Duration is the only specification that matters in use, and it is the one least often published. Here is how to read what you are given.

Same chemistry, different tuning

Nearly all disposable warmers use iron powder, salt, water, carbon and vermiculite. What differs is the particle size, the carbon content and how breathable the pouch is — all of which control how fast the oxidation runs.

Faster is hotter and shorter. Slower is cooler and longer. There is no formulation that is both, whatever a listing implies.

Average temperature is the honest figure

A published maximum is measured for a moment, usually inside a glove, under ideal conditions. An average temperature over a stated number of hours describes what somebody actually experiences.

When comparing two products, put them on the same basis: hours to fall below a stated temperature. If one supplier will not give you that, the comparison is not close.

Conditions change the answer

Airflow speeds a warmer up and shortens it; a sealed pocket slows it down and can stall it. Cold slows the reaction, which is mildly ironic and worth knowing.

This is why a warmer that lasted all day in a glove seems to fail in a boot: it is the oxygen, not the product.

Test before you commit

Order a small quantity, open one at the start of a shift and check it at the point in the day that matters to you. Twelve warmers is enough to settle an argument that a specification sheet will not.

Test in the conditions they will be used in, not on a desk. The desk result is always better and always irrelevant.