Question: How do cheap nasty handwarmers work?

Jonny Brooks-Bartlett answered on 23 Jun 2015:

Handwarmers work because of rust, the same rust that destroys your bike in the winter. The key ingredient is iron. Small pieces of iron are scattered in heat packs, isolated by the wrapping on the pack from any oxygen. When the wrapping comes off, oxygen comes in and makes it “oxidize” the iron. Oxidation is what causes a lot of the heat on earth. When a tree burns down, the carbon in the tree is oxidizing, too. Both iron and carbon give off heat when they oxidize.

To heat the handwarmers quicker salt is added to make the reaction faster and so you get the handwarmers working.

I imagine that cheap, nasty handwarmers work in a similar way but take longer to heat because they don’t put as much iron in or salt, or maybe the oxygen doesn’t get to the iron quickly enough because they use a cheaper, less efficient wrapping material. I’m not entirely sure about this bit though.