Winter whiteouts, freezing weather and power outages everywhere from Europe to Washington and Beijing have been making headlines these last few months, but as fingers by the million have been going numb, Japan has one nifty weapon of mass defrostation to alleviate the seasonal problem: self-warming pads.

Popularly known as kairo, literally meaning “fire inside the pocket,” these pads emitting neither flames nor smoke nonetheless warm the cockles of many a Japanese heart when the mercury takes its seasonal dive.

With the disposable types, all you need to do is to rip off the outside plastic wrap and shake the bag a few times — though some types nowadays even don’t need shaking. Then the pads automatically warm up to 50 or 60 C and continue exuding their heat for up to 20 hours.

Sold across the country at convenience stores or drug stores, and normally costing less than ¥50 apiece, disposable kairo have long been a must-have item in this seasonally frigid land — especially for this cold-sensitive reporter, who recently survived a working tour of sub-zero Austrian cities with these pads slipped into her leather boots and stuck on her back between many layers of shirts. (Yes, there are adhesive types that you can stick to your clothes to warm anywhere you like.)

While these pads are not unknown in other parts of the world, they are particularly popular in Japan. According to Nihon Kairo Kogyokai, an industry association of 18 major disposable kairo manufacturers, a total of 1.4 billion kairo pads were sold during what they call the 2008 season, from May 2008 through April 2009. That’s more than 10 pads each for every one of the 120-odd million people in the country.

But how do these magic warmers work? What’s inside? And how did the kairo culture start and catch on here to such a degree?

One man who holds the key to these nagging questions is Yasumasa Usui, the third-generation president of Mycoal Co., a major kairo manufacturer in Tochigi Prefecture. Warming to my questions in a recent interview, Usui said it was his grandfather who brought the concept of body warmers to Japanese consumers more than 100 years ago. Back then, in 1904, the company launched the first commercial kairo, using a form of coal made from hemp stems as a heating device. The coal, once lit, would be placed in a portable tin case that people could tuck inside their kimono, allowing them to feel its moderate heat for hours, he said.

“We used hemp stems to make coal, which, once ignited, would get warm more quickly than other kinds of coal,” explained Usui, who, at 81, boasts that he is the only man alive in Japan who knows the history of coal-burning kairo firsthand.

Warm tins were most often used by the elderly and the sick, but they also helped soldiers withstand the cold during Japan’s military campaigns in China in the 1930s and ’40s, and then again in World War II on battlefields in subarctic regions of Russia and China, Usui said.

After the war, Usui said, the firm’s business shifted in large part from soldiers to sportspeople, as it started shipping hemp coal to Europe and the United States, where golfers used the tins to warm their golf balls in order to make them fly further.

But with Japan’s rapid economic growth through the early 1970s, natural hemp — which had primarily been cultivated to extract fiber — became less popular and was replaced by synthetics. Then came the “iron revolution.”

Most of the kairo currently sold in Japan, including all of Mycoal’s kairo products, use iron powder. Interestingly enough, as with many inventions, it came about through an experiment that went wrong, explained Yuichi Nitanai, an official in the marketing department of another major kairo manufacturer, Lotte Health Products Co.

Back in the mid-1970s, Tokyo-based Lotte, whose main business is making chewing gum, chocolates and other sweets, was working with an outside contractor to develop a more powerful oxygen absorber, using iron powder, to put in small sachets in its bags of confections to stop them going stale. By accident, water got mixed into the iron — and, lo and behold, it got warm. So what exactly happened?

Well, for the scientifically-minded, here is the reaction that occurred: Fe+3/4O2+3/2H2O → Fe(OH)3+96Kcal/mol

Put simplistically for the equationally challenged (such as myself), it seems that what Lotte’s scientists stumbled on that day more than 30 years ago — and what’s still happening in millions of people’s pockets and inside their jackets and socks every cold winter day — is the rusting of iron (Fe), which takes place when it is mixed with oxygen (O2) in the air and water (H2O).

Lotte quickly turned the mishap into a business opportunity. In 1978, the company became the first in Japan to launch iron-filled, portable heating pads it named Hokaron — a brand that soon became a household word.

Today many companies market kairo that work through the same chemical reaction. To the dismay of Lotte engineers, the iron-powder kairo itself could not be patented because its chemical formula is considered too general, Nitanai said.

Along the way, however, manufacturers have popped various other things into the pads and also carefully studied the combination of substances in them to make them heat up faster and last longer. So in addition to iron powder, many now have salt mixed in, which serves as a catalyst and speeds up the heating process. Many, too, now include what’s called “activated carbon” — an extremely porous processed coal that can absorb oxygen, while another common addition is bits of expanded vermiculite that are excellent at holding water. These materials are mixed in advance to allow the iron to rust gradually and consistently, industry officials say.

Finally, in today’s high-tech heat pads, all these ingredients are wrapped in nonwoven fabric with myriad invisible holes that permit the iron oxidization process to kick off the moment the outside plastic wrapping is opened and air gets in.

One of the biggest drawbacks of these iron-fueled warmers, however, is that they cannot be recycled. And whether disposable kairo should be burned or buried along with other unburnable waste depends on each municipality.

Mycoal’s Usui said his company has tried for years to develop other materials to make kairo recyclable, but so far in vain. “However, we are now trying to make our products less damaging to the environment by making each pad lighter and more powerful with less material,” he said.

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