest setting is called “deodorize” because it
produces small amounts of ozone ( < 0.05 ppm). This 1.8 kg shielded wand was
attached to a wheeled power pack and produced a radiant dose of 100 mJ/cm 2 after
a 5 s exposure at the highest setting. The unit produces far-ultraviolet radiation
(185-230 nm) and was tested for germicidal effect against C. difficile , MRSA,
and vancomycin-resistant Enterococcus (VRE). The unit manufacturer used the
far-ultraviolet spectrum in the belief that the higher photon energy would increase
the speed of microbial inactivation. The distance from the wand to the surface was
not specified, and no mention was made of ambient conditions such as RH and
temperature. The highest reduction was demonstrated against VRE (6.9 log 10 CFU),
and the lowest reduction was against C. difficile (4.4 log 10 CFU). In the presence of
organic matter, the unit was less effective with only a 3.4 log 10 CFU reduction of
C. difficile . This is not surprising given the limited penetration ability of UV in the
185-230 nm range. The authors concluded that additional research was needed
to determine if this technology could serve as a useful adjunct to routine environ-
mental services in healthcare facilities.