So when you see this you should probably take into consideration the whole body of evidence. A synopsis of a recent PHAC rapid review can be found on PHO's website. I read the whole rapid review but it isn't publicly posted.

https://www.publichealthontario.ca/-/media/documents/ncov/research/2021/02/synopsis-young-phac-rapid-review-masks.pdf?la=en https://twitter.com/CDCgov/status/1359536382180544517

These are all lab based studies, not real world, but let me break this down.

When evaluating masks, there are 3 factors we are concerned about:

1. Filtration (otherwise what is the point)
2. Fit (leaks are bad)
3. Breathability (It is useless if you don't wear it)

When it comes to filtration, medical masks (of all types) offer great filtration. Cloth masks are a crap shoot ranging from 10% to >95%. This depends on materials, layers, and construction.

Short version is 3 layer >2 Layer >>>> 1 Layer

Fit is huge, and why I harp on people not bending the nose bar over their nose. A loose fitting mask can reduce effectiveness by more than 50%.

There are lots of tips to improve fit, and the CDC guidance covers some of those, but I'm not sure double masking is the best answer.

I should also add that these appear to have been stationary mock ups and we know that movement is huge. I am skeptical that double masks stay in place very well. A good mask is one you don't have to adjust.

Comfort is a big knock against double masking. The evidence indicates that breathability pretty much dies above 3 layers. So if you put a 3 layer mask over a level 1 or 2 mask, you're not going to have a good time.

I also advise against putting a plastic bag over your head.

Finally, donning and doffing is something that isn't looked at here but I do have some concerns. When you double mask, you are complicating the donning and doffing process and increasing your risk of contamination.

TLDR:
1. Wear a medical mask or a 3 layer non-medical mask.
2. Don't throw out your 2 layer masks since they are still pretty good.
3. Make sure your mask fits properly.
4. Don't neglect the rest of your IPAC strategies.