Is the blue gel in ice pack poisonous?

Do reusable gel ice packs absorb any latent heat?

• Ice packs made from water "pack" a massive wallop of cold because water requires a very large amount of heat to power the transformation of solid ice back into water. It takes about 145 times as much heat to turn a block of 31ºF ice into the same amount of 32ºF water as it does to turn it into a 32ºF block of ice. Ice packs therefore stay cold a long time as all of this "enthalopy of fusion" energy is required, and gel packs simply don't match that. A popular reusable gel pack contains these ingredients: Water                                            60 - 70% by weight Propylene glycol                          20 - 35% Nylon Film                                         < 5% Sodium carboxymethyl cellulose     < 5% Polyproplyene Cover.                        < 3%  Do the ingredients above absorb any "latent" heat at typical freezer temperatures, and if so, how does the total heat that it can absorb compare to a comparable volume of ice?

• Answer:

  Water+cold=ice. That's most of what's going on here. Propylene glycol is antifreeze, which depresses the freezing point of water. The solid becomes liquid at a lower temperature, absorbing more heat in the transformation (the latent heat of melting). The gel pack doesn't contain any more "cold" than an equivalent amount of water, but it absorbs heat faster, so it may feel colder. The rest of the "ingredients" are just keeping the thing from splashing around too much when liquid.