Comparison of Commercially Available Disposable Chemical Hand and Foot Warmers

Winter recreation enthusiasts, hunters, campers, hikers, winter sport athletes, ski patrol members, and search and rescue units often find themselves in staggeringly cold environments. A common denominator among these groups is the need to protect extremities against the cold. One popular method involves disposable hand, body, and foot warmers (HFWs) that rely on chemical reactions with air to produce heat and that are simply inserted by the user inside gloves and boots. These warmers are relatively inexpensive, disposable, light, and easily carried. However, there is a paucity of information on the thermal behavior of these devices.

These warmers rely on a chemical reaction involving iron powder, water, salt, activated charcoal, and vermiculite to produce heat. A US Patent Office search showed that different types of warmers go back in history to at least 1924 (eg, patent 4 366 804, 1983). Most warmers use a simple chemical reaction that is similar to rusting when the chemicals within the warmers are exposed to air. The iron in the mixture is exposed to air when the plastic packaging is opened, the iron oxidizes with salt as a catalyst, and carbon helps disperse the heat while vermiculite helps act as an insulator to control the rate of the reaction. The reaction is slow enough that it can last for hours. However, the iron eventually converts to iron oxide and the reaction stops.

In preparation for the 2006 Torino Olympics, pilot work was undertaken to determine the thermal behavior of these devices. Anecdotally, the primary author is also a member of the Teller County, CO, Search and Rescue Unit, which had tried both iron-air–based warmers and sodium acetate–type warmers and found the iron-based warmers simply more practical in a fast-moving search and rescue effort. Both sodium acetate–type warmers and the warmers used in “Meals Ready to Eat” were considered for this experiment, but both were deemed impractical for use during the Games because of the danger of puncturing of a liquid or semi-liquid inner material, mass, the duration and magnitude of heat production, the odor (Meals Ready to Eat), and the impractical nature of tasks required to reuse the sodium acetate–type warmers. The simple nature of the iron-air–based devices, prior search and rescue experience, and pilot laboratory work showed that the timing and duration of heat production were widely variable in spite of the fact that this type of warmer is the preferred type of hand and foot warmer. Information obtained from packaging rarely matched measured results. However, given their ubiquitous use, determining how long these devices take to reach peak temperature and the duration and magnitude of heat production are important characteristics that users should be aware of in order to apply these devices effectively. Athletes, coaches, officials, volunteers, and many others are exposed to hours of extreme cold during winter sport events. Portable, lightweight, and effective cold countermeasures are important to the success of Olympic performance and participation.

The purpose of this investigation was to characterize the thermal behavior of a variety of disposable chemical iron-air–based HFWs. The resulting descriptive characteristics of thermal behavior, peak temperatures, and durations of heat production should help those who use these devices make more informed choices, understand the behavior and limitations of these devices, and be able to predict thermal behavior with greater precision.

Reliability values were assessed via intraclass correlations of measurement trials; technical errors of measurement and statistical differences between trials were calculated.^{,  ,}  Reliability of all warmers’ trial data of maximal temperatures resulted in an intraclass correlation of 0.95, with a relative technical error of measurement (TEM) of 5.1% and an absolute TEM of 3°C. A matched-pairs t test of the difference between trials was not statistically significant (P > .05). These values indicate that our measured temperature values showed excellent reliability and relatively small measurement errors. The reliability of the time periods with temperatures exceeding 30°C showed an intraclass correlation of 0.98 and no statistical differences between trials (P > .05). However, the TEMs were large. The relative TEM was 29%, and the absolute TEM was 3.6 hours. The TEM values point to the intramanufacturer variation that was seen in several of the devices.

Table 1 shows descriptive information on the various types of warmers and the packaging indications of the warmers’ expected performance. Table 2 shows the results of the temperature measurements performed on the various warmers.

By consulting Tables 1 and 2 one can see that there are variations in measured behaviors. In some cases the warmers met and exceeded the expectations laid out in the packaging information, while in others the warmers fell short. The average temperature on packaging materials was not clear with regard to the duration over which the average was obtained, making further analysis impossible (beyond duration description). However, maximum temperatures from both measured values and from packaging were comparable. The difference between packaging and measured values for maximum temperature was not statistically significant (matched-pairs t test, P > .05) across all warmers. A comparison of maximum temperatures of the hand and body warmers with the foot warmers showed that the manufacturers’ listed values were statistically different (t test, P = .005), while the measured values of maximum temperature were not (t test, P > .05).

Figure 1 shows the most typical—and probably most desirable, in terms of predictability—thermal behavior of one type of hand warmer tested. Figure 2 shows an example of variability within a manufacturer's model. Figure 3 shows a less common temperature-time pattern, but serves as an example of the range of temperature changes. The correlation between the masses of the devices and the mean of the temperature trials greater than 30°C was r = 0.94, indicating that the amount of reactive material in the device was highly predictive of the duration of heat production.

Although there was a dominant form to the temperature-time curves (Figure 1), there was also variability between warmers of the same manufacturer and warmers from different manufacturers. In an effort to depict the variety of temperature-time patterns observed, 3 examples are shown in Figure 1, Figure 2, Figure 3. These examples show that these types of warmers 1) reach a near-maximum range that is held for a period, and then heat production decays rapidly (Figures 1 and 2), or 2) produce heat rapidly to a shorter duration peak and then decay rapidly (Figures 2 and 3). Unfortunately, consistent behavior was not the hallmark of these types of warmers.

Cold injuries are a serious risk to people who engage in winter sports or winter wilderness activities, particularly the military. Historically, the morbidity from frostbite, frost nip, and misdiagnosed trench foot reached as high as 27% to 33% in British soldiers during World War I. World War II showed similar problems, with approximately 10% of the total fighting force on both sides lost to cold injuries via death or incapacitation. The Korean War later resulted in a similar percentage of cold injuries, and even more recently the British claimed trench foot as a major medical problem in the Falklands conflict.

Cold injuries are the result of a combination of time and temperature. The colder the environment, the less the time required to produce tissue damage. Wetness, such as soaking or dropping the devices in water, will likely preclude the use of the small thermal devices studied here. Experience has shown that dropping these devices into water during trekking renders them unusable. Although we did not study these devices in water, it is unlikely the chemical reactions will occur to any helpful extent if the devices are wet. Thus, chilbain, pernio, and trench foot are unlikely candidates for prevention via these types of chemical warmers. However, dry/cold injuries such as frostbite and frost nip may be prevented by the judicious use of thermal adjuncts to the extremities (eg, the chemical warmers studied here). Experience with Olympic athletes has shown that perspiration during winter outdoor competition is minimal and should not reach levels that would incapacitate these types of warmers.

After searches of PubMed and Sport Discus using “hand warmer,” “foot warmer,” “toe warmer,” and “chemical warmer” as search terms, the authors were unable to find any relevant research articles on these types of devices. However, there are a few web sites that provide nonresearch information on chemical hand and foot or toe warmers, such as Wikipedia and ScienceIQ.com. The lack of research data was surprising, given the common use of these warmers by people who venture into the winter wilderness or participate in winter sports. Unfortunately, comparisons with existing literature are not possible.

The primary findings of this study were that 1) the warmers vary in thermal behavior, sometimes in small amounts and sometimes greatly; 2) the variability occurred within and between manufacturers; 3) thermal patterns over time were sometimes astonishingly different; and 4) there is a strong relationship between duration of heat production and the mass of the device. All but two of the hand warmers showed different temperature durations in terms of hours. All hand warmers and body warmers showed longer durations of heat production than did the foot and toe warmers.

Those people who use iron-air–based warmers should probably not rely on a single warmer to reduce cold exposure. Instead, people should have several warmers with them so that if one fails or its duration of heat production is exceeded it can be replaced. Finally, the strong relationship between the mass of the devices and their heat production indicates that as a simple rule, the heavier the device, the longer the heat production. Of course, this is predictable based on simple chemistry. However, the relationship is especially important with the foot and toe warmers that are constrained in size by virtue of having to fit within a shoe or boot. The constraints of a boot or shoe on the size and mass of the devices should caution users to perhaps bring more of these types of warmers than of those for hands and body for winter sports and winter wilderness activities.

This study was limited to the specific HFWs immediately available to us in the local Colorado Springs area and to heat production measurement while in a laboratory environment with a typical ambient temperature between 22°C and 25°C. No iron-air–type devices were intentionally excluded.

Future research should try to broaden this study to include a wider variety of warmers using different chemical processes and the influence of insulation on user warmth and comfort.