Scientific research shows that drinking water contains minerals and trace harmful heavy metal ions, such as cadmium, mercury, arsenic, etc. When water is heated, as the temperature rises, the solubility of certain calcium and magnesium ions decreases, and the concentration of insoluble salts continuously increases. When the water is concentrated to a certain extent, precipitation occurs.

These precipitates can enter the teacup along with the brewed tea. Unfinished tea or tea left standing for extended periods exposed to air allows tea polyphenols in the tea leaves to oxidize with metal elements in tea residue, forming brown tea stains, commonly known as tea scale, which adheres to the inner wall of the cup. Tea stains contain various harmful metals including cadmium, lead, mercury, and arsenic, making them the primary culprits endangering human health.