All Pex is sensitive to UV light in varying degrees. Manufacturers warn against it being exposed to direct or indirect sunlight, and fluorescent lighting can give off UV light as well. Some have UV inhibitors added during manufacturing but the protection from that is temporary, usually limited to several months of exposure.
As I recall, PVC has these same limitations with regard to UV damage. I can imagine a roll of pex left out in the Arizona sun for a few years getting chalky but I haven't seen it and I don't expect to see degradation from interior lighting such as fluorescent. Lots of folks with interior pex radiant heat systems that have portions of pex exposed to the interior lighting.
Is this situation a condition of the blue/red colored PEX (& white for that matter)? Years ago (20 or so) when my brother engineered both residential and commercial/industrial radiant systems, the PEX was a beige color... wonder if that older stuff responded as you're saying?
It would have to do with the specific resin and additives used on a specific model of tube. It could and probably does vary from mfg to mfg as far as "how" UV resistant the tubing is. As Highbeam pointed out, the residual exposure from fluorescent lighting or indirect sunlight is likely minor to negligible. Of far greater concern is the exposure from storage in between manufacturing and installation. Hence why the UV additives were added in the first place. (Remember, they are only UV resistant for several months.... not indefinitely.) Orange PEX for heating/radiant systems and AlumaPex have oxygen barriers to prevent oxygen from permeating the tube, which is an issue with closed loop systems. The beige/white stuff is still around too, as far as I've ever seen it's the same as the red and blue. The OCD in me loves the idea of doing a house with blue cold, red hot, and white filtered/drinking water lines.
It kills me to use red pex for a cold water line so instead I just buy all white now. My radiant slab has all coated "barrier" pex in red!
Nearly all of the plumbing in my house is plastic of some sort. PVC for waste/drain lines, PEX for domestic supply. I have copper manifolds coming off my hot and cold supplies. All of the hot water is home-run from each fixture to the manifold with red PEX, and likewise for the cold with blue PEX. All of it is exposed to the light in the basement (although not direct sunlight), has been fine for 12 years now. I expect a lifetime out of the plumbing in the house. My basement and garage slabs both have O2 barrier PEX radiant, red in the house and white in the garage (2 different manufacturers). At work, we are just transitioning from cast iron for all of our waste lines, to Sched 80 PVC. The cast iron was getting eaten by corrosive drain cleaners from the inside, and corrosive soils on the outside. All water supply lines are copper, and the geothermal heating system is a mixture of CPVC and copper (ProPress). Holy crap that stuff is expensive. Our next major renovation project, we will probably start using PEX for water supply lines. We use recirc loops because the building is too big to homerun supply lines.
