A question from Twitter: can you wear glasses under a full face respirator? The answer is no, but I wanted to see just how bad the results would be.

The reason you can't wear glasses under a full face respirator is that the mask seals to your face, not to your nose and mouth. The arms of your glasses break the face seal and let the bad air in. It only takes a small leak to completely ruin the safety of your respirator. The PortaCount test shows just how badly wearing glasses under your respirator compromises the air tight seal.

The correct way to wear prescription lenses with a full face respirator is to buy the appropriate prescription lens insert and have new prescription lenses fitted into the insert. This is annoyingly expensive, so it is too bad that you can't wear regular glasses under your full face respirator but you can't, so don't.
For example, 3M sells a prescription lens insert for the 6800 series respirators:

https://www.3m.com/3M/en_US/p/d/v0000...

Different respirators need different inserts.

There are some universal inserts you can buy, like these ones that suction cup to the inside of the mask, but I'd hate to have the suction cup fail while I was wearing the mask and couldn't take it off to reseat them.

https://rxsafety.com/productcategor...