Tea leaves pull heavy metals from water, significantly lowering the amount of lead and other dangerous compounds that people may be unknowingly drinking, a new study found.
Recent research has highlighted potential applications for used tea leaves, from biofuels to gluten-free cookies. But the new study shows a public health benefit from something that countless people are already doing. About five billion cups of tea are consumed each day around the world, according to one estimate.
“You can see the implications,” said Vinayak Dravid, a materials scientist at Northwestern and an author of the study, which was published this week. “How often do we touch billions of people?”
In many countries, the water used to steep tea is contaminated with lead from aging pipes. In the United States, nine million homes get their water through pipes that contain lead, according to the Environmental Protection Agency. Lead is especially dangerous to children. Exposure can lead to developmental delays and behavioral issues.
Dr. Dravid and his team tested how different types of tea — black, white, oolong, green, rooibos, herbal, loose leaf and plain old Lipton — behaved in water with varying amounts of lead.
The tea was then allowed to steep for variable periods of time. Afterward, the scientists measured how much lead remained in the water.
Compounds called catechins in tea leaves act like “little Velcro” hooks to which lead molecules latch, said Michelle Francl, a chemist at Bryn Mawr College and the author of a book on the chemistry of tea. Dr. Francl also said that the “ridges and valleys” of the tea leaves provided the necessary surface area for that interaction.
While those properties have been known for some time, Dr. Dravid and his colleagues were the first to look at the lead-detoxifying powers of a single cup of tea. They found that black tea leaves became wrinkled after roasting and were thus best equipped to absorb heavy metals.
“Green tea and black tea had fairly equivalent amounts of metal absorbed,” said another author, Benjamin Shindel, who was a doctoral candidate at Northwestern while working on the study.
White tea, on the other hand, undergoes a much more gentle preparation. Its leaves remain smooth, offering less surface area from which to draw heavy metals from water. Herbal tea enthusiasts may be disappointed to learn that chamomile tea does a poor job of heavy-metal filtering, too, probably because it is made with chamomile flowers, not tea leaves.
Still, these distinctions between different types of tea were not the most relevant factor. “It’s much more important how long you’re brewing the tea for,” Dr. Shindel said.
The researchers found that steeping a cup of black tea for five minutes could remove 15 percent of lead from the water, which is helpful, but there is “no safe level” of lead exposure, according to the E.P.A.
“With lead and other contaminants, any decrease is meaningful to some extent, especially if you have a lack of resources or infrastructure that would already remediate some of these problem materials,” said Caroline Harms, who was an undergraduate student of Dr. Dravid at Northwestern while working on the study.
The longer the steeping time, however, the more bitter the resulting tea. Last year, Dr. Francl caused a minor international scandal by suggesting that adding salt to tea could mitigate its bitterness. But even that controversial chemistry hack has its limits. “It’s not really drinkable after 10 minutes of steeping tea, and no amount of salt is going to help that,” she said.
Some samples in the study had to be steeped for 24 hours, which would render the tea undrinkable.
The study’s authors said they were less motivated to make public health prescriptions for policymakers than to study a hidden benefit of a global habit.
The researchers estimated that a country where people drank large amounts of tea would have about 3 percent less lead ingestion from the water supply than a (hypothetical) identical country that did not drink any tea.
“How wonderful,” said Henrietta Lovell, the founder of the Rare Tea Company. Ms. Lovell, who supplies teas to exclusive restaurants, pointed out that tea had been used in China for medicinal purposes for thousands of years. “The more I learn about tea, the more fabulous and fascinating it becomes,” Ms. Lovell said.
Dr. Francl believes that the new findings could eventually pave the way to a scalable method of making sure people aren’t ingesting heavy metals. “Given that clean water is such a global issue,” she said, “if there was a way to take this proof of concept and tweak it to produce potable water at the end, that would be pretty good.”
