Painfully new noob has a backer board question....

[Ogami37]

Hey all! First post and not sure this is the right place for it.... so if its not please forgive my noobieness.
I started collecting comics about 30 years go... I got away from it from a while but recently have pick it up again. About 3 years ago I swapped out all my poly bags for Mylars. At the time I decided to go with BCW and I'm sure most of you can relate... it took days to swap all the bags and boards out.
Today I was pulling some books for an up coming signing and I noticed that the boards were yellowing and browning in spots. Would this be from the comic or from the boards? I thought BCW boards were acid free.... or at least had (as they claim) the lowest PH boards on the market.
Any thoughts?
Thanks!

[Mr.Majestic]

Mylars are great and the fancy boards too but unless you're vacuum sealing too there's always some contamination. I personally put all in mylar, have the boards outside the bags, spaced out to give rigidity and have those, well over 10 000 comics, in a cedar closet.