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Why does cool tap water help a small burn injury?

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Cross-section diagram of human skin showing burn depth by degree, with arrows indicating that first-degree burns affect only the epidermis, second-degree burns reach the dermis, third-degree burns extend into subcutaneous tissue, and fourth-degree burns penetrate to muscle. — burn This diagram shows how burn injuries are classified by severity. First-degree burns damage the outer skin, known as the epidermis. First-degree burns are the least severe form of burn injury. Second-degree burns damage the outer skin and some of the underlying skin, known as the dermis, which also includes some connective tissue. Third-degree burns damage all the layers of skin, including subcutaneous tissue. Fourth-degree burns, which are the most severe, destroy every layer of skin and also damage muscle, tendons, and bones.

For small first-degree or small second-degree burns, immersing the wound in cool tap water is the recommended first-aid measure. This action is effective because it stops the burning process and dissipates the heat energy from the injury.1

It is important to avoid applying home remedies such as butter or petroleum jelly to the area, as these substances trap heat within the tissue and can lead to further damage.1