To the Editor: The ability to disinfect and reuse disposable N95 filtering facepiece
respirators is urgently needed during the current COVID-19 pandemic because supplies
are running low in hospitals throughout the United States and abroad. Ultraviolet
(UV) germicidal irradiation (UVGI) is one possible method for respirator disinfection
to facilitate the reuse of dwindling supplies. Dermatology offices often use narrow-band
UVB to treat skin diseases. If necessary, we propose a possible repurposing of phototherapy
devices, including these UVB units, to serve as a platform for UVC germicidal disinfection.
UVGI is a disinfection method that uses UVC radiation to inactivate microorganisms
by causing DNA damage and preventing replication. Previous studies have shown that
UVC can inactivate coronaviruses, including severe acute respiratory syndrome coronavirus
(SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV).
One study of respirators contaminated with H1N1 influenza A found significant reductions
(≥3-log reduction) in viable influenza virus under substantial artificial soiling
conditions after being treated for 60 to 70 seconds at an irradiance of 17 mW/cm2,
resulting in a UVGI dose of ∼1 J/cm2 measured at 254 nm.
The efficacy of this dose has been verified in additional studies, and higher doses
(up to 2 J/cm2) have been shown to provide diminished benefit after 1 J/cm2.
It is recommended to treat used masks, but not visibly soiled, to allow the 3-log
reduction reported in the literature to be sufficient to achieve safe reuse levels.
It is important to note that the time to deliver 1 J/cm2 depends on the irradiance;
hence, it can be longer or shorter depending on the delivery device's capabilities.
In a prototype model that has been developed (Fig 1
), this dose can be delivered in 1 minute and 40 seconds at an irradiance of 10 mW/cm2.
The distance from the lamp to the top of the table in Fig 1 is approximately 14 cm.
Image of prototype being developed by Daavlin. The field of irradiation is approximately
15 inches × 45 inches, and depending on the manufacturer of the mask, this would allow
for the treatment of ∼18 to 27 masks (2 minutes per side). (A) With ultraviolet light
on and (B) ultraviolet light off.
(Photographs used with permission of Bob Golding, Daavlin, Byron, Ohio.)
However, UV radiation does degrade polymers, which presents the possibility that UVGI
exposure, while decontaminating, may also reduce the efficacy of the respirator and
decrease protection to workers. Lindsley et al
exposed 4 different models of N95 filtering facepiece respirators to UVGI doses of
120 to 950 J/cm2. Results of the study showed that UVGI exposure led to a small increase
in particle penetration (up to 1.25%) and had little effect on the flow resistance.
However, at higher UVGI doses, the strength of the layers of the respirator material
was substantially reduced (in some cases, >90%), but this significantly varied among
the different models. UVGI had less of an effect on the respirator straps: a dose
of 2360 J/cm2 reduced the breaking strength of the straps by 20% to 51%.
It should be noted that the dosages used in the study above are 100- to 1000-times
higher than those shown to disinfect H1N1 influenza A–contaminated respirators. Therefore,
considering that many of our health care providers are using substitutes for N95 filtering
facepiece respirators that offer very limited degree of protection, using UVGI and
repurposing phototherapy devices could be the best practical solution at this time.