Diarrhoea-inducing waterborne microbes often go undetected in parts of the world with the highest rate of under-five deaths from gastrointestinal infection.
According to the UN Children's Fund (UNICEF), lack of water safety regulations, inter-ministerial coordination and surveillance can paint a deceptively benign portrait of water quality.
"There are different interpretations of water safety among the line ministries [working on water issues], which makes it hard to draw a conclusion about water quality," Rolf Luyendijk, senior statistician for water and sanitation at UNICEF, said.
Taps, boreholes, covered wells or springs, as well as rainwater, are considered "improved" and "safe" water sources but they do not guarantee safe drinking water, he said.
"Water from a dug well may not meet microbiological standards and may still be deadly," he told IRIN. According to the World Health Organisation (WHO), contaminated water contributes to more than two million deaths from diarrhoea each year, plus millions of other cases of waterborne diseases.
In 2004, UNICEF and WHO piloted rapid water assessments in Bangladesh, China, Ethiopia, India, Jordan, Nicaragua, Nigeria and Tajikistan, which showed that only piped came close to meeting international guidelines. Other water sources labelled as "improved" were about half-way compliant with the international guidelines.
Luyendijk told IRIN ministries working on water and sanitation need to improve data coordination and water quality surveillance to find out if investments are reaching the neediest.
"There is an enormous amount of money invested in boosting access [to safe water and sanitation] and those improvements have not reached the poorest quintile [20 percent]," he said.
While 84 percent of people living in low-income countries are reported to use improved water sources, eight out of 10 people without access live in rural areas, according to the latest WHO-UNICEF report on water and sanitation coverage.