Is it OK to boil water more than once, or should you empty the kettle every time?
But is it okay to re-boil water that's already in the kettle from last time? While bringing water to a boil disinfects it, you may have heard that boiling water more than once will somehow make the water harmful and therefore you should empty the kettle each time.
Such claims are often accompanied by the argument that re-boiled water leads to the accumulation of allegedly hazardous substances including metals such as arsenic, or salts such as nitrates and fluoride.
This isn't true. To understand why, let's look at what is in our tap water and what really happens when we boil it.
What's in our tap water?
Let's take the example of tap water supplied by Sydney Water, Australia's largest water utility which supplies water to Sydney, the Blue Mountains and the Illawarra region.
From the publicly available data for the January to March 2025 quarter for the Illawarra region, these were the average water quality results:
pH was slightly alkaline
total dissolved solids were low enough to avoid causing scaling in pipes or appliances
fluoride content was appropriate to improve dental health, and
it was 'soft' water with a total hardness value below 40mg of calcium carbonate per litre.
The water contained trace amounts of metals such as iron and lead, low enough magnesium levels that it can't be tasted, and sodium levels substantially lower than those in popular soft drinks.
These and all other monitored quality parameters were well within the Australian Drinking Water Guidelines during that period. If you were to make tea with this water, re-boiling would not cause a health problem. Here's why.
It's difficult to concentrate such low levels of chemicals
To concentrate substances in the water, you'd need to evaporate some of the liquid while the chemicals stay behind. Water evaporates at any temperature, but the vast majority of evaporation happens at the boiling point — when water turns into steam.
During boiling, some volatile organic compounds might escape into the air, but the amount of the inorganic compounds (such as metals and salts) remains unchanged.
While the concentration of inorganic compounds might increase as drinking water evaporates when boiled, evidence shows it doesn't happen to such an extent that it would be hazardous.
Let's say you boil one litre of tap water in a kettle in the morning, and your tap water has a fluoride content of 1mg per litre, which is within the limits of Australian guidelines.
You make a cup of tea taking 200ml of the boiled water. You then make another cup of tea in the afternoon by re-boiling the remaining water.
On both occasions, if heating was stopped soon after boiling started, the loss of water by evaporation would be small, and the fluoride content in each cup of tea would be similar.
But let's assume that when making the second cup, you let the water keep boiling until 100ml of what's in the kettle evaporates. Even then, the amount of fluoride you would consume with the second cup (0.23mg) would not be significantly higher than the fluoride you consumed with the first cup of tea (0.20mg).
The same applies to any other minerals or organics the supplied water may have contained. Let's take lead: the water supplied in the Illawarra region as mentioned above, had a lead concentration of less than 0.0001mg per litre. To reach an unsafe lead concentration (0.01mg per litre, according to Australian guidelines) in a cup of water, you'd need to boil down roughly 20 litres of tap water to just that cup of 200ml.
Practically that is unlikely to happen — most electric kettles are designed to boil briefly before automatically shutting off. As long as the water you're using is within the guidelines for drinking water, you can't really concentrate it to harmful levels within your kettle.
But what about taste?
Whether re-boiled water actually affects the taste of your drinks will depend entirely on the specifics of your local water supply and your personal preferences.
The slight change in mineral concentration, or the loss of dissolved oxygen from water during boiling may affect the taste for some people — although there are a lot of other factors that contribute to the taste of your tap water.
The bottom line is that as long as the water in your kettle was originally compliant with guidelines for safe drinking water, it will remain safe and potable even after repeated boiling. — Reuters
Hashtags
Try Our AI Features
Explore what Daily8 AI can do for you:
Comments
No comments yet...
Related Articles
Free Malaysia Today
18-07-2025
- Free Malaysia Today
AI-powered blood test allows for early breast cancer detection
The new AI-powered lipid-based blood test enables earlier and less invasive breast cancer detection. (Freepik pic) SYDNEY : Australian scientists have developed the first artificial intelligence-powered lipid-based blood test, enabling earlier and less invasive breast cancer detection, Xinhua reported. The test is already in use in specialist clinics across Sydney and Melbourne after transitioning from the research phase in March, according to a statement from the University of New South Wales (UNSW) published on Wednesday. Unlike traditional imaging and biopsies, which can miss tumours and carry risks, the new AI-powered blood test detects subtle molecular signals in the blood. This allows for cancer diagnosis months or even years before symptoms appear, said UNSW associate professor Fatemeh Vafaee. AI drives the breakthrough by analysing millions of molecular markers to detect early cancer signs, according to the expert from Vafaee Lab, a biomedical AI laboratory at UNSW. 'By integrating explainable AI techniques, we ensure the models provide not only accurate outcomes but also clinically interpretable insights, which are crucial for building trust and supporting decision-making in real-world healthcare settings,' Vafaee said. Contributing to global efforts to make AI-driven blood tests part of standard breast cancer screening, the lab offers a valuable new option, especially for women with dense breast tissue where traditional imaging may be less effective. The team is now developing multi-analyte tests that combine various biomarkers for more precise cancer detection across multiple types, including lung, liver, and brain tumours. They are also exploring tests using other biofluids like urine and saliva, while working to integrate diverse data for a complete view of patient health.
Malay Mail
17-07-2025
- Malay Mail
Is it OK to boil water more than once, or should you empty the kettle every time?
SYDNEY, July 17 — The kettle is a household staple practically everywhere — how else would we make our hot drinks? But is it okay to re-boil water that's already in the kettle from last time? While bringing water to a boil disinfects it, you may have heard that boiling water more than once will somehow make the water harmful and therefore you should empty the kettle each time. Such claims are often accompanied by the argument that re-boiled water leads to the accumulation of allegedly hazardous substances including metals such as arsenic, or salts such as nitrates and fluoride. This isn't true. To understand why, let's look at what is in our tap water and what really happens when we boil it. What's in our tap water? Let's take the example of tap water supplied by Sydney Water, Australia's largest water utility which supplies water to Sydney, the Blue Mountains and the Illawarra region. From the publicly available data for the January to March 2025 quarter for the Illawarra region, these were the average water quality results: pH was slightly alkaline total dissolved solids were low enough to avoid causing scaling in pipes or appliances fluoride content was appropriate to improve dental health, and it was 'soft' water with a total hardness value below 40mg of calcium carbonate per litre. The water contained trace amounts of metals such as iron and lead, low enough magnesium levels that it can't be tasted, and sodium levels substantially lower than those in popular soft drinks. These and all other monitored quality parameters were well within the Australian Drinking Water Guidelines during that period. If you were to make tea with this water, re-boiling would not cause a health problem. Here's why. It's difficult to concentrate such low levels of chemicals To concentrate substances in the water, you'd need to evaporate some of the liquid while the chemicals stay behind. Water evaporates at any temperature, but the vast majority of evaporation happens at the boiling point — when water turns into steam. During boiling, some volatile organic compounds might escape into the air, but the amount of the inorganic compounds (such as metals and salts) remains unchanged. While the concentration of inorganic compounds might increase as drinking water evaporates when boiled, evidence shows it doesn't happen to such an extent that it would be hazardous. Let's say you boil one litre of tap water in a kettle in the morning, and your tap water has a fluoride content of 1mg per litre, which is within the limits of Australian guidelines. You make a cup of tea taking 200ml of the boiled water. You then make another cup of tea in the afternoon by re-boiling the remaining water. On both occasions, if heating was stopped soon after boiling started, the loss of water by evaporation would be small, and the fluoride content in each cup of tea would be similar. But let's assume that when making the second cup, you let the water keep boiling until 100ml of what's in the kettle evaporates. Even then, the amount of fluoride you would consume with the second cup (0.23mg) would not be significantly higher than the fluoride you consumed with the first cup of tea (0.20mg). The same applies to any other minerals or organics the supplied water may have contained. Let's take lead: the water supplied in the Illawarra region as mentioned above, had a lead concentration of less than 0.0001mg per litre. To reach an unsafe lead concentration (0.01mg per litre, according to Australian guidelines) in a cup of water, you'd need to boil down roughly 20 litres of tap water to just that cup of 200ml. Practically that is unlikely to happen — most electric kettles are designed to boil briefly before automatically shutting off. As long as the water you're using is within the guidelines for drinking water, you can't really concentrate it to harmful levels within your kettle. But what about taste? Whether re-boiled water actually affects the taste of your drinks will depend entirely on the specifics of your local water supply and your personal preferences. The slight change in mineral concentration, or the loss of dissolved oxygen from water during boiling may affect the taste for some people — although there are a lot of other factors that contribute to the taste of your tap water. The bottom line is that as long as the water in your kettle was originally compliant with guidelines for safe drinking water, it will remain safe and potable even after repeated boiling. — Reuters
Free Malaysia Today
12-07-2025
- Free Malaysia Today
Emergency vaccines slash deaths by 60%
Vaccine-preventable diseases such as measles, meningitis, and yellow fever are on the rise globally amid misinformation and cuts to international aid. (AP pic) GENEVA : Emergency vaccination during outbreaks of diseases like cholera, Ebola and measles have over the past quarter-century reduced deaths from such illnesses by nearly 60%, according to a new study. A similar number of infections are also believed to have been prevented, while billions of euros have been generated in estimated economic benefit. The Gavi vaccine alliance, which backed the study, said it collaborated with researchers at Burnet Institute in Australia to provide the world's first look at the historical impact of emergency immunisation efforts on public health and global health security. 'For the first time, we are able to comprehensively quantify the benefit, in human and economic terms, of deploying vaccines against outbreaks of some of the deadliest infectious diseases,' Gavi chief Sania Nishtar said in a statement. 'This study demonstrates clearly the power of vaccines as a cost-effective countermeasure to the increasing risk the world faces from outbreaks.' The study, published this week in the British Medical Journal Global Health, examined 210 outbreaks of five infectious diseases – cholera, Ebola, measles, meningitis and yellow fever – in 49 lower-income countries between 2000 and 2023. Vaccine roll-outs in these settings had a dramatic impact, with the study showing they reduced both the number of infections and deaths by almost 60% across the five diseases. For some of the diseases the effect was far more dramatic. Vaccination was shown to decrease deaths during yellow fever outbreaks by a full 99%, and 76% for Ebola. At the same time, emergency vaccination significantly reduced the threat of outbreaks expanding. It also estimated that the immunisation efforts carried out during the 210 outbreaks generated nearly US$32 billion in economic benefits just from averting deaths and years of life lost to disability. That amount was however likely to be a significant underestimate of overall savings, it said, pointing out that it did not take into account outbreak response costs or the social and macro-economic impacts of disruptions created by large outbreaks. The massive Ebola outbreak that hit West Africa in 2014, before the existence of approved vaccines, for instance saw cases pop up worldwide and is estimated to have cost the West African countries alone more than US$53 billion. The study comes after the World Health Organisation warned in April that outbreaks of vaccine-preventable diseases such as measles, meningitis, and yellow fever are on the rise globally amid misinformation and cuts to international aid. Gavi, which helps vaccinate more than half the world's children against infectious diseases, is itself currently trying to secure a fresh round of funding in the face of the global aid cuts and after Washington last month announced it would stop backing the group.
