Although health benefits have been assumed throughout the history of using Camellia sinensis as a common beverage, there is no high-quality evidence that consuming tea confers significant benefits other than possibly increasing alertness, an effect caused by caffeine in the tea leaves. In clinical research conducted over the early 21st century, tea has been studied extensively for its potential to lower the risk of human diseases, but there is no good scientific evidence to indicate that consuming tea affects any disease or improves health. In regions without access to safe drinking water, boiling water to make tea is effective for reducing waterborne diseases by destroying pathogenic microorganisms. By constituents or substances Aluminum, iron and other metals Tea drinking accounts for a high proportion of aluminum in the human diet. The levels are safe, but there has been some concern that aluminum traces may be associated with Alzheimer's disease. A 2013 study additionally indicated that some teas contained lead (mostly Chinese) and aluminum (Indian/Sri Lanka blends, China). There is still insufficient evidence to draw firm conclusions on this subject. Most studies have found no association between tea intake and iron absorption. However, drinking excessive amounts of black tea may inhibit the absorption of iron, and may harm people with anaemia. Concerns have been raised about the traditional method of over-boiling tea to produce a decoction, which may increase the amount of environmental contaminants released and consumed. Fluoride exposure All tea leaves contain fluoride; however, mature leaves contain as much as 10 to 20 times the fluoride levels of young leaves from the same plant. The fluoride content of a tea leaf depends on the leaf picking method used and the fluoride content of the soil from which it has been grown; tea plants absorb this element at a greater rate than other plants. Care in the choice of the location where the plant is grown may reduce the risk. It is speculated that hand-picked tea would contain less fluoride than machine-harvested tea, because there is a much lower chance of harvesting older leaves during the harvest process. A 2013 British study of 38 teas found that cheaper UK supermarket tea blends had the highest levels of fluoride with about 580 mg per kilogram, green teas averaged about 397 mg per kg and pure blends about 132 mg per kg. The researchers suggested that economy teas may use older leaves which contain