Does boiling remove everything harmful from water?

"I boiled it, so it's fine" is one of the most reasonable-sounding things an Indian household says about its drinking water — and for most of the country's public health history, it's been correct advice. Boiling is what municipal advisories tell you to do during monsoon contamination, it's what hostels and offices do with tanker water, it's what generations of parents have done with formula water. It works, it's free, and it needs no equipment beyond a stove.

The problem is what "it works" is actually doing. Boiling handles what's alive in your water. It does nothing to what's dissolved in it — and that gap is where most of the confidence goes wrong. Water that's perfectly sterile can still be loaded with fluoride, arsenic, nitrate, or heavy metals, and boiling won't tell you that, or fix it.

What boiling actually does

Boiling works by heat, not chemistry. Above roughly 70°C, the proteins that hold bacteria, viruses, and protozoa together start to lose their structure — the technical term is denaturation. Enzymes stop functioning, cell membranes and viral envelopes break down, and the organism can no longer survive or reproduce. At a full rolling boil (100°C at sea level), this happens fast and comprehensively across the range of pathogens found in contaminated water.

This is why boiling is the WHO- and CDC-recommended method for emergency and household water disinfection: bring water to a rolling boil for 1 minute, extending to 3 minutes above about 2,000 metres altitude, where lower atmospheric pressure means water boils at a slightly lower temperature. Let it cool before drinking or storing. There's no benefit to boiling longer than that — the pathogens are already dead within the first minute at a full boil.

• Bacteria: E. coli, Salmonella, Shigella, Vibrio cholerae (cholera)
• Viruses: Hepatitis A, rotavirus, and most other waterborne viruses
• Protozoa: Giardia and Cryptosporidium, notably resistant to chemical disinfectants like chlorine but not to heat

What it leaves completely untouched

Dissolved contaminants — fluoride, arsenic, nitrate, heavy metals like lead or iron, most pesticide residues, and general dissolved salts — have no cell wall, no protein structure, nothing for heat to disrupt. They're simply dissolved in the water, the same way salt stays dissolved in water you boil for cooking. Heat doesn't separate them out; only evaporation, filtration, or chemical treatment can.

Evaporation, in fact, cuts the wrong way for anyone hoping boiling helps. As water boils off as steam, whatever is dissolved in it stays behind in the pot — so extended boiling doesn't dilute chemical contamination, it concentrates it. This is a real, specific risk with nitrate: if source water already carries nitrate near or above the safe limit, boiling it down for tea, formula, or cooking raises the nitrate concentration in what's left.

Nitrate matters more for infants than adults. Infants convert nitrate to nitrite far more efficiently than adults do, and high nitrite intake causes methemoglobinemia — "blue baby syndrome" — which impairs the blood's ability to carry oxygen. Preparing formula with boiled water from a nitrate-contaminated borewell doesn't remove the risk; extended boiling can make it worse.

Where this goes wrong in Indian households

This isn't a hypothetical edge case here. A 2023 Central Ground Water Board assessment of over 15,000 groundwater samples nationally found 19.8% exceeded the permissible limit for nitrate, 9.04% for fluoride, and 3.55% for arsenic. Fluoride contamination is present across 469 districts in 27 states; arsenic across 230 districts in 25 states, with Uttar Pradesh, Bihar, and Assam accounting for the most affected districts and West Bengal a long-documented hotspot on the Bengal delta; and nitrate contamination — largely from agricultural runoff and fertiliser use — spans 440 districts.

Under Indian drinking water standards (IS 10500), the acceptable limits are 1.0 mg/L for fluoride (relaxable to 1.5 mg/L only if no alternate source exists), 45 mg/L for nitrate, and 0.01 mg/L for arsenic, with no permissible relaxation on either of the latter two. None of these numbers move when you boil the water — a borewell sample that fails on fluoride or arsenic still fails after fifteen minutes at a rolling boil.

Households relying on a private borewell in a fluoride- or arsenic-affected region, or on tube-well water in a heavily fertilised agricultural belt, can be boiling religiously for microbial safety while the actual long-term risk — fluorosis, arsenicosis, or nitrate exposure — sits untouched in the same glass. Old plumbing adds another layer: iron and other metals picked up from corroded fittings aren't affected by heat either.

What to actually do

Boiling is the correct, sufficient response to microbial risk: tanker water of uncertain origin, monsoon-season contamination, a municipal boil-water advisory, or any situation where the concern is what might be alive in the water rather than what's dissolved in it. Keep doing it for those situations — nothing else at home matches it for reliability against pathogens.

It stops being sufficient the moment the source itself is suspect for dissolved contamination — a borewell in a fluoride or arsenic belt, groundwater in a heavily farmed area, or any source you've never had tested. There, boiling should be paired with, not substituted for, a proper water test and, if the test comes back high, treatment suited to what's actually there: reverse osmosis for a broad range of dissolved salts and metals, activated alumina or ion-exchange for fluoride specifically, or arsenic-specific adsorption media.

• If you rely on a borewell or tube well regularly, get it lab-tested for fluoride, nitrate, and arsenic at least once — a TDS meter alone won't tell you which of these is present, since it only measures total dissolved solids, not composition.
• For infant formula, treat nitrate as a separate, non-negotiable check on the source water, independent of boiling.
• Don't over-boil hoping for extra safety margin — past the first minute at a rolling boil, you're only concentrating whatever chemical contamination is present, not adding disinfection.