This is not a question about the merits of BHA or BHT (butylated hydroxyanisole or butylated hydroxytoluene). It is a question about how BHA/BHT work when "added to packaging material".
The BHA is often added directly to food, where it has an antioxidant effect keeping fats from turning rancid during storage. But often, especially on whole grain breakfast cereals, BHA is listed as "added to packaging material".
What mechanism allows the BHA/BHT in the packaging to protect the food? Does it adsorb free radicals for example? Or "added to packaging material" just a marketing trick, because the BHA/BHT actually must migrate to the food to work?
[Ref 1] Encyclopedia of Food and Color Additives, edited by George A. Burdock