The chemistry inside every hand warmer (since 1923)

In 1923, a Japanese chemist named Niichi Matoba patented a chemical reaction that's been inside almost every disposable hand warmer ever since. Five ingredients. One reaction. A hundred years of warmth.

The reaction

Iron + oxygen → iron oxide + heat. That's the entire chemistry. The same reaction that makes nails rust over decades, sped up by a thousand inside a hand warmer pouch.

What's actually inside

Iron powder. The fuel. About 50% of the pouch by weight. Reacts with oxygen to form iron oxide (rust) and release heat.

Salt (sodium chloride). The accelerant. Salt water creates an electrolytic environment that dramatically speeds up the iron oxidation reaction. Without salt, iron rusts over years. With salt, it rusts in hours.

Water. Required for the reaction. About 5% by weight. Without moisture, oxidation stops.

Activated charcoal. Distributes heat evenly across the pouch. Absorbs and releases small amounts of moisture to keep the reaction stable.

Vermiculite. A naturally occurring mineral that expands when heated. Acts as a porous carrier — holds the iron + salt + water mixture in place inside the pouch and helps regulate reaction speed.

Why air-activated, not water-activated

Inside an unopened pouch, the iron is sealed away from oxygen. The chemistry is dormant. The moment you rip the foil wrapper, air reaches the pouch and the reaction begins.

Water alone won't activate the pouch — it needs oxygen. That's why pouches won't activate in your sealed pocket without breathable fabric, and why they accelerate when shaken (more air contact).

What changed in 100 years

Almost nothing — and that's the point. The chemistry is settled. Every brand from HotHands to PREF*RE uses Matoba's 1923 reaction. The differences come from everything around the chemistry: the pouch material, the iron blend, the manufacturing precision, the brand experience.

PREF*RE's iron blend is more refined (99% biodegradable contents that break down to soil). Our pouch is spun-lace nonwoven instead of crinkly plastic. The reaction inside is identical to what HotHands has been making since 1991.

Why it matters

Understanding what's inside a hand warmer changes how you think about "disposable." PREF*RE pouches don't contain plastic, batteries, or electronics. They contain iron and salt and water and minerals. After 8 hours of use, the contents are mostly rust + sodium chloride solution + organic carriers — all of which break down in soil.

It's the most low-tech, time-tested, environmentally honest way to make heat we know.

RIP OPEN. SHAKE WELL. GET HEATED.