A Better Way to Boil: Comparing Methods of Purifying Water at Home

Abstract

Despite major gains in access to water, sanitation, and hygiene services around the world, an estimated $2.2\text{billion}$ people still lack access to safe drinking water.1 In places without a potable supply, some households treat their own water, often by boiling. Others use bottled water. But little is known about the relative effectiveness of different boiling methods, the microbiological safety of bottled water compared with public water sources, and the relationship between home water treatment and indoor air pollution. A study recently published in Environmental Health Perspectives explored these questions in rural Chinese communities.2

Looking at a representative cross section of households in 15 villages each in Guangxi and Henan Provinces, the authors of the new study collected a variety of data around water sources, sanitation, health, and gender equity. They measured thermotolerant coliforms (TTCs) in samples of drinking water as an indicator of fecal contamination. As TTC counts increase, generally speaking, so too does the expected risk of pathogenic infection.3

Proper sterilization requires bringing water to a rolling boil—not a simmer—for one full minute, or three minutes at elevations above 6,500 feet. Water should then be allowed to cool and stored in a sanitized container with a tight lid.4 Image: © Salineekapui/Shutterstock.

The researchers placed temperature sensors on pots and kettles to corroborate self-reported data on boiling times. These data, in turn, were used to estimate household exposure to fine particulate matter (${\mathrm{PM}}_{2.5}$) released by burning biomass to heat the water.

Approximately 61% of the survey participants boiled their water, 30% used bottled water, and 9% drank untreated water. Overall, some level of TTC was detected in 13.6% of the households using electric kettles and 11.6% of those using open pots, compared with approximately half of both the households using bottled water and those consuming untreated water. Cohen says the frequency of TTC contamination in bottled water suggests the water may be coming from contaminated sources.

Electric kettles appeared to be more effective at purifying water than boiling in pots. “Most electric kettles heat the water until the boiling point, so this method typically ensures full inactivation of pathogens,” explains lead author Alasdair Cohen, an assistant professor of Environmental Epidemiology at the Virginia Polytechnic Institute and State University. Kettles also typically have a built-in top that reduces the opportunity for post-boiling secondary contamination. Water in pots, on the other hand, may not be fully brought to boiling because users may mistake a vigorous simmer for a full boil. Pot-boiled water also may become contaminated after treatment.

The researchers found it took twice as long to boil water in pots compared with kettles. In most cases, Cohen says, this was due to comparative heating efficiency in biomass- versus electricity-fueled boiling. Across both provinces, the researchers estimated that boiling water in pots resulted in an average household ${\mathrm{PM}}_{2.5}$ concentration of $79{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$. In Guangxi households, where data were available for multiple seasons, average concentrations were much higher in winter than in summer.

“This paper brings together primary data and analysis on safe water and household air pollution, fields that are often discussed in completely different literatures,” says Karen Levy, an associate professor of public health at the University of Washington who was not involved in the research.” It is important to break down the silos between these two fields of study because water and energy are so highly interconnected.”

“Use of temperature sensors for verification of self-reported boiling data is a new and interesting approach,” Levy adds. “Sensors can add objective, quantitative estimates to study phenomena that are hard to get at with self-reported survey data.” She cautions that some characteristics of the study region—such as near-universal electricity access and common access to piped water—may not apply to other low- and middle-income countries or even other regions in China.

“This paper tackles an important topic. Boiling with electric kettles provides a viable alternative with built-in safe water storage for many settings, such as urban slums, where electricity is available but water quality is poor,” says Ayse Ercumen, an assistant professor of Global Water, Sanitation, and Hygiene at North Carolina State University who also was not involved with the study. “Boiling is already practiced for water treatment in many parts of the world, but there have been few efforts to capitalize on this existing practice or rigorously study its impact on health, including trade-offs with household air quality.” Ercumen says a follow-up randomized controlled trial would strengthen the findings.

The high level of TTC contamination found in the bottled water has important public health implications for populations that use this commodity. “We were surprised by the relatively high proportion of rural households that used bottled water as their primary source of drinking water,” Cohen says. “International agencies and governments focused on expanding safe drinking water access need to address the various negative impacts associated with increasing bottled water use in China and in other low- and middle-income countries.”

Biography

Wendee Nicole is an award-winning science writer and editor based in Houston, Texas. She has also written for Discover, Nature, Scientific American, and other publications.