Earlier in the COVID-19 pandemic, decontaminating and reusing N95 FFRs was an important emergency approach for facilities and individuals to maintain adequate supplies of this essential type of personal protective equipment. It may again be necessary to apply this approach, and UV-C irradiation can be an effective and inexpensive decontamination method. Accurate UV-C transmittance data of the FFR layers are needed to inform proper dosing throughout the thickness of the FFR. Without knowledge of the work by Purschke et al. [1], we independently built experimental setups in our two separate laboratories to perform UV-C transmittance measurements on various N95 FFRs, including the 3M 1860, which is one of the most widely used FFRs and one also measured by Purschke et al.

Figure 1(a) shows the Agrawal transmittance measurement setup, comprising a low-pressure mercury lamp as a UV-C source (6035, Newport Corp., Irvine, CA), two 35 mm focal length fused silica plano–convex lenses (LA4052, Thorlabs, Newton, NJ) with an intervening 254 nm narrowband filter (39-312, Edmund Optics, Barrington, NJ), black-anodized aluminum aperture, and 254 nm irradiance sensor (XSD140T254, International Light Technologies, Peabody, MA). The irradiance sensor includes its own 254 nm narrowband filter in front of the photodiode. The additional 254 nm filter between the lenses further ensures that the transmittance of only that wavelength band is measured. The FFR sample is placed on top of the sensor, and the aperture is lowered to make gentle contact with the sample without compressing it. Figure 1(b) shows the Syphers transmittance measurement setup, which is similar in concept to Agrawal’s, without refractive optics and with a lateral sample translation mechanism. It also uses a low-pressure mercury lamp (GS411, General Electric, Boston, MA), and the UV sensor (TOCON ABC3/ABC5 by sglux, Boston Electronics, Brookline, MA) has a broader spectral response of 227–360 nm peaking at 290 nm, used with a narrowband 254 nm filter (67-809, Edmund Optics) to confirm UVC as the only measured component. The sensors in both setups include a light-diffusing Teflon disc in front of the photodetector, which allows for a broad angular acceptance of light similar to an integrating sphere, which was used in the spectrophotometric transmittance measurements performed by Purschke et al. Having a broad angular acceptance range is necessary when measuring the total transmittance of scattering materials such as the layers of an FFR.

 

Fig. 1. UV-C transmittance measurement setups in the (a) Agrawal laboratory and (b) Syphers laboratory.

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Agrawal took a single transmittance measurement through an 8.4 mm diameter area of each individual layer of three FFRs from one manufacturing lot. Syphers measured transmittance through 24 spatially separated points across each layer of four FFRs from four different manufacturing lots, including one FFR provided by Agrawal and one provided by Purschke. Each of Syphers’ measurement points covered a 3 mm diameter area.

Table 1 shows 1 in² samples of the four individual layers of the 3M 1860 FFR and our transmittance estimates for each layer alongside what Purschke et al. reported. Agreement between our two laboratories’ transmittance values is excellent, within one standard deviation for all layers except filter layer 1. However, our transmittance values are generally lower than Purschke et al.’s, especially for the outermost and innermost layers, which we observed to be the least transmissive of the four layers.

Table 1. Mean UV-C Transmittance of 3M 1860 FFR Layers, with Uncertainty in Parentheses

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The innermost layer is 0.5–1.5 mm thick and visibly dense, so <1% UV-C transmittance is quite reasonable. Purschke et al. used an integrating sphere spectrophotometer, the accuracy of which can be hindered in the UV-C range by weak output of the illumination source, low photodetector responsivity, integrating sphere losses, stray light, and sample autofluorescence. We speculate that one or more of these instrument limitations caused the apparent transmittance to be considerably elevated for the outermost and innermost layers of the 3M 1860.