Fogger tube is yellowing

[kairi]

I have a Zoomed ReptiFogger and I keep it on most of the time for my tree frog enclosure. I use Reptisafe to condition the water used in it. I've noticed a part of the tubing turning yellow. What's causing this?

[Lacibeth]

Heat and UV exposure will yellow plastics.

Iron content in the water can lead to orangey/rust discoloration as well.