For teas to be labelled decaffeinated, the caffeine content should not exceed 0.4% by dry weight, which is equivalent to approximately 4 mg of caffeine per 170 ml serving.
The process of decaffeination extracts the caffeine in tea. The current commercially available methods for decaffeinating black tea are solvent based extraction using ethyl acetate or methylene chloride, and extraction using supercritical (solid) carbon dioxide. All three methods extract caffeine with minimum effect to the quality of tea.