Hand warmers were used for several years before becoming commercialized in 1971. After many years’ research, including testing by an Antarctic expedition team, Niichi Matoba created the company Hakkin Warmers. While Matoba’s hand warmers were activated through oxidation (exposure of iron to oxygen), lighter-fuel, charcoal, and battery operated hand warmers have also become available. I’m a huge fan of the latter, as they are rechargeable, large enough to fully grasp (yet remain pocket-sized!), and last up to 6 hours with a single charge. Perfect for any trek outside during the Boston winters!