Written and maintained by CASRAI Editorial Board
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General product information for laboratory and clinic staff. It is not medical advice and does not replace the safety data sheet (SDS) or instructions that come with a specific pack.
Short answer
An instant cold pack is a sealed package holding water and a chemical that dissolves in it. Squeezing the pack breaks the water bag, the two mix, and the contents turn cold. The chemical is not the same in every product: safety data sheets name ammonium nitrate for some packs and urea for others, and the National Capital Poison Center lists calcium ammonium nitrate as a third variant.
What is inside an instant cold pack
The National Capital Poison Center describes instant packs as filled with water and a “chemical reactor”. Squeezing the pack releases the reactor, which mixes with the water and lowers the temperature of the water to almost freezing. The three reactors it discusses are ammonium nitrate, calcium ammonium nitrate and urea.
Two manufacturer safety data sheets show what that looks like in practice:
- Ammonium nitrate pack. A Cardinal Health sheet for its Instant Cold Pack (revised January 2004) describes “a waterproof package containing small white ammonium nitrate pellets surrounding a small rupturable plastic bag filled with water.” Its composition section lists water (CAS 7732-18-5) and ammonium nitrate (CAS 6484-52-2) without percentages.
- Urea pack. A Nortech Labs sheet for its COOL Instant Cold Pack Urea (issued 30 July 2014) lists, by weight, urea (CAS 57-13-6) at 42-52%, water at 40-50%, outer film at 5-10% and inner film at 1-3%.
How a cold pack works
The mechanism the sources describe is simple: the pack keeps the water and the solid chemical apart until it is squeezed. Breaking the inner bag lets the two combine, “the liquid becomes cold” (Cardinal Health), and the poison center describes the water temperature falling to almost freezing.
Ammonium nitrate, calcium ammonium nitrate and urea compared
Poison-control guidance and manufacturer SDSs describe and classify these reactors differently. The table sets each reactor beside what the poison center and the safety data sheets state.
| Reactor | What the poison center says | What the SDS says |
|---|---|---|
| Ammonium nitrate | “The most toxic ingredient if swallowed.” It may dilate blood vessels and cause methemoglobinemia and hemolysis. | Cardinal Health (2004): unmixed pellets “may explode from heat or contamination”, can accelerate burning in a fire and can react explosively with hydrocarbons. Broken-pack solution may irritate skin, eyes and throat; swallowing may cause vomiting, stomach upset and gastritis. Solution pH 3.0. |
| Calcium ammonium nitrate | Toxic properties match ammonium nitrate, but a larger amount must be swallowed for the same effect because calcium carbonate is nontoxic. | A McKesson Medical-Surgical sheet for a product listed as a calcium ammonium nitrate cold pack (file dated 2018-2019) classifies it as acute oral toxicity category 4 (H302) and serious eye damage category 1 (H318), signal word “Danger”. |
| Urea | Called “the least concerning reactor” of the three. If enough is swallowed: nausea, vomiting, dizziness, drowsiness, confusion. | Nortech Labs (2014): not classified as hazardous under the OSHA Hazard Communication Standard (29 CFR 1910.1200). Liquid at pH 7; no special storage requirements. |
The sheets are of different ages and come from different manufacturers, so the rows describe those specific products, not every pack of that type.
Reading a cold pack’s SDS
For a stock manager or lab safety lead, the sheet tells you more than the carton does. A checklist of four points drawn from the sheets above:
- Section 3, composition. Check the CAS number. Ammonium nitrate is 6484-52-2 and urea is 57-13-6, so two products sold as the same item can hold different chemicals.
- Section 2, hazard classification. Compare the GHS class and signal word. The urea sheet above is not OSHA-hazardous; the McKesson sheet carries H302, H318 and “Danger”.
- Revision date. The sheets above were revised in 2004, issued in 2014 and filed in 2018-2019. Confirm that the sheet you hold is the current one for the product you stock.
- Storage and handling. The Cardinal Health sheet gives room temperature not exceeding 104 F (40 C) and protection from freezing, and asks for safety glasses and impervious gloves if the packaging is broken. The Nortech sheet gives no special storage requirement.
For the clinical choice between cold and heat, see the guide on hot and cold therapy selection. The companion explainer how a hot pack works covers the heating counterpart, and the items inside a first aid kit guide shows where instant packs sit among other supplies. The cryotherapy cuff guide covers a related cold-therapy product.
Common misunderstandings
All instant cold packs contain the same chemical
They do not. The sheets and poison-center article above name ammonium nitrate, calcium ammonium nitrate and urea, and each has a different hazard profile.
The contents are harmless because the pack is sold for first aid
The poison center names ammonium nitrate as the most toxic ingredient if swallowed, and the McKesson sheet carries an oral toxicity classification. Urea is ranked least concerning, but it can still cause symptoms if enough is swallowed.
FAQ
What are the ingredients in an instant cold pack?
Water plus a solid chemical: ammonium nitrate in the Cardinal Health sheet, urea in the Nortech sheet. The Nortech sheet gives 42-52% urea and 40-50% water by weight, plus plastic film.
How does a cold pack work?
Squeezing ruptures the inner water bag, the water mixes with the chemical, and the liquid becomes cold. The poison center says the water temperature falls to almost freezing.
Do all instant cold packs contain the same chemical?
No. The sheets and poison-center article name ammonium nitrate, calcium ammonium nitrate and urea as the chemical that dissolves in the water, and the CAS numbers differ between products.
Where do I find what is in a specific pack?
Section 3 (composition) of the pack’s safety data sheet lists the chemicals and their CAS numbers, and the Nortech sheet also gives percentages by weight. Section 2 gives the hazard classification.
Sources
- National Capital Poison Center, “What’s inside an ice pack?”, https://www.poison.org/articles/whats-inside-ice-packs-201, checked 30 October 2026.
- Cardinal Health, Safety Data Sheet, “Instant Cold Pack” (product code 80B), revised January 2004, https://www.shepherd.edu/app/uploads/2016/04/Instant-cold-pack-for-first-aid-GraingerCardinal-Health.pdf, checked 30 October 2026.
- Nortech Labs, Safety Data Sheet, “COOL Instant Cold Pack Urea” (INST56U/57U/68U), issued 30 July 2014, https://hdsupplysolutions.com/wcsstore/ExtendedSitesCatalogAssetStore/product/fm/additional/11/113640-113644_Cold%20Pack-Urea_SDS_Original.pdf, checked 30 October 2026.
- McKesson Medical-Surgical, Safety Data Sheet, “Instant Cold Pack” (MFR 16-9703), file dated 2018-2019, https://scchc.org/wp-content/uploads/2025/09/Instant-Cold-Pack-McKesson-General-Purpose-6-X-9-Inch-Plastic-Calcium-Ammonium-Nitrate-Water-Disposable.pdf, checked 30 October 2026.
