Is it true that boiled water left for over three days causes cancer? Even drinking water gets caught in the crossfire.Recently, a netizen posted on Guokr.com targeting overnight water, claiming that consuming water containing various chemicals like zero alcohol, oxygen phenols, hydroxyl hydrogen, and hydroxy acids may cause symptoms such as stomach bloating, nausea, vomiting, and loss of appetite. Another netizen even claimed that boiled water stored for three days produces carcinogens.Is overnight water really that dangerous? Is the carcinogenic claim true or false? Fan Zhihong, Associate Professor at the Department of Nutrition and Food Safety, College of Food Science, China Agricultural University, clarified yesterday that carcinogens don't materialize out of thin air. Therefore, whether overnight water is safe to drink ultimately depends on whether the water quality itself meets safety standards. Expert: This is Purely Netizen Satire"Can't you tell this is a hoax post?" When a reporter sought verification regarding the April 8th post by netizen "Youqinruoshui" on Guokr.com, Fan Zhihong, Associate Professor at the Department of Nutrition and Food Safety, School of Food Science, China Agricultural University, immediately exposed the post's nature: The substances mentioned in the post—such as "zerol alcohol, oxygen phenol,hydroxy hydrogen, hydroxy acid, caustic hydrogen, and di-zero ether" in overnight water are simply everyday terms for "water."
Regarding this widely discussed online post, Fan Zhihong stated: "Such posts exist precisely because they exploit people's fear of chemical elements."
However, despite being a hoax, many people have fallen for it. The widespread sharing of well-intentioned warnings among friends has left those in the know both amused and exasperated.
Harmful Substances Don't Appear Out of Thin Air
This "chemophobia" has unfairly tarnished the reputation of overnight water. Regarding claims that carcinogens form, experts clarify that without a material foundation in the water, carcinogens cannot materialize spontaneously.
The most alarming rumor circulating online is that "boiled water stored for three days produces carcinogens."Fan Zhihong counters: "The most feared carcinogen, nitrite, requires nitrate as a precursor. But take ordinary mineral or purified drinking water—it contains either minerals and trace elements, or nothing at all. Where would carcinogens come from under these conditions?"Regarding the vaguely defined concept of "overnight water," Fan Zhihong clarified that as long as the water source itself meets safety standards, theoretically, boiling it repeatedly won't produce carcinogens. As for "steaming water," whether it contains carcinogens depends entirely on what was steamed in it.If used repeatedly for steaming food, nitrates and nitrites from the food may leach into the steaming water. Through repeated heating and concentration, this can indeed elevate nitrite levels. However, if steaming is done gently over low heat, ingredients are less likely to leach into the water, and minimal evaporation occurs. In such cases, nitrite levels in the steaming water typically do not rise to toxic levels.
Additionally, compared to overnight boiled water, overnight tea should not be consumed. Tea leaves steeped in water produce substances like amino acids, which over time can promote microbial growth, posing health risks.
Water Source Quality Matters More
Fan Zhihong explains that concerns about water contamination outweigh worries about drinking overnight water."In rural areas, this is a major problem. Contamination of drinking water sources by nitrogen fertilizers, livestock manure, or nitrogen-rich industrial wastewater is common in villages and small towns. Not only surface water but even groundwater can sometimes be affected."She added that urban landfill sites can also cause such groundwater contamination. Nitrate pollution in water sources is converted by microorganisms into nitrites, leading to poisoning incidents among humans and livestock in rural and small-town areas. Furthermore, water treatment plants struggle to effectively remove nitrates, making the assurance of water source quality a critical issue.
Is it true that water boiled over three days can cause cancer? We hope our explanation has provided clarity. Finally, we sincerely wish you good health and happiness.