Caffeine constitutes about 3% of tea's dry weight, translating to between 30 mg and 90 mg per 8-oz (250-ml) cup depending on type, brand, and brewing method. A study found that the caffeine content of 1 g of black tea ranged from 22 to 28 mg, while the caffeine content of 1 g of green tea ranged from 11 to 20 mg, reflecting a significant difference.
Tea also contains small amounts of theobromine and theophylline, which are stimulants and xanthines similar to caffeine.
Because of modern environmental pollution, fluoride and aluminium also sometimes occur in tea. Certain types of brick teamade from old leaves and stems have the highest levels.