Apart from dressing in warm clothes and trying to stay out of the cold, there is one more answer to staying warm in adverse weather. These are hokkairo. Hokkairo are small pouches that contain chemicals that heat up when exposed to Oxygen in the air. Hokkairo are inexpensive (about 100 - 150 yen per packet) and provide up to 8 hours of continuous heat. They also come in different temperature ranges, with the hottest hitting peak temperatures of 70C. Most operate at around 50 - 63C. Hokkairo are designed to be placed in a jacket pocket, however those people with poor circulation will be pleased to know that there are some hokkairo available that are designed to be placed in shoes to keep your toes warm. Some also come with an adhesive backing so that they can be placed on your back under a jacket.
Although they may not feel that hot, hokkairo can cause serious burns if used incorrectly. The gentle heat that the hokkairo give off has a numbing effect on the skin. The unfortunate side effect of this is that the user may not be aware that they are being burnt until serious damage has been done. A good rule to avoid burns is to keep a couple of medium thickness layers of clothing between the hokkairo and your skin. Purchase cooler operating hokkairo if this is unavoidable. Placing the hokkairo in an internal jacket pocket will warm the air that the jacket traps close to your body, thus keeping your whole torso warm. Toe warmers are generally an exception to this rule, as they are designed to run at cooler temperatures.
Hokkairo are small pouches containing iron, water, cellulose, vermiculite, activated carbon and salt. Opening the plastic wrapping exposes the slightly moist iron to the Oxygen in the air, forming Iron Oxide (FE2O3, commonly referred to as 'rust'). 'Rusting' isn't the fastest chemical reaction on the planet (one which would normally take days, or even weeks), so salt is added into the mix to act as a catalyst, and thus, considerably speeding up the reaction. Now that the reaction is giving off heat, the carbon is used to evenly distribute the heat around the packet and the vermiculite (an inert mineral with insulating properties) is used to retain the heat, thus giving off a gentle warmth. Cellulose is added as a filler, to give the pouch a bit of 'volume'.
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