My manga are turning yellow just from the top

[Guillermo31]

But I didn't expose them to the sunlight

[† ParasiteEve]

Yeah I have some older ones that do this and they don't get exposed to sunlight (hell I rarely get exposed to sunlight)

It could be from smoking (I don't smoke, some in my house do but not in my room where I keep my manga)

Or it's just an age thing.

Edited September 17, 2014 by ParasiteEve

[Nintendon't]

Paper does that when it ages.

[Moodkiller]

(hell I rarely get exposed to sunlight)

 

Haha! I will admit I laughed out loud at that (due to that fact that I can relate) 

 

@OP - It could also be due to moisture in the air. (not knowing where you live) it would vary on the humidity levels. If you want a bit more of a scientific reason, here you go

[Tsukumo Yuma]

A lot of books do that, moisture can do that, also the oils and moisture that are in your skin, overtime will start to yellow the paper.

[Xegoog]

The real reason is a chemical they use in the paper that makes it deteriorate like that (It's the same thing used in plastics, hence why gray game consoles can yellow). One of my 2 wiimote jackets has yellowed but the other one has remained clear. Heat accelerates this process and literally "cooks" the thing (paper), hence the change in color. It does the same thing with paint on cars. My car's body is 95% plastic and the paint is a semi-thick layer of it too, which is why it has started cracking.