Types
Air-activated hand warmers work through the process of oxidation. When the sealed package is opened, air enters the tiny holes in the the warmer's polypropylene bag. The air interacts with the iron that is in the bag and creates iron oxide---rust.
Supersaturated solution warmers contain a metal device in the center. Users snap the metal disc, which causes the crystals to precipitate. The heat energy of the warmer is locked into the solution; precipitation releases the stored energy to produce a temperature of up to 130 F (54 C).
Contents
In air-activated warmers, the bag traps moisture. Vermiculite in the warmer helps keep the heat from dispersing. Carbon may be included to help distribute the heat throughout the warmer. Salt is a catalyzing agent. Cellulose acts as a filler. Air-activated warmers can also contain charcoal and sawdust.
Supersaturated warmers contain water, food grade salt (sodium acetate) and a metal disc. An initial boil dissolves the salt. The contents become a clear, supersaturated liquid.
Benefits
Air-activated hand warmers are nontoxic and they are safe for airline travel. They often come in single-use items, so they can be thrown out following each use. Some air-activated hand warmers contain nontoxic biodegradable components.
Supersaturated warmers are reusable (hundreds of times) if you boil the device for 5 to 10 minutes, then allow it to cool. The heat of boiling converts the crystals back into liquid form.
Time Frame
Grabber, a brand of air-activated warmers, may last 12, 20 or 40 hours. Other brands may last between 4 to 40 hours. Special mini-warmer packets are made with adhesive for use in shoes, boots and inside clothing. Heat Factory makes a line that is safe for children.
Supersaturated warmers do not last as long as iron-oxide warmers. Hot SnapZ is a brand that provides warmth for 30 minutes to 2 hours.
Considerations
While all of the warmers mentioned can be used at sporting events, by hunters, fishermen and while camping, Hot SnapZ can also be used underwater because their heat is not dependent on oxygen, making them valuable for divers.
