What causes ponies to yellow?

[Matcha]

Is it the sun or something else that turns ponies yellow? I've heard that sunfading can be used to make ponies less yellow though. I recently purchased a rather white Glory, and I'd like to know if there's any way I can preserve her whiteness! I'm mostly worried about her because I know Glories are quite prone to yellowing 

[lovesbabysquirmy]

LIIIIIIIIIIIIIIIFE!

Yeah and also the sun.

/InvaderZim

[dragonfly]

Some chemical in the plastic which deteriorates over time  and causes discoloration.

[shabbychicdee]

perfume
a lot of scented ponies have badly discoloured over the years.

cigarette smoke also.

[Matcha]

Some chemical in the plastic which deteriorates over time  and causes discoloration.

oh yikes, looks like this isn't preventable then ): that sucks!

perfume
a lot of scented ponies have badly discoloured over the years.

cigarette smoke also.

would the scents from the scented pony affect other ponies near it? o__o

[Mewtwofan1]

     I like to think of it like this. Aging is inevitable. You will age. But granted you don’t expose yourself to dangerous chemicals and stuff you shouldn’t be, treating your body right and you won’t age as fast or look as bad.
     Yellowing of plastic is inevitable, as some of the chemicals do break down and decay. But things like sun exposure, cigarettes smoke and other chemicals would make the process faster. It will keep that glory white for a long time if you treat her right.
     And I’m not sure if this is the correct answer, but I do not believe the chemicals used in scented ponies that cause damage can be transferred to others simply by being near them. But, I’m not a chemist. So uh...best to take that advice with a grain of salt.

[starstrider]

There are a few causes of discolouration in ponies.  External causes such as sunlight, perfumed ponies (sweet berry ponies!), adhesives (so-soft ponies!), exposure to cigarette smoke.  Internal causes are generally plastic oxidisation and ageing), internal rust damage.

Oftentimes there is mild discolouration of all ponies regardless of their body colour, but white simply shows it the most so it appears to be a more severe yellowing when compared to other colours.