Ski season is upon us. There is no greater relief on a frigid winter day than warming cold, painful fingers and toes. In a recent issue of Wilderness and Environmental Medicine (2009;20:33-38), William Sands, Ph.D. and colleagues authored an article entitled, “Comparison of Commercially Available Disposable Chemical Hand and Foot Warmers.” The objective of their study was to characterize the thermal behaviors of 14 commercially available hand and foot warmers.
The warmers were studied in pairs in a laboratory setting, not in frigid conditions. Each warmer was monitored with a rapidly-responding thermister to determine its external temperature. One of each pair of warmers was placed in a boot or glove. Temperature was recorded until the heat output of the devices ceased and the temperature was determined to be identical to ambient temperature.
The results were quite interesting. There was variability both within and between manufacturers and types of warmers. Some of the devices exceeded packaging claims, while others fell short. The greater the mass of the warmer, the longer the duration of heat production.
Commercially available disposable warmers rely on a chemical reaction involving iron powder, water, salt, activated charcoal and vermiculite. One such brand is Grabber warmers. The iron in the mixture is exposed to air when the packaging is opened. The iron oxidizes with salt as a catalyst. Carbon helps disperse the heat, while the vermiculite helps act as an insulator to control the rate of the reaction. Once the iron converts to iron oxide (similar to rusting), the reaction ceases.
Based on the variability of the warmers in their thermal behavior, and the fact that all hand warmers and body warmers showed longer durations of heat production than did the foot and toe warmers, the authors made recommendations:
1. Do not rely upon a single warmer to reduce cold exposure. Carry several warmers to compensate for failures and short duration of warming.
2. Do not expect the devices to work if they become wet via submersion into liquid.
3. The heavier the device, the longer the heat production. This has implications for foot and toes warmers, that must be constrained in size in order to fit within a shoe or boot. For foot and toe warmers, a greater number of warmers should therefore be carried, to replace warmers that are no longer warm.