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When my son had a cast put on his arm after he broke it, the cast felt very warm when it was wet. Why is that?

When someone is having a plaster cast applied, it is done in stages. The limb is first covered with a type of "sock" and then wrapped with cotton. A splint is usually put next to the limb to keep the limb straight and the plaster product is dipped in water and then the wet plaster is wrapped around the limb and splint.

When the plaster comes in contact with the water, a chemical reaction occurs and heat is generated. As the plaster is wrapped around the limb, this heat is retained between the layers as the plaster remains wet. Once the plaster dries, the temperature drops and the plaster feels cold to touch.

Reference: 

Matthew A. Halanski, MD, et al. Thermal Injury with Contemporary Cast-Application Techniques and Methods to Circumvent Morbidity. In The Journal of Bone and Joint Surgery. November 2007. Vol. 89. Pp. 2369-2377.

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