I read it in the book Peter Atkins
The endothermic dissolution of ammonium nitrate in water is the basis of the instant cold-packs that are included in some first-aid kits. They consist of a plastic envelope containing water dyed blue (for psychological reasons) and a small tube of ammonium nitrate, which is broken when the pack is to be used.
Could not understand why die water blue would have any physiological effect on a body. What type of physiological effect is he refering to?