A PTC (positive temperature coefficient) heater is an element whose electrical resistance rises sharply as it warms. As it heats, less current flows, so heat output drops on its own near a designed switching temperature. In wearables, PTC films and inks provide built-in temperature limiting without a separate sensor, but the set point is fixed at manufacture and does not replace good design.

A heated vest works like a small, flexible electric blanket. A rechargeable battery pushes current through thin heating elements, usually carbon fiber or metal alloy wire, sewn into panels at the chest, back or collar. The elements resist the current and turn electrical energy into heat. A controller sets the heat level, and the vest's insulation holds that heat near your core.

Once opened, most air-activated hand warmers are rated for about 7 to 12 hours of heat, while larger body warmers are rated up to 18 hours. Unopened, they typically carry a shelf life of 3 to 4 years. Expired but sealed warmers often still work, though they may run cooler or shorter. Airflow, moisture and storage all change real-world runtime.

Disposable hand warmers work by controlled rusting. When you open the wrapper, oxygen reaches a mix of iron powder, water, salt, activated carbon and an absorbent filler. The iron oxidizes to iron oxide and releases heat. Salt speeds the reaction, carbon spreads the heat, and the porous pouch limits airflow so warmth lasts for hours instead of minutes.

Every warming technology does one of two things: it adds heat to the body or it slows the heat the body loses. Chemical warmers and battery heaters add heat. Insulation, heat-reflective linings, space blankets and aerogel slow loss. Phase-change fabrics only store and return heat briefly, and far-infrared textiles only redirect it. The warmest systems combine a heat source with insulation and reflection.