Have just returned from local Pharmacie without an (for tetchy old me) adequate explanation for the following conundrum.
Everyone is likely to be familiar with the masques jetables many people are wearing so as not to bathe adjacent others in a humid miasma of exhaled breath loaded (potentially) with Covid virus.
I have a packet (which comes without instructions as to its use) which on the face of it 
seem pretty straightforward. Except that each mask has two surfaces, one is pale blue, one white.
A d they are not identical. The blue surface is covered with a soft pleated tissue and looks and feels moisture absorbent. The white surface is smooth and pleated too.
Further examination shows that one length of the soft blue surface has a turned-back hem about 1cm deep, forming a a shallow pocket or gutter that can be lifted with a fingertip. I assume this feature was included in the design and intended to fulfil a purpose.
I was drawn by the design to infer that the blue Ć bsorbent surface was intended as the interior surface, where it might absorb some moisture in the exhaled breath, and incidentally make wearing the mask more comfortable, avoiding a clammy build up. I could see little purpose in designing a mask with a moisture absorbing outer surface to trap germs, and no reason for making it blue (other than identifying which surface, perhaps, should be worn inside).
So far, I thought, so reasonable. But for certainty, and out of curiosity, I asked in the pharmacy for an informed opinion. I had also noticed that all the pharmacy staff were wearing masks blue-side-out, including the head pharmacist.
My question, mildly put, was which side should go outside, and which inside, and why were they different in colour and texture? My question was received with blank astonishment. āBlue is outsideā, was the reply. I asked politely, āParcequeā? " Cāest un filtreā.
I said that I understood that, and that I also understood that it operated to prevent the escape of contaminated breathed-out air, and perhaps the blue absorbent surface tissue was intended to serve that purpose? āCāest un filtre, monsieur. Bleu Ć lāextĆ©rieur !ā No explanation offered, or (presumably) needed.
But I am not convinced, being narkily inclined, and thought of googling it, but had second thoughts: āWhy not put the question to the SF community?ā
So there it is! 
PS The upper edge of the mask (that fits over the nose) incorporates a pliable metallic insert that can be moulded to confirm to the profile of the wearerās nose. That presupposes that the shallow pocket on the blue surface is intended to configure the wearers chin, especially during chin-wags, when a looser fit might otherwise allow the escape of vocalised air.
on Dadās trousered lap. I know who comes off wetter. 