Be careful not to puncture the foil. Activated charcoal can be purchased as a cartridge filter in any store that sells kitchen vent fans. Monitor the air space with polished metal samples. Any improvement to indoor air quality is valuable. There is no such thing as a threshold limit for objects.
All damage is cumulative.
Seeing the damage is just a matter of time. Cecily Grzywacz is a Conservation Scientist at the Getty Conservation Institute. She is part of an international team of scientists studying indoor-generated pollutants in museums. Helen Alten is an object conservator specializing in treatments for historical, archaeological and ethnographic materials. She can be reached at Northern States Conservation Center, P.O. Box 8081, St. Paul, MN 55108.