This article suggests you should consider never storing your personal items in the seat-back pocket on airplanes:
Martha Stewart ~ “Being highly aware of germs and trying to stay healthy on flights has always been an issue for travelers. Travelers are not only keeping some Purell in their carry-ons but also disinfecting some of the dirtiest places on their flight.
There is one thing that most travelers might ignore, though: your seat-back pocket.” ~ Martha Stewart
Please click the link below to view the full article.