I had the Cables Boxed in the Garage for probably 18 years, they were not exposed to light but probably changes in temperature. It is very dry in here and the temperature in the garage can vary from 40F to 90F
In general I use soapy water to clean anything electronic, sometimes I use GooGone to get rid of more stubborn gunk. Most of the Pomona cables were used when I got them so I don't know in what environment they were exposed to. I know that they aged differently so probably they were exposed to different elements or I got a particular batch that wasn't very good to start with.