//I thought I kept a copy of the earlier WHO advice but I cannot seem to find it.//
Found it. It's in this document:
https://apps.who.int/iris/bitstream/handle/10665/331693/WHO-2019-nCov-IPC_Masks-2020.3-eng.pdf?sequence=1&isAllowed=y
A couple of passages I dragged out of Page 2:
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Advice to decision makers on the use of masks for healthy
people in community settings
As described above, the wide use of masks by healthy
people in the community setting is not supported by current
evidence and carries uncertainties and critical risks. WHO
offers the following advice to decision makers so they apply
a risk-based approach.
However, the following potential risks should be carefully
taken into account in any decision-making process:
• self-contamination that can occur by touching and
reusing contaminated mask
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Note the mention of "critical risks" and the one in particular that I am concerned with.
I'll try to find the latest WHO advice to see how it differs a bit later.