Repeatedly boiled water refers to water that has been brought to a boil multiple times. A common rumor claims such water is undrinkable, on the grounds that its nitrite levels exceed safety limits and long-term consumption causes poisoning or even cancer. Is this claim well-founded?

Nitrites Can Form Carcinogens

Nitrites and nitrates are ubiquitous nitrogen-containing inorganic compounds in nature. The natural nitrogen cycle and human activities serve as major sources of nitrates. Microbes often convert nitrates into nitrites simultaneously. As a general term for inorganic compounds, common nitrites include sodium nitrite and potassium nitrite. They resemble salt in appearance: white to pale yellow powder or granules, odorless, slightly salty, hygroscopic and highly soluble in water. Widely used as food additives in meat products, they enhance color and preserve freshness.

Nitrites themselves are not carcinogenic. However, under gastric acid conditions, they react with secondary amines, tertiary amines and amides in food to produce N-nitrosamines, potent carcinogens recognized worldwide. Toxicological data shows acute nitrite poisoning occurs after ingesting 0.2–0.5 grams, while 3 grams can be fatal.

The national standard GB 2760-2014 National Food Safety Standard – Standards for the Use of Food Additives stipulates a maximum usage of 0.5 g/kg for sodium nitrate and potassium nitrate as color fixatives and preservatives, with residual nitrite (calculated as sodium/potassium nitrite) limited to no more than 30 mg/kg.

Repeatedly Boiled Tap Water Does Contain Trace Nitrites

Chinese people are accustomed to drinking hot water. Boiling creates a high-temperature, low-oxygen environment that accelerates the conversion of nitrates into nitrites.

Scientific experiments tested standard-qualified tap water boiled different times and measured nitrite concentrations:

1 boil: 0 mg/L

2 boils: 0.011 mg/L

3 boils: 0.027 mg/L

4 boils: 0.073 mg/L

5 boils: 0.098 mg/L

Even water boiled five times would require drinking 120 liters to exceed safety limits. Still, nitrite levels rise noticeably after three boils, and after ten boils the concentration hits 1.390 mg/L, exceeding the limit. It is therefore recommended to boil tap water no more than three times.

Another experiment measured nitrite in boiled water left standing for different durations:

Freshly boiled: 0 mg/L

1 hour after boiling: 0.0011 mg/L

2 hours after boiling: 0.056 mg/L

Overnight boiled water: 1.09 mg/L

Nitrite accumulates the longer boiled water sits, with a sharp jump overnight. Drink boiled water promptly after preparation instead of storing it for long periods.

Poor Source Water Leads to Higher Nitrite Levels

Water quality exerts a greater influence on nitrite content than boiling frequency or standing time. Some rural areas rely on bitter well water rich in nitrates. If boiled and stored for too long, nitrates reduce into nitrites and may trigger poisoning. Rural drinking water may be contaminated by nitrogen-rich fertilizers, livestock manure and industrial wastewater; urban tap water can also be polluted by improperly disposed landfill waste, driving up nitrate concentrations.

Repeatedly boiling substandard high-nitrate water produces far more nitrite. Residents in regions with poor local water quality are advised to install water purifiers or buy reputable bottled water. Barreled water dispensers require regular cleaning; choose small-capacity barrels and finish the water quickly after opening.

Is Tap Water Safe to Boil and Drink?

China’s 2006 Standards for Drinking Water Quality sets limits of 10 mg/L nitrate (calculated as nitrogen) for surface water and 20 mg/L for groundwater, with a nitrite ceiling of 1 mg/L.

The FAO and WHO establish acceptable daily intake (ADI) values: 5 mg per kg body weight for nitrates and 0.2 mg per kg body weight for nitrites. For an adult weighing 60 kg, the ADI for nitrite equals 12 mg. With compliant tap water, a person would need to drink 12 liters to hit this limit. The Dietary Guidelines for Chinese Residents (2016) recommends a daily water intake of only 1.5–1.7 liters.

In summary, properly boiled tap water within three heating cycles and consumed fresh poses no health risks to humans.