Why does boiling water make it safe to drink?

Whether you’ve seen news about boil water advisories after a natural disaster or watched outdoor survival experts boil water while camping in the woods, you’ve probably heard before that boiling water can help purify it. But how does boiling water make it safer to drink? And what kinds of contamination does boiling water eliminate?

How boiling water kills germs

To answer these questions, we need to take a closer look at how water gets contaminated. Finding the right water treatment solution depends on what kind of hazards are present in the water to start. If your water is muddy with dirt, for example, boiling it won’t remove the particles like a physical filter would. Likewise, if chemical or radiological hazards are present in the water, boiling it will not make it drinkable.

However, when it comes to microbial hazards—disease-causing microorganisms like bacteria, viruses, and parasites—boiling or heating water is the most widely used and effective method to make water safe. Infectious diseases caused by microbial pathogens, such as cholera, typhoid, and Shigella, are the most common and widespread health risk associated with drinking water. Like humans, microorganisms are sensitive to temperature, and boiling water kills most pathogens by damaging their protein structures, disrupting their cell membranes, and inactivating essential enzymes. The same principle is behind pasteurization, the process that makes products like milk, eggs, and juice safer to consume.

Heating water to a high temperature—212°F or 100°C—will eliminate most of these pathogenic organisms, particularly the viruses and bacteria that cause waterborne diseases. To eliminate microbial hazards, the CDC recommends bringing clear water to a rolling boil for 1 minute, or 3 minutes at elevations above 6,500 feet to compensate for the lower boiling point of water. After boiling, allow the water to cool before use and store it in clean, sanitized containers with tight covers.

Photo: A water station at Kilingili Health Centre in Kakamega County, Kenya. One in five of all health care facilities have no basic water services. Credit: PATH/Anthony Karumba

Treating water to prevent diarrheal disease

Boiling water is an accessible and effective way to treat water for microbial pathogens, but it’s often impractical or inefficient given the amount of water a person uses each day. Boiling is also energy intensive and does not provide residual protection of water during storage. For these reasons, boiling water is not typically a common outbreak response strategy for diseases.

Chlorination, a type of chemical disinfection that involves adding chlorine-based products to water, is another type of disinfection method that is cheap, reliable, and widely available in several forms. Distributing chlorine to households during a disease outbreak or in the aftermath of flooding or natural disasters can provide a layer of protection against waterborne diseases like cholera.

Increasing access to clean water

Globally, more than 2 billion people lack access to safe drinking water, and one in five of all health care facilities have no basic water services. Gaps in clean water access have far-reaching consequences: poor access to water, sanitation, and hygiene (WASH) is responsible for more than half of diarrhea cases. Expanding access to clean water is about more than just preventing dehydration and thirst—it’s about stopping the spread of disease and improving people’s lives and livelihoods globally.

Between 2000 and 2022, 2.1 billion people gained access to safely managed drinking water, but there are wide disparities in access—from 94 percent regional coverage in Europe and Northern America to just 31 percent in sub-Saharan Africa. While improvements in WASH infrastructure are the ideal long-term solution, water treatment solutions like boiling and chlorination are fast, effective ways to prevent disease in areas where WASH services are inadequate.

Cover photo: In Kakamega County, Kenya, Metrine Mutonye makes several trips a day to a water kiosk with her jerrycans. Credit: PATH/Anthony Karumba