I recently had surgery for skin cancer, and my doctor recommended using frozen peas rather than ice packs to reduce swelling. She said peas were not only much cheaper, but in bags contoured to the face better than ice packs.
Furthermore, when my wife broke her ankle this spring, both the emergency room physician and her doctor recommended bags of frozen peas rather than ice packs. Again this was primarily because the bags would contour around her leg.
OK, as long as you don't eat the peas.
While it's true that frozen objects can harm tissue, I don't believe vegetables kept in a home freezer or a cooler reach dangerous temperatures. They also tend to thaw quickly.
Actually anything that has been in the freezer long enough (eg peas or ice) will initially be at the temp of the freezer-- ~0F in my freezer. So if it can suck the heat out of tissue fast enough, it can freeze tissue. Thus initially there is a race until the peas/ice begin to melt.
A mixture of ice and water or a mixture of frozen peas and water/unfrozen peas will tend to stay at their respective melting/freezing temps until totally thawed.
Once the peas/ice have warmed to their melting/freezing point (slightly below 32F for peas, 32F for pure ice) then there is steady-state condition. Since flesh also freezes at slightly below 32F, the risk depends on the freezing temp of the peas vs the freezing point of flesh. (The freezing point of ice is always higher than the freezing point of flesh.) It is possible that the freezing temp of the peas is higher than the freezing point of flesh--I don't know the exact numbers.
So maybe there is a problem with peas (or other frozen objects), maybe not. I prefer not to take the risk. Pure water ice or commercial ice packs (once they have begun to melt) are known to be safe.
BTW1, my commercial ice packs soften up after a few minutes and will then conform to the shape of the body quite well.
BTW2, if you thaw peas and then refreeze them, you will have a solid block. Not reusable--you probably have to eat them...
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BTW3, yes you can actually eat the peas...
Any thing that significantly cools the flesh without damaging it (or causing hypothermia) will do. Simple cold water (eg a mountain stream) will do quite well.
Doug