In fact, studies have proved that copper, lead and iron in tea will stabilize after 8- 10 hour after being brewed overnight at 25℃, and the amounts at this time are not much different from those at the beginning.
More importantly, they are far from "exceeding the standard":
(1) copper: the maximum copper content in 24 hours is 22.49 mg/kg. If a person drinks 10g tea a day (a cup of tea is about 1~3g), the copper intake is 0.2249mg, which is far lower than the daily copper intake of adults stipulated by the Food and Nutrition Bureau (FNB) of the National Academy of Sciences.
(2) Lead: the maximum content in 24 hours is 2.4ug/L, which is far lower than the current lead content standard of drinking water in China: 50 ug/L;
(3) Iron: The highest content in 24 hours is 30.46 mg/kg, which is far lower than the lethal dose of iron poisoning: 130 ~ 250 mg/kg, so there is no need to worry about iron poisoning in tea.
Therefore, the metal content in overnight tea will not cause harm to the body. In fact, as long as we don't soak tea in it for the night, we don't have to worry about simply letting tea stay overnight!
Statement 2: Overnight tea contains secondary amines, which can be converted into carcinogenic nitrosamines.
Secondary amine itself is an unstable compound, which widely exists in all kinds of foods. The daily intake of secondary amine in staple food is much higher than that of overnight tea.
In addition, secondary amine is not the same as nitrosamine. Secondary amine itself is not a carcinogen, but only needs to react with nitrate under certain conditions to generate nitrosamines. The so-called specific conditions include many factors such as pH, microorganism and temperature. However, the human digestive tract does not have these factors, so secondary amines will not be converted into nitrosamines after drinking into the gastrointestinal tract.
According to the research, nitrosamines can only cause cancer if they are absorbed100 ~ 200mg per kilogram of body weight. Although the overnight tea may contain nitrosamines, the content is extremely small and will not cause such great harm at all.