A couple of points in the thread below indicate things to keep in mind when using "dry heat" packs that depend on rapid oxidation of iron powder. The best short discussion of the chemical reactions involved that I could find in a brief search can be found at http://www.madsci.org/posts/archives/mar98/890260329.Ch.r.html
The basic point, correctly mentioned by several of the posts, is that this chemical reaction uses oxygen; as I understand the equations in the website listed above, it uses a LOT of oxygen, much more than would be contained in the airspace in a typical shipping box. The reaction is also aggressive enough to scavenge basically all of the oxygen out of a closed container rather quickly; worse, in a perforated container the higher temperature inside will retard inflow of cooler outside air into the box. Solving that problem by placing holes such that the box "draws" air also defeats the retention of the heat being produced. In short, my understanding of the oxygen demands of the reaction vs. the amount of heat given off indicates that any box that is going to stay reasonably warm inside (say, 20-30 F above outside air temperatures) is also going to have significant oxygen depletion problems.
So why do experienced keepers find that this shipping method "usually works"? One answer could be that reptiles are able to tolerate very low oxygen levels for much longer than a warm-blooded animal could. This anaerobic capability is better at lower temperatures, however, and it might thus be a mistake to try to keep animals any warmer in transit than they absolutely need to be. Don't forget that monitors have less ability, physiologically, to withstand anoxia than do most other reptiles. What goes first under anoxic conditions are those cells with the highest metabolic demands, which are generally those in the heart and in the more "cognitive" parts of the brain. Unlike heart muscle cells, neurons typically don't repair damage.
This isn't intended as a "don't do it or your lizard will arrive retarded" post. Instead, people should be aware of what is going on with dry heat packs, and consider both the costs and the benefits.




