What’s inside disposable hand warmers?

For winter sports enthusiasts, hand warmers can mean the difference between calling it a day early and playing outside for as long as possible. In fact, anyone who braves cold temperatures might be tempted to try the little disposable pouches that emit warmth within seconds of being exposed to air.

Hand warmers date back centuries to when people in Japan would use hot stones to warm their hands, says Keiko Ishikawa, a marketing manager at hand warmer maker Mycoal USA. Portable hand warmers filled with hot ash were the version that followed, she says. These days, you can buy a variety of hand warmers based on battery packs and lighter fuel, but disposable hand warmers rely entirely on chemistry.

Disposable hand warmers turn up the heat in your mittens by means of an exothermic reaction that, in essence, just creates rust. Each pouch typically contains iron powder, salt, water, an absorbent material, and activated carbon. When the pouch is removed from its outer packaging, oxygen drifts across the pouch’s permeable covering. With salt and water present, the oxygen reacts with the iron powder located inside to form iron oxide (Fe₂O₃) and release heat.

The absorbent material can be pulverized wood, a polymer such as polyacrylate, or a silicon-based mineral called vermiculite. It helps retain the moisture so that the reaction can occur. The activated carbon helps to evenly disperse the heat produced, which can average 135 °F.

Although the chemistry of disposable hand warmers is simple, their engineering is more complicated. “You want to make this thing act quickly because people like to open up the packet and feel warm right away, but you also want it to last a long time,” says Joe Vergona, manager of engineering and product development at Grabber Performance Group, a Grand Rapids, Mich.-based company that sells hand warmers. For example, some hand warmers last seven hours, and others can last more than 24 hours.

To lengthen the time a hand warmer lasts, some companies opt to increase the amount of iron in the packet, Vergona says.

Another strategy is to experiment with the iron powder. “If you vary the raw materials in the warmer, you can change how quickly the reaction happens or how much of the warmer is reacted at one time,” Vergona adds. For example, the greater the surface area of the iron, the more it can react with oxygen to produce heat, he says.

The pouch material also affects the performance of the hand warmers. “It’s a balance of the ingredients inside the pack and the performance characteristics of the pouch itself,” Vergona says. The iron powder and other ingredients are contained in a blended nonwoven material that has specific permeability characteristics. If the pouch admits more oxygen, the reaction occurs more quickly. Toe warmers, for example, use a nonwoven material that lets in more oxygen to compensate for the low-oxygen environment inside a shoe. “The level of perforation, the size of the holes—all that’s going to govern how much oxygen enters the warmer,” Vergona says.

To extend the shelf life of hand warmers, the outside wrapper is specially chosen to ensure that minimal amounts of oxygen get in and minimal water gets out. “Any old plastic, and the hand warmers will last a week and die,” because oxygen can get in and spoil the product, Vergona says. The outside wrappers are usually made of polymers such as the plastic polyethylene.

The main difference between disposable hand warmers and some reusable versions is the chemicals used to produce the heat-releasing reaction. Reusable hand warmers don’t contain iron but instead use a supersaturated solution of sodium acetate that releases heat as it crystallizes. Boiling the used packet restores the solution to its supersaturated state. Air-activated hand warmers can’t be reused.

Disposal hand warmers don’t just keep humans from getting too cold. Grabber also sells heavy-duty warmers that can help tropical fish survive transportation through chilly climates.

UPDATE: This article was modified on December 12, 2016 to refresh its information and data.