...open cell plastic foam which starts deteriorating within 1-2 years after manufacture; thereafter its disintegration accelerates. Fine foam dust and flakes will quickly penetrate through the screen and be deposited all over the (electrostatically charged and therefore attractive) capsule area. The final stage of this material is that of a semi-liquefied goo, and then the fun begins...
For form-fitting foam lined microphone cases the same rules apply as for the wind screens:
Once the foam visibly flakes off when rubbing it hard, it's time to at minimum use a particle barrier (i.e. plastic bag) to prevent the foam particles from hitting and melding onto the capsule.
