Use of Silica Beads to Dry N95 Respirators During Storage

Abstract

Introduction:

The 2019 novel coronavirus, SARS-CoV-2, has caused an unprecedented use of personal protective equipment (PPE), especially the disposable filtering facepiece respirator (FFR), N95. The Centers for Disease Control and Prevention has provided crisis capacity strategies for FFR decontamination and reuse, including the drying of N95s in paper bags for 5 days to remove the moisture that could maintain virus viability.

Methods:

We tested the ability of food-grade silica bead packets to accelerate moisture removal from N95s during 24-hour time periods.

Results:

Briefly, N95s sprayed with water were completely dehydrated by silica beads within 24 hours, as measured with a commercial moisture meter. Importantly, repeated wetting and drying (with silica) did not decrease the N95 ability to exclude the bitter taste of Bitrex during standard fit testing.

Conclusions:

We conclude that food-grade silica beads can desiccate wet N95s over 24 hours, removing moisture.

Limited reuse (ie, a fixed number of use, remove, and repeat events) while maintaining physical integrity and exposure reduction of the same disposable filtering facepiece respirator (FFR) during a pandemic has been suggested as a crisis strategy to conserve supplies. Importantly, strict compliance with engineering and administrative controls when reusing FFRs is essential to reduce the risk of exposure to respiratory pathogens that may reside on the FFR. ^{1 -4} At the time of writing, the US Food and Drug Administration had not approved routine physical or chemical decontamination standards of care for FFRs. However, the Centers for Disease Control and Prevention has released crisis capacity strategies for FFR decontamination and reuse. One of these decontamination strategies is FFR storage in a clean, breathable paper bag for a minimum of 5 days between uses. ⁵ This action protects the FFR during storage and, we believe, permits the FFR to be dried of any moisture it may have acquired.

A recent study suggests that SARS-CoV-2 viruses can remain viable between 3 hours (when aerosolized) and up to 24 hours (when spotted) on cardboard (40% relative humidity [RH], 21-23°C). ⁶ Another study identified that SARS-CoV-2 remained viable on paper for up to 3 hours at 65% RH at 22°C. ⁷ One additional study suggested that 70°C dry heat inactivates SARS-CoV-2 in approximately 55 minutes. ⁸ Although other studies suggest longer viability times depending on surface, the data from these studies support the concept that enveloped viruses are inactivated when dehydrated. However, overnight brown-bag storage is not a means of decontamination. ^{5 -7}

We tested the ability of food-grade silica beads to remove moisture from FFRs over 24 hours. Briefly, 3 N95 FFRs (3 M 8200 NIOSH, St Paul, MN) were each sprayed with approximately 3.75 mL (5 squirts from a household spray bottle) of tap water delivered from 12 linear inches. Ideally, many FFRs would be tested with single water treatments. However, we could not justify the use of additional FFRs while scarce during the pandemic crisis. Thus, we chose to use fewer FFRs in a repeated-measures treatment model. RH content of each FFR was measured with a moisture meter (General MMD7NP, New York, NY) using the “drywall setting.” Percentage RH of each FFR was measured at 5 positions using a plus-sign pattern (measurements at the top, middle, and bottom or the FFR, centered vertically; and left and right sides of the FFR, centered horizontally) immediately after wetting. The highest percentage RH reading was recorded to determine if all moisture would be desiccated by this method. FFRs were then respectively placed into 1-gallon, air-tight plastic bags. The percentage RH of 3 food-grade silica bead packets (50 g, Dry & Dry, Brea, CA) was similarly measured at 5 (plus sign) positions on the bead packet, with the highest percentage RH reading recorded. Bead packets were each placed inside a small brown bag to prevent direct FFR-bead packet interaction and respectively placed into a plastic bag containing an FFR. Plastic bags remained at ambient temperature (16°C) and humidity (56% RH) for 24 hours. Percentage RH of FFRs and bead packets was remeasured, approximating the previously recorded positions. This testing cycle was repeated daily for 7 days. The geometric median percentage RH differences of FFRs and bead packets were evaluated by the Wilcoxon signed-rank test because the data are nonparametric. Additionally, qualitative fit testing was assessed to determine if repeated wetting and drying altered Bitrex exclusion (ie, failure to taste the Bitrex). ⁹ Qualitative fit testing was supervised by a certified nurse. Differences between the initial and 24-hour median percentage RH time points and fit testing results are discussed.

Regardless of individual percentage humidity of FFRs, the geometric median percentage RH of the FFRs decreased from 90% (95% CI for the median, 81-100) to 0% (P < .001, Wilcoxon signed-rank test) after 24-hour exposure to the silica beads (ie, no moisture remained). Concurrently, the geometric median percentage RH of the bead packets increased from 6% (95% CI for the median, 5-11) to 11% (95% CI for the median, 9-15; P < .001, Wilcoxon signed-rank test) after 24 hours. The median percentage RH of 3 control FFRs respectively sealed in plastic bags without beads (2 replicates) did not significantly lose moisture after 24 hours (96%, 95% CI for the median, 69, 100 vs 91%, 95% CI for the median, 69, 97). The 3 FFRs wetted and dried 9 times were also subsequently found to exclude Bitrex upon fit testing.

These data suggest that exposure to 50 g of food-grade silica beads for 24 hours can remove 100% of approximately 3.75 mL of moisture applied to an N95 FFR with no effect on qualitative fit testing. This study does not speak to inactivation of SARS-CoV-2 by desiccation. Desiccation has not yet been tested as a means of decontamination for FFR; a tightly controlled study using coronavirus would need to be conducted to determine if decontamination can be achieved through desiccation. This study does demonstrate that silica can be used to desiccate an N95, removing moisture that may be generated during the decontamination process using an autoclave or ionized/vaporized hydrogen peroxide, thus enabling the N95 to be more rapidly returned for use. Our findings suggest that a quantitative study of coronavirus-spiked biological fluid, similar to saliva, could demonstrate if desiccation is a suitable means of decontamination in settings when other proven methods are unavailable or when resources are scarce.