What is the harmful substance in tea?

December 23, 2025 •

4 min read

Globally, people drink more tea than any other beverage besides water, according to a 2014 report from the Food and Agriculture Organization of the United Nations. While celebrated for its antioxidant benefits, tea also contains several substances that, if consumed in excess or under specific conditions, may pose health concerns.

Quick Summary

Several substances in tea can be harmful when consumed excessively, including tannins that inhibit iron absorption and cause nausea, caffeine that affects sleep and anxiety, and contaminants like heavy metals and fluoride, particularly in low-quality teas.

Key Points

Tannins Inhibit Iron Absorption: The polyphenols in tea can bind to non-heme iron from plant-based foods, potentially worsening iron deficiency in vulnerable individuals.
Caffeine Can Cause Anxiety and Sleep Issues: Overconsumption of caffeine in tea can lead to anxiety, restlessness, and disturbed sleep patterns, especially in sensitive people.
Low-Quality Teas May Contain Heavy Metals: The tea plant accumulates heavy metals like lead and aluminum from the soil, with higher concentrations often found in cheaper teas made from older leaves.
Excess Fluoride is a Risk: Chronic, excessive intake of fluoride from tea, particularly cheaper brands or instant varieties, can lead to skeletal or dental fluorosis.
Some Tea Bags Release Microplastics: Certain tea bags made from synthetic materials can release microplastic particles when steeped in hot water, though the health implications are not yet fully understood.
Mindful Brewing Matters: Brewing techniques like shorter steeping times can reduce the amount of tannins, heavy metals, and fluoride extracted into your tea.

Common 'Harmful' Substances in Tea

When we ask, 'what is the harmful substance in tea?', the answer is not a single culprit but rather a few common compounds that can have negative effects depending on dosage, preparation, and individual sensitivity. For most people, moderate tea consumption is not a health concern, but understanding these substances is important for informed choices, especially for those with pre-existing conditions.

Tannins

Tannins are a class of polyphenolic compounds found in tea and many other plants. They are responsible for the tea's bitter, dry, and astringent taste and can provide antioxidant benefits. However, their ability to bind with other compounds is also the source of their potential downsides.

Reduced Iron Absorption: Tannins can bind to non-heme iron (the type found in plant-based foods), making it unavailable for absorption in the digestive tract. For individuals with iron deficiency or those following a vegan/vegetarian diet, excessive tea consumption with meals can exacerbate the problem.
Digestive Irritation: Consuming high-tannin tea on an empty stomach can irritate the digestive tissues, leading to nausea or stomach aches in sensitive individuals. This effect can be minimized by drinking tea with food or adding a splash of milk.

Caffeine

Tea contains a natural stimulant called caffeine, although generally in lower amounts than coffee. Still, overconsumption can lead to several adverse effects.

Sleep Disruption: Caffeine is known to inhibit melatonin production, the hormone that signals sleep, which can lead to lower quality sleep. This is particularly relevant when consuming caffeinated tea close to bedtime.
Increased Anxiety and Restlessness: Some individuals are more sensitive to caffeine's stimulant effects, which can manifest as increased anxiety, stress, or restlessness. Limiting intake or switching to caffeine-free herbal teas can help.
Dependency and Withdrawal: Regular, high intake of caffeine can lead to a mild dependence. Abruptly stopping can cause withdrawal symptoms like headaches, irritability, and fatigue.

Heavy Metals and Fluoride in Tea

Beyond naturally occurring compounds, tea can also contain contaminants depending on the plant's growing environment and processing. The Camellia sinensis plant is a known bioaccumulator, meaning it readily absorbs elements from the soil, including heavy metals and fluoride.

Heavy Metals: Studies have detected heavy metals such as lead and aluminum in tea leaves, especially in blends from certain regions. Lower-quality, cheaper teas that use older leaves tend to have higher concentrations of these metals. Over time, these can bioaccumulate, posing long-term health risks.
Fluoride: The tea plant accumulates fluoride from the soil, with older leaves containing higher levels. While moderate intake of fluoride is beneficial for dental health, chronic overexposure from excessive consumption, especially of low-quality or instant teas, can lead to skeletal or dental fluorosis. The amount of fluoride infused into your cup is also influenced by brewing time and the mineral content of your water.

Potential Harms from Tea Bags

Even the vessel used to steep the tea can be a source of potential harm. Research has highlighted issues with some tea bags themselves.

Microplastics: Some tea bags, particularly those made from nylon or polyethylene terephthalate (PET), have been shown to release microplastic particles when steeped in hot water. While the long-term health effects are still being studied, this is a valid concern for regular tea drinkers.
Chemical Additives: Some paper tea bags are treated with chemicals like epichlorohydrin to increase wet strength. The Environmental Protection Agency (EPA) has reported this chemical as a potential carcinogen, though studies on leaching into brewed tea are ongoing.

Mitigating the Risks: A Comparison Table


Potential Harmful Substance	Primary Concern	How to Mitigate Risk	Teas with Higher Concentrations	Teas with Lower Concentrations
Tannins	Reduced iron absorption, digestive upset	Drink between meals, add milk, consume in moderation	Black tea, long-steeped tea	White tea, green tea, herbal teas
Caffeine	Anxiety, sleep disruption, dependence	Limit daily intake, avoid before bed, switch to decaf/herbal	Black tea, long-steeped tea	White tea, green tea, herbal teas
Heavy Metals	Long-term bioaccumulation, neurotoxicity	Choose high-quality, reputable brands; consider loose leaf	Cheaper, lower-grade teas; blends from contaminated areas	High-quality, organically grown teas from reputable sources
Fluoride	Skeletal/dental fluorosis with chronic overexposure	Use younger leaves (fine picking), shorter steep time, filter water	Cheaper, older leaves (coarse picking), instant tea powders	White tea, high-grade loose leaf teas with younger leaves
Microplastics	Potential release of plastic particles	Use loose leaf tea or cloth-based tea bags	Nylon or PET-based tea bags	Loose leaf tea, paper or cloth tea bags

How to Enjoy Tea Safely

For most people, the health benefits of tea, such as its antioxidants and anti-inflammatory properties, outweigh the risks, as long as consumption is moderate. However, paying attention to the quality of your tea and how you prepare it can significantly reduce potential exposure to harmful substances. The recommended daily caffeine intake for most healthy adults is up to 400 mg. As a reference, a cup of black tea contains about 25-110 mg of caffeine, and green tea contains 30-50 mg. Individuals with conditions like iron deficiency, anxiety, or kidney issues should be more cautious and may need to consult a healthcare provider.

Conclusion

While tea is generally a safe and healthy beverage, understanding potential risks from substances like tannins, caffeine, heavy metals, and fluoride is important for responsible consumption. By choosing high-quality, whole-leaf teas, controlling brewing time, and moderating intake, you can enjoy the many benefits of tea while minimizing exposure to potentially harmful components. Making informed choices about your tea and brewing habits ensures that your daily cup remains a source of comfort and health, not a hidden risk.

Frequently Asked Questions

Tea is not inherently harmful, but it contains several substances like tannins, caffeine, heavy metals, and fluoride that can have negative effects, particularly when consumed in excess or from low-quality products.

Tannins are the compounds in tea that can inhibit the body's absorption of iron, specifically non-heme iron from plant sources. Individuals with iron deficiency should be mindful of this, especially when drinking tea with meals.

Yes, for some people, the caffeine content in tea can lead to feelings of anxiety, stress, or restlessness, especially with excessive intake. Sensitivity to caffeine varies among individuals.

The tea plant naturally absorbs elements, including heavy metals like lead and aluminum, from the soil. These metals are more concentrated in older leaves often used in lower-grade tea products.

Chronic and excessive consumption of tea with high fluoride levels can potentially be dangerous, leading to skeletal or dental fluorosis. The risk is highest with cheaper, instant teas and long brewing times.

Not all tea bags are completely safe, as some made from nylon or PET can release microplastics when heated. Opting for loose leaf tea or specific cloth/paper bags is a safer alternative.

To reduce harmful substances, choose high-quality, loose-leaf tea, use younger leaves (often found in more expensive teas), shorten your brewing time, and use filtered water, especially if your local tap water has high fluoride.