Cold is a useful aid in reducing swelling in an
accident. However carrying ice cubes in a first aid kit has some drawbacks. A practical
answer to cold on demand uses endothermic heat of solution. A plastic container containing
water in one compartment and a salt such as Ammonium Nitrate in the other is kept separate
by a plastic seal. When a coldpack is required the seal is broken and the water and salt
mix. The endotherm of solution provides the cooling.