When the bubbles are tingling/tickling your face. IT IS NOT IRRITATING, and NOT BURNING. Just tingling because bubbles are forming, and popping/moving. You know, like someone is running feather toys on your face -- that kind of feeling. But it only happens once in a while and doesn't go on for the entire time. So you can still definitely relax, lie down for 5 minutes with eyes closed, with this on.
And don't forget to take a picture after the mask carbonates, it will be very very funny!
Note: Allergic skin with caution. I used this on myself and it did not cause redness or any problems. If make you itchy, please stop using it.