Low cost snorkelling masks are being investigated as improvised full-face PPE during the current CoVid crisis using medical filters attached via downloadable 3D printed adapters.
The idea probably does have some merit.
I was worried about possible excessive re-breathing and CO2 build-up with some connector designs, so performed an experiment with a sidestream ETCO2 analyser and a no-name mask. Despite superiority claims of some versus others I suspect they are all moulded in the same factory.
NOTE: This not a clinical trial, it is a single test on one person - myself.
These are my personal views only, expressed below:
Specifically:
- These masks have 3 ports in the top connection. The central one is for inhaled gas and the lateral ones are for exhaled gas. There is also an additional exhale valve on the front of the mask, probably for purging water out of the mask.
a) Some 3D printed adapters occlude the lateral ports, the idea being that you will inhale through the filter and exhale through the front valve of the mask.
b) Other adapters do not do this, all ports meet together at the filter.
You might expect less CO2 build up with (a) than with (b).
I tried both on myself, taking slow, measured breaths just as a diver would do using any kind of equipment similar to this. To reduce CO2 re-breathing this is generally good advice anyway.
1) With lateral top ports blocked, more exhaled gas came out through the front port as expected. However some gas still did emerge through the top filter (possible poor seal across the lateral ports of my 3D print).
2) My ETCO2 was a little higher with the design that did not block the lateral ports, but the difference was marginal.
3) Work of breathing was lower with the design that did not block off the lateral ports.
4) It has been suggested that a Y with 2 filters does help with work of breathing. I would agree that reducing the work of breathing by changing from a single filter to two would be desirable. It was not that pleasant after a while with a single filter.
5) The mask partially steamed up with BOTH arrangements. Not severely so but an anti fog spray or something to help with this would be desirable.
6) The plastic, although very clear, is a little wavy and does distort vision slightly.
7) A good seal check on the overall mask is to put your finger over the top of the filter and check that you cannot then easily breathe in. Note that this does not necessarily tell you that the inner mask is sealing well.
8) In the design with lateral ports blocked, some exhaled gas did still emerge from the top of the filter, i.e. it did not create a fully one way system of valves as intended, only partially so. Therefore I would NOT suggest putting a long hose on this with a filter on the end with either of these designs of adapter, if you should ever be tempted to do so.
In conclusion:
- Inner mask has to be a good fit. If you breathe gas back and forth from entire mask area, including around your eyes, this would be a big problem.
- Assuming this, and assuming you take slow moderate sized breaths (i.e. not small breaths) the ETCO2 did not appear to excessively increase with either design.
- Keeping the internal volume of anything you attach to the top connector between the mask and the filter(s) as low as possible would be a sound idea.
- I suspect if I physically blocked off the side-ports, not relying on the 3D print to achieve that, then there would be no re-breathing at all up through the filter and the mask would then not steam up at all, which would be my next experiment.

свернуть