Oxygen absorbers have totally changed the food storage process. It used to be a lot more work (and mess) to dry pack for home storage.
But these U.S. Military-spec O2 absorbers remove up to 99.8% of the oxygen from your sealed containers. That leaves a nitrogen atmosphere of 95%, which is exactly what you want for safe long term food storage.
Several people have asked how to use oxygen absorbers, so let's answer it here. The ones I prefer are oversized oxygen absorber packets (500cc or 2000cc) because I need fewer of them per container.
IMPORTANT NOTE: Oxygen Absorbers should be used within 15 to 30 minutes after opening the package for maximum oxygen absorption. If used within that time frame, you will avoid wasting any packets. Seal those you will not be using right away back in their own bag or seal them with a vacuum sealer. (Do not use Ziploc bags - they will never be air tight.)
Do not plan on storing oxygen absorbers - they will only last about one year after they are manufactured. So don't buy a whole bunch unless you are planning to use them right away. They are great because they have increased the storage life of foods and made the job of dry canning food products much easier. Just don't buy more than you can use.
It's a chemical process - they contain iron powder and salt which react with the oxygen in the air causing the iron powder to rust. Once the iron powder has oxidized, they are "loaded" and the absorption stops.
The number of oxygen absorbers required varies by:
Using the 500cc size oxygen absorber:
Remember that it depends on what you are packing, i.e. macaroni would have more air in the container than wheat. It is not harmful if you use more than what is required.
For those who like a little more technical answer:
A full 6-gallon bucket of grain or beans has about 1791cc oxygen left inside the container at sea level. If the bucket contains food such as pasta, it will have more air and need more O2 packs; a powdered food will have less air and need fewer.
You will need 3-4 of the 500cc oxygen absorbers if you are packing your food at sea level to about 4,000' elevation, and just 2-3 for 4,000' to 7,000' elevations.
In short, no. Hand/foot warmers won't reduce the O2 down to the levels that proper O2 absorbers will. There were tests done back in the 90s comparing their effectiveness to O2 absorbers and they came up short.
There is a threshold level where food can be safely stored and over that, oxidation can occur, bug growth can flourish, etc. Hand warmers will indeed bring down the 02 level, but short of testing the 02 level, how would we know if it lowered the level down into the safe zone? I would never trust them for food storage.
No. They are non-toxic but it's not a good idea to eat them and they don't taste good. (Some have mistaken them for a seasoning package - seriously!) So when you open a container of dehydrated or freeze-dried food, find the absorber inside and toss it in the garbage.
In summary, oxygen absorbers are over rated to give you a protection factor if you should leave them out in the air too long. Generally, you have about 15 to 30 minutes before they reach this advertised minimum. Using more than required is better than less and not harmful.