Methods and materials

The testing equipment consisted of a PortaCount 8030 Respirator Fit Tester (TSI, Minnesota, USA), Particle Generator 8026 Tester (TSI, Minnesota, USA), Surface Pro (Microsoft, Redmond, WA), Nellcor OxiMax N-65 Pulse Oximeter (Medtronic, Minnesota, USA) (Figure 1), and a gauge manometer (Instrumentation Industries, Inc., Pennsylvania, USA). The PortaCount 8030 was calibrated March 6, 2020 by TSI. 

Figure 1. From left to right: A) pulse oximeter, B) SurfacePro, C) PortaCount 8030, D) ARIA QR+ snorkel mask, and E) particle generator 8026. 

Testing was done in an approximately 9 ft. x 9 ft. room with the particle generator on. A 3M 6800 series (3M, St. Paul, MN) full-faced mask served as the benchmark. 

Full face snorkel-mask

An Aria QR+ (Ocean Reef, Inc., California, USA) Medium/Large full-faced snorkel mask was tested since the design minimizes fogging problems and CO₂ rebreathing.

A hole was drilled into the mask and a PortaCount grommet for testing N95 masks was inserted. Silicone was added to seal the external and internal surfaces. Crazy glue was used to secure and seal the contact between the metal grommet and plastic tube to reduce air leaks that can impair accurate quantification of the fit factor.

Filter 

For filtration we selected a 3M P100 particulate filter with 99.97 % filter efficiency meeting NIOSH P100-series test criteria. 

3D printed adapter 

The adapter was a modification of the APA – Aria Protection Adapter available at the Ocean Reef website (https://oceanreefgroup.com/covid19/).  The original adapter was designed to accept a 40 mm particulate air filter with 1/7 inch thread. The design was modified to accept two 3M P100 filters.  We made an earlier prototype with a single filter but found it required too much breathing effort to use for long durations. The CAD design is available here: https://www.tinkercad.com/things/7dZnjwUKZRr The design reduced the printing material required compared to a vertical alignment of the filter. 

The adapter prototype was a black polylactic acid (PLA) print. PLA prints are porous and without an airtight coating they will not work in an air filtration application. Two coats of XTC-3D (Smooth-On, Inc., Pennsylvania, USA) were applied to make it airtight. One 3M 3PRG7 (3M, St. Paul, MN) inhalation port gasket was placed at each of the two inhalation ports. Finally, one P100 filter was attached at each of the two inhalation ports. Sticky putty (Alcolin, Cape Town, South Africa) was used to ensure an airtight seal between the mask and the snorkel connector.

Fit testing 

Fit testing was performed using the OSHA 29CFR1910. 134 protocol in the PortaCount 8030. The fit test exercises include normal breathing, deep breathing, turning the head side to side, moving the head up and down, talking, grimace, bending over, and normal breathing. Each test exercise lasts one minute except for the grimace exercise which is 15 seconds. A passing fit factor for full face masks like this design is 500, while for half face masks is 100. The fit factor is expressed as the challenge aerosol concentration outside the respirator divided by the challenge aerosol concentration that leaks inside the respirator during a fit test. Therefore, the higher the number the better.

Test scenarios

Daily QA for the PortaCount with the particle generator active was performed and passed. We tested the 6600 series 3M mask as our benchmark and three experimental configurations of the snorkel mask. These four experimental setups were named as follows:

• 3M 6800 series mask: benchmark
• Snorkel mask duct tape: In this setup the front plastic cover was removed as shown in this video (skip to 1 minute): https://www.youtube.com/watch?v=ewrsJ4lTgj4&feature=emb_logo
• Snorkel mask no modifications: This setup preserves the original configuration of the snorkel mask including the mushroom valve and protective cover in front of it.
• Snorkel mask mouth cover removed: This setup preserves the mushroom valve, but removes the plastic protective cover in front of it as shown in the previous video.

The mushroom valve by the mouth was also removed. Two square pieces of duct tape covered the resulting opening to seal the port’s opening. 

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Results

3M 6800 series mask

The 6800 series 3M mask served as our benchmark and reached its maximum fit factor in 1:15 minutes. O₂ Saturation remained stable. Real time test results can be seen here:

https://drive.google.com/file/d/1hY2ZT3B8HeySWEC7M7wPVEo2ACfaUtaZ/view?usp=sharing

The 3M mask passed the fit test results with a fit factor of 333867 (see Table 1). Fogging or humidity were not an issue.

Snorkel mask duct tape

This configuration reached its maximum fit factor in approximately 2:11 minutes. O₂ Saturation remained stable. Real time test results can be seen here:

https://drive.google.com/file/d/1ETWlNk0VjiZnSMaRXsm5R3HmVk0Zrd-M/view?usp=sharing

This configuration passed the fit test results with a fit factor of 32281 (see Table 1). Increased humidity decreased the comfort of the mask although fogging was minor.

Snorkel mask no modifications

This configuration reached its maximum fit factor in approximately 1:56 minutes. O₂ Saturation remained stable. Real time test results can be seen here:

https://drive.google.com/file/d/1Hpwc90kCozlu2EnfLrXCWGt5bk2bqQw8/view?usp=sharing

This configuration passed the fit test results with a fit factor of 15448 (see Table 1). Fogging or humidity were not an issue. Further evaluation of this configuration generated during inspiration a negative pressure of 2 cm of water and no positive pressure during exhalation.

Snorkel mask mouth cover removed

This configuration reached its maximum fit factor in approximately 1:08 minutes. O₂ Saturation remained stable. Real time test results can be seen here:

https://drive.google.com/file/d/1kPSIH0cFRrhGwltuDOD-RODMJAn9U_OG/view?usp=sharing

This configuration passed the fit test results with a fit factor of 1105 (see Table 1). Fogging or humidity were not an issue.

Table 1. Fit testing results for the different mask configurations

	3M 6800 series mask	Snorkel mask duct tape	Snorkel mask no modifications	Snorkel mask mouth cover removed
Normal breathing	297913	44910	18511	1123
Deep breathing	288127	40394	27612	1412
Head side to side	442017	63628	38360	1541
Head up and down	349546	66749	63259	1234
Talking	266729	21200	7933	1953
Bending over	326150	12676	7534	929
Normal breathing	454406	76511	16065	593
Overall fit factor	333867	32281	15448	1105
Fit Test*	Pass	Pass	Pass	Pass

*OSHA fit factor passing value is 500 or greater

Subjective user experience

A radiation therapist wore the non-modified snorkel mask from 9 AM to 3 PM while performing daily work activities which require an increased level of exertion while positioning and moving patients to the treatment couch of the linear accelerator. She took off the mask for lunch and for a break to have a drink. She treated between 25 and 30 patients that day. Visibility was great and comfort better than other PPE she had used. Near the end of the daily with increased physical activity she felt some breathing discomfort. She measured the O₂ sat at the time which was 100 %. Overall her feedback was that it was a comfortable option she could tolerate for prolonged periods of time. The only negative feedback was that the patients and the other therapists had a hard time hearing her, requiring speaking up or using hand gestures. Talking on the phone revealed no issues.