35
are adequate to fully inactivate the coronavirus (>6 Log
10
reduction) at an exposure of 30 minutes
without damage to mask components or to the mask’s performance. Significant inactivation of MERS
Co-‐V has also been reported by Leclercq et al. at 65°C for an exposure at 15 minutes (> 4 Log
10
;
below the limit of virus detection.). Darnell et al. found similar results with SARS-‐CoV with > 5 Log
10
at 65°C for a 10-‐minute exposure.
Although all the above-‐cited viral references were conducted in heated liquid suspensions, it has
been shown by Zhou et al. in a comparison of dry heat inactivation on solid surfaces versus liquid
suspensions, an equivalency of inactivation at 65°C. This study supports the inactivation data
gleaned from liquid suspension studies may also be comparable for solid carrier inactivation rates.
In a recent report conducted thought the Department of Mechanical Engineering at Rice University
(Yap et al.) a thermodynamic model for dry heat inactivation is presented which has synthesized
data from peer-‐reviewed literature to accurately predict temperature-‐dependent inactivation of
Coronaviruses. This paper states: “virus inactivation occurs primarily due to thermal denaturation of
the proteins that comprise each virion.” Using published data from dry heat inactivation studies,
their model demonstrated that inactivation is indeed “a thermal denaturation process, inactivated by
thermally-‐driven protein denaturation.” Time-‐temperature inactivation data derived from this
model is shown in Table I (adapted from Yap et al).
Table I
Temperature
Average
Coronavirus
Sterilization Time,
6 Log
10
Maximum Human
Coronavirus
Sterilization Time,
6 Log
10
Average Human
Coronavirus
Sterilization Time,
6 Log
10
60°C
48 min
9 min
8 min
80°C
3 min
< 1 min
< 1 min
This data supports the work presented above for similar time-‐temperature parameters and the
study by Zhou et al. in which no differences were found between solid surface and liquid suspension
inactivation of viruses at 65°C.
Similarly, data supports that Mycobacteria is very heat sensitive requiring only a few minutes at 60-‐
65°C for complete inactivation (Sung and Collins, Hammer et al., Grant et al., Keswani and Frank).
Literature searches have demonstrated that
Mycobacterium paratuberculosis
, can be inactivated in
solution to a > 6 Log
10
reduction in 6 minutes at 65°C (149°F) and at 68°F (155°C) in 2.2 minutes.
Mycobacterium
paratuberculosis
is more thermally resistant than
M. bovis
, the heat-‐resistant
surrogate designated for Mycobacterium challenge inactivation studies. The time-‐temperature
profiles selected for N95 mask re-‐processing exceed those profiles required to inactivate > 6 Log
10
reduction of Mycobacterium.
This dry heat process does not use any chemicals, using only low-‐temperature air at high-‐velocities
to decontaminate. Organic residues residing on a used mask do not significantly impede the delivery
of heat to microorganisms nor affect the inactivation efficacy of this process. The low temperatures
used in this process do not affect the material integrity or composition of the N96 masks tested to
date.
