With summer weather arriving in a blaze of glory, your trusty ice packs might start getting more use. Whether for keeping food and drinks cool, icing injuries, or relieving the discomfort of a sunburn, ice packs have a multitude of uses at home, never mind elsewhere, where they’re vital for keeping medications, chemicals, and other spoilable items safe. But what’s inside that trusty, usually blue, often rectangular, object?
Ice packs, sometimes known as gel packs, freezer packs, or cold packs, contain water and some additives. Which additives depend specifically on the type of ice pack, but there are some general trends. Almost all ice packs contain water and some amount of propylene glycol, a chemical commonly used to reduce the freezing temperature of water. It is commonly found in antifreeze and airplane de-icers. Including it in ice packs means that these can then stay squishy while below 0˚C, the better to mould to an injury or plastic container. In the past, some ice packs were made using ethylene- or diethylene glycol, but these have been discontinued due to toxicity. Thankfully, ice packs now are liable to cause only a bit of an upset tummy if their contents are ingested.
Many ice packs also contain some combination of hydroxyethyl cellulose (a thickening agent that’s a main component of water-based lubricants), sodium polyacrylate (a super absorbent polymer that is used in diapers and bandages) or silica gel (those beads that come in those packets that say ‘do not eat’). Some also contain agents to avoid bacterial growth, and many include blue dye, just for aesthetic reasons.
Instant ice packs are an entirely different story, both chemically and toxicologically speaking. They are designed to provide coldness on demand—although not for as long—without needing a freezer. They do this through a chemical reaction. Inside the package are two bags, one with water and one with another chemical, that break open and combine when squeezed and shaken. Common candidates include urea, ammonium nitrate and calcium ammonium nitrate. The dissolution of these compounds in water is an endothermic reaction, which is to say energy is absorbed from the surroundings in the form of heat, making the environment it is in feel cold. This is as opposed to something like the combustion of wood, which is an exothermic reaction and releases heat to its surroundings.
While properly nifty for inclusion in a first aid kit, instant ice packs pose a much more significant risk with exposure. Ammonium nitrate is the most concerning amongst the three common reagents. Ingesting it can cause many issues, including methemoglobinemia, wherein your blood’s oxygen-carrying capacity is compromised.
But should we be applying ice packs to injuries at all? In the past few years, there has been a lot of debate over whether icing an injury is the right move. Even the creator of the Rest, Ice, Compression, and Elevation (RICE) protocol, Dr. Gabe Mirkin, has seemingly gone back on his initial assertion, saying in 2015 that “icing delays recovery.”
But as Paul Ingraham from PainScience.com explains, it’s not as simple as throwing the ice out with the bathwater. “Icing is primarily an analgesic — a pain-reliever — and not an actual treatment. That is, it doesn’t “fix” anything. Use it like you use ibuprofen. It may help to resolve chronic problems… but it’s mostly intended to simply numb painfully inflamed or other hurting tissues.” Dr. Mirkin also agrees: “Since applying ice to an injury has been shown to reduce pain, it is acceptable to cool an injured part for short periods soon after the injury occurs.”
As for the idea that icing delays recovery, we really need more evidence before making that call. As Paul Ingraham discusses, clinical evidence supports icing having none or even a positive effect on healing while definitely working for acute pain relief. It is also possible that a slight decrease in the healing rate is worthwhile for ice’s analgesic effects, particularly if no other pain remedies are on hand.
