What is the main difference between using this for cooking versus for my scalp or skin?

The core difference lies in the application's purpose and required caution. In cooking, the mustard powder is used as a seasoning or emulsifier in recipes like sauces, dressings, or rubs. Its role is to contribute flavor, a mild heat, and a slightly bitter note. When used externally on the scalp or skin, it's chosen for the warming effect created by the active compounds (glucosinolates) that become active when mixed with water. This stimulates the skin's surface, increasing local warmth. Because this effect can be strong, external use demands greater care. It involves a direct application, carries a higher risk of irritation, burning, or redness, and should always be preceded by a patch test. Culinary use integrates the ingredient into food, diluting its intensity, while external use applies its full potency directly.