Written and maintained by CASRAI Editorial Board
Last updated
General product information only, not medical advice. For pain, injury or a condition you are worried about, ask a qualified health professional.
A hot pack works by getting heat from somewhere other than the wall socket. A chemical pack makes heat in a sealed bag through a chemical change: a reusable sodium acetate pack releases heat when its solution crystallizes, and a single-use iron pack releases heat as iron reacts with air. A moist heat pack, such as a hydrocollator pack, does not make heat at all; it is warmed in water first.
What you need: the three kinds of hot pack
People use “hot pack” for several different products. They get warm in different ways, so the first step is to know which one you hold.
| Type | Where the heat comes from | Reusable? |
|---|---|---|
| Reusable chemical pack (sodium acetate) | Crystallization of sodium acetate, an exothermic process, starts when you bend the metal disc inside the bag | Yes. Boiling the bag in water redissolves the sodium acetate |
| Single-use chemical pack (iron and water) | Exposure to air oxidizes the iron (it rusts), and that reaction gives off heat | No reset step is described; the reaction is one-time |
| Moist heat pack (hydrocollator type) | Warmed beforehand in a water-heated unit; no chemical reaction | Yes. See moist heat pack for what it is and how it is regulated |
How each type makes heat
Reusable sodium acetate pack
The bag holds a sodium acetate solution in a reversible solution and crystallization system. A small bendable metal disc inside the bag gives the solution a place to start crystallizing. Once crystals start, they spread through the bag, and crystallization is an exothermic process: the pack becomes quite hot. University chemistry demonstration manuals describe it this way.
In short: bending the disc starts crystallization, and the heat is released as the crystals form. The University of Wisconsin-Madison chemistry demonstrations describe resetting the bag by placing it in boiling water, which redissolves the sodium acetate so the bag may be reused. A pack that has just reacted can be quite hot, so it should be covered rather than placed on bare skin (see below).
Always follow the maker’s instructions for your pack, including how long to heat it and how to handle it afterwards.
Single-use iron pack
A single-use chemical pack contains iron that reacts with air. According to the University of Colorado Boulder lecture demonstration manual, exposing the contents to air results in oxidation of the iron, which creates rust, and the reaction releases heat. The sources describe no reset step for this type.
Moist heat (hydrocollator) pack
This type stores heat from an outside source instead of making it. Its definition, regulatory class and a supply checklist are on the moist heat pack guide, so they are not repeated here.
Are chemical hot packs regulated as medical devices?
A chemical pack sold for medical purposes falls under 21 CFR 890.5710. The regulation defines a hot or cold disposable pack as a device intended for medical purposes that consists of a sealed plastic bag incorporating chemicals that, upon activation, provides hot or cold therapy for body surfaces. It is Class I (general controls). Except when intended for use on infants, it is exempt from the premarket notification (510(k)) procedures in subpart E of part 807, subject to 21 CFR 890.9. In the FDA classification database, product code IMD is “Pack, Hot Or Cold, Disposable” and product code MPO is “Infant Heel Warmer (Chemical Heat Pack)”, both Class I under 890.5710.
A water-circulating hot or cold pack is a different device: 21 CFR 890.5720 classifies it as Class II (special controls), exempt from premarket notification subject to 890.9. It is a pumped-water system, not a bag that heats itself. For the wider idea of what counts as a device, see what is a medical device.
What can go wrong
- Contact burns. NSW Government (Australia) consumer guidance says not to place a heat pack directly onto exposed skin, because it may cause contact burns, especially in the very young and the elderly.
- Microwaved grain packs. The NSW guidance covers fabric heat packs filled with wheat or another grain. For those it says to monitor heating and not heat for longer than instructed, to lay the pack out evenly in the microwave to prevent hot spots, and not to put a glass of water in the microwave with the pack because it can cause scalding. These microwave rules apply to grain packs, not to sodium acetate or iron packs.
When to get professional help
If heat therapy is part of treating an injury, a medical condition, or a baby’s care, a clinician or the product’s instructions for use should decide how it is used. For how clinical settings choose between heat and cold, see the hot and cold therapy selection guide and, for the cold side, cryotherapy cuffs.
FAQ
How does a reusable hot pack work?
Bending the metal disc starts crystallization of the sodium acetate solution, and that crystallization releases heat. Boiling the bag in water dissolves the crystals again so it can be reused.
How does a disposable hot pack work?
Air reaches iron in the pack and the iron oxidizes (rusts), releasing heat. It is a single-use reaction.
Is a hydrocollator unit the same as a hot pack?
No. A hydrocollator is a water-heated container that warms moist heat packs; the packs do not produce heat by a chemical reaction. See the moist heat pack guide. Athletic training rooms use both heat and cold options, summarized in athletic training room equipment.
Can I put a hot pack directly on my skin?
NSW Government guidance says not to place a heat pack directly onto exposed skin because of the risk of contact burns, especially for the very young and the elderly.
Sources
- 21 CFR 890.5710, Hot or cold disposable pack. https://www.law.cornell.edu/cfr/text/21/890.5710 (checked 2026-10-30)
- 21 CFR 890.5720, Water circulating hot or cold pack. https://www.law.cornell.edu/cfr/text/21/890.5720 (checked 2026-10-30)
- FDA product classification database via openFDA, product codes IMD and MPO. https://api.fda.gov/device/classification.json (checked 2026-10-30)
- University of Colorado Boulder, Lecture Demonstration Manual T520, exothermic and endothermic reactions, hot and cold packs. https://www.colorado.edu/lab/lecture-demo-manual/thermochemistry/t520-exothermic-and-endothermic-reactions-hot-and-cold-packs (checked 2026-10-30)
- University of Wisconsin-Madison Chemistry Demonstrations, reusable heat pack. https://demolab.chem.wisc.edu/general-chemistry-demonstrations-topic-page/thermochemistry-reusable-heat-pack/ (checked 2026-10-30)
- NSW Government, Heat pack and hot water bottle safety. https://www.nsw.gov.au/legal-and-justice/consumer-rights-and-protection/safety/other-consumer-products/heat-pack-and-hot-water-bottle-safety (checked 2026-10-30)
