Acrylic print turning yellow?

I have a voicemail from a client saying an acrylic print I sold them a few years ago, is turning yellow.  To what extent?  I have no clue, I haven't called her back.  I wanted to get some information first to know what I'm working with before jumping on this call.  

Other than sunlight, what else would cause yellowing? 

UV will do that, the sun is no joke on prints. 

Is she or someone in the house a smoker?  This is #2 in the likely cause of yellowing.
In both cases a layer of glass over the photo would help reduce yellowing.

Is the photo being protected from these? What other environment might exist that could be affecting the photo? What kind of printer are you using and what type of ink was used? Discount inks could have been a factor.  The more modern photo inks from the better brand photo printer manufacturers are far superior to those of 10-25 years ago.

My photo printer is a 3 year old Canon ix6820 with Canon ink and I only print photos with it.

I have an Epson XP-830 printer that's 5 years old that I use for general needs printing. It's supposed to be capable of photo printing, but the quality of the photos from it are terrible, and the Epson black ink cartridges leak droplets onto the photos during printing. I have owned several HP printers in the past that didn't live up to being able to print quality photos using their ink too, although they were supposed to be able to print photos too.
This is proof enough to me that not all photo printers are up to the task of "quality photo printing". These are all general use color printers at best. Are you using a quality photo printer with quality ink? 

There are other good quality photo printers on the market, and I don't have experience with all of them. I looked at sample photos from several brands of true photo printers before deciding on my Canon printer, and it's the lowest cost version of their true photo printers.  

Charley

OMG, that would have to be a LOT of smoke  , for their sake, I hope it's UV doing it.  

Didn't know on the smoke, but know for a fact that if you have the image in the sun over a period of time, that will do the damage to the colors and acrylic will turn yellowish.  

If you plan on keeping the print in the sun, check with the printer you are getting from.  They should have measures for UV exposure.  

More reason to get metal prints

Doesn't the quality of the acrylic play a role in this as well?