Methylene blue applied to N95 respirators and medical masks for SARS-CoV-2 decontamination: What is the likelihood of inhaling methylene blue?

Abstract

Background: Global shortage of personal protective equipment (PPE), as consequence of the COVID-19 global pandemic, has unmasked significant resource inequities prompting efforts to develop methods for safe PPE decontamination for reuse. The World Health Organization (WHO) in their Rational Use of PPE bulletin cited the use of a photodynamic dye, methylene blue, and light exposure as a viable option for N95 respirator decontamination. Because WHO noted that methylene blue (MB) would be applied to surfaces through which health care workers breathe, we hypothesized that little to no MB will be detectable by spectroscopy when the PPE is subjected to MB at supraphysiologic airflow rates.

Methods: A panel of N95 respirators, medical masks, and cloth masks were sprayed with 5 cycles of 1,000 uM MB solution. Mask coupons were subjected to the equivalent of 120 L/min of 100% humidified air flow. Effluent gas was trapped in an aqueous solution and the resultant fluid was sampled for MB absorbance with a level of detection of 0.004 mg/m3.

Results: No detectable MB was identified for any mask using Ultraviolet-Visible spectroscopy.

Conclusions: At 500-fold the amount of MB applied to N95 respirators and medical masks as were used for the decontamination study cited in the WHO Rational Use of PPE bulletin, no detectable MB was observed, thus providing safety evidence for the use of methylene blue and light exposure for mask decontamination.

Keywords: COVID-19; Coronavirus; Global health; Personal protective equipment (PPE); Photodynamic therapy; Singlet oxygen.

Fig 1

Panel of N95 respirators, Medical Masks (MM) and a community cloth mask tested. From left to right: Halyard Fluidshield Duckbill 46,727 N95 respirator, 3M 1,860 N95 respirator, Kolmi Duckbill Filtering Face Piece (FFP) 2 NR Type 2R, 3M VFlex 1804 N95 respirator, 3M 1,870 + N95 respirator, EN 14,683 Type 2 medical mask (generic), cloth community mask.

Fig 2

MB solution. Appearance of 1,000 µM MB in spray bottle.

Fig 3

Sprayed and dried N95 respirator. Example of mask immediately after MB spray application with 1,000 µM MB (left) and after drying for 24 hours (right). MB is visible to the unaided eye on the respirator but with less intensity compared to immediately after application.

Fig 4

Mask coupon bolt. Experimental design of the mask coupons and the bolt through which air was passed. (A) Coupon bolt cross section; (B) Coupon bolt side view; (C) Components of bolt and a cut out mask coupon (yellow arrow) in the center of the image.

Fig 5

Testing apparatus. Diagram of how air was humidified and then passed through the various mask samples and trapped within a solution for subsequent spectroscopy.