Pvc sched 40 DWV pipe has been used outside as a vent pipe for years and I have never seen it deteriorate as others have mentioned but i live in upstate NY. PVC is also used for furnace venting on high eff. furnaces and ran outside with the same results of nothing happening. It might be right that the pipe needs to be painted or sealed but I have only seen the pvc painted to make it blend into the roof color and even then after a few years the paint chips off and looks like ship. I know I should right it up, but it is so common that even if I do write it up I will be the only one doing so and every contractor and Code inspector will laugh it off and say I have rocks in my head. That being said it is your call depending on your area.

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Well thats my 2 cents.
Brian Kelly
Kelly Home Inspections