Water is the foundation of life and one of the most essential substances for the survival of all living organisms1. In the human body, water plays vital roles, including regulating physiological functions such as body temperature, blood pressure, and pH balance; transporting essential nutrients and compounds like oxygen, glucose, sodium, and potassium to cells; aiding digestion; maintaining cellular health and integrity; and supporting overall well-being2. Water plays a crucial role in ensuring food safety throughout various stages of food production. However, the importance of water quality and its impact on both food products is often underestimated, which can compromise food safety and product quality3.

Tea is one of the most widely consumed beverages worldwide and is primarily brewed with water. Tea contains a wide range of bioactive compounds, including catechins and epicatechins, theaflavins, flavonol glycosides, L-theanine, caffeine, theobromine, and various volatile organic substances. These components are known for their significant antioxidative, antimicrobial, antihypertensive, anti-inflammatory, anticarcinogenic, neuroprotective, cholesterol-lowering, and thermogenic properties4. Scientific researches—including laboratory experiments, animal studies, epidemiological analyses, clinical trials, and meta-analyses—have consistently associated tea consumption with a reduced risk of various diseases, include cancer, diabetes, arthritis, cardiovascular disease, stroke, genital warts, digestive disorders, and metabolic conditions such as obesity5. The health benefits of tea stem from its diverse biological mechanisms. For example, flavonoids function as powerful antioxidants, while caffeine and other methylxanthines influence intracellular second messenger systems. Catechins, in particular, exhibit strong anti-inflammatory effects. Together, these actions contribute to neuroprotection, cardioprotection, and cancer prevention6. In addition to its physiological benefits, tea is widely enjoyed for its calming and meditative qualities, which are often cited as key factors in its global popularity7,8.

Along with the beneficial properties of tea consumption, there are concerns about contamination with chemical and toxic compounds that may pose risks to human health. In a study by Pourfarzi et al., it was concluded that there was a significant association between hot tea consumption (OR = 2.85) and an increased incidence of gastric cancer9. In another study, Canqing Yu et al. reported that drinking tea at high temperatures—especially when combined with excessive alcohol consumption or tobacco use—is associated with an elevated risk of esophageal cancer10. In a population-based case–control research, Yang et al. also found that consuming very hot tea significantly elevates the risk of esophageal squamous cell carcinoma (ESCC) in Chinese men, particularly among alcohol drinkers11. In a meta-analysis conducted by Luo and Ge, a significantly increased risk of ESCC was found among individuals who drank hot tea, though no significant association was observed for esophageal adenocarcinoma12.

Nitrate (\({NO}_{3}^{-}\)) is an essential plant nutrient that is naturally present in the environment. In general, vegetables and meat are the main sources of human exposure to nitrates, and drinking water are known as a secondary route13. However, several studies have reported a positive association between elevated nitrate levels in drinking water and pregnancy-related complications, including low birth weight, intrauterine growth retardation, prematurity, spontaneous abortion, fetal deaths, and neonatal deaths14,15,16. Additionally, In a systematic review, Picetti et al. found a positive association between high levels of nitrate in drinking water and gastric cancer, but there was insufficient data to establish a link with other types of cancer17. The World Health Organization (WHO) has established a maximum contaminant level (MCL) of 50 mg/L as nitrate in drinking water. This guideline is set to protect general populations, especially vulnerable groups like bottle-fed infants, and it does not account for other risks, such as cancer or reproductive complications associated with chronic nitrate exposure1. Notably, the International Agency for Research on Cancer (IARC) classifies nitrate as a probable human carcinogen (Group 2 A), citing its role in forming N-nitroso compounds, which are strongly linked to gastric cancer18,19.

Golestan Province is a northeastern region of Iran with a high incidence of esophageal and gastric cancers20. The results of a meta-analysis study have reported a significant link between dietary nitrate, nitrite, and N-nitroso compounds and the increased occurrence of gastrointestinal cancers in this area21. Additionally, concerns about the carcinogenic potential of repeatedly boiled water and prolonged boiled water have increased, causing widespread anxiety22. Although numerous studies have demonstrated an association between habitual hot tea consumption and increased cancer risk, the potential health implications of progressive nitrate accumulation in repeatedly boiled water remain unexamined in experimental research. Given the evidence suggesting elevated nitrate concentrations in repeatedly boiled water, this study aims to analyze how nitrate levels fluctuate with multiple boiling cycles. Additionally, we employed the Hazard Quotient (HQ) approach—a method endorsed by the U.S. Environmental Protection Agency (EPA)—to evaluate the health risks associated with nitrate exposure in infants, children, and adults.