Quality of drinking water
Authors: Stephen P Luby
Journal: BMJ (2007)

People who drink water that is contaminated with human faeces are at risk of diarrhoea, a condition that results in 1.8 million deaths in children each year.1 In this week's BMJ, a systematic review and meta-analysis by Clasen and colleagues2 finds that household interventions to improve the microbiological quality of drinking water reduce the occurrence of diarrhoea. Their results show that the quality of water has an impact on health. They also highlight the value to public health of achieving the targets outlined in the seventh millennium development goal—to reduce by half the proportion of people without sustainable access to safe drinking water.

Two articles published in 1985 and 1991 are the most cited reviews on the effectiveness of interventions to prevent diarrhoea.3 4 These considered only improvements to the water source, however. They did not assess the microbiological quality of the water at the point of use, and they did not include any of the recent studies that evaluated microbiologically effective treatment of drinking water at the point of use. They concluded that improvements in the quality of drinking water at source reduced diarrhoea by 15-17%, in contrast to larger reductions as a result of increased water supply, promotion of hand washing, and improved sanitation.

During the past two years, two reviews with meta-analyses have included many new studies on water treatment at point of use and re-evaluated the effect of improved water quality on diarrhoea.5 6 The first concluded that in high quality studies, water treatment at point of use reduced diarrhoea by 39% compared with 11% (not statistically significant) for interventions that improved the quality of water at source.5 The second reviewed interventions that used water treatment with sodium or calcium hypochlorite, it analysed only the youngest age group reported in each study, and it concluded that chlorine treatment at point of use reduced diarrhoea by 29%.6

The meta-analysis by Clasen and colleagues of 42 controlled trials and 56 000 participants is the most rigorous review to date.2 Studies that met their criteria included those assessing all water disinfection techniques at point of use and improvements to the water source, and published and unpublished studies. Clasen and colleagues conclude that interventions to improve the microbiological quality of drinking water are generally effective in reducing diarrhoea in adults and children under 5 years, and that household interventions are more effective than water source interventions. However, they caution that heterogeneity between trials means that effectiveness may vary according to the setting.

These promising results suggest that the measurement used to assess progress towards the millennium development goal should be changed to reflect the importance of microbiological water quality. Currently, the global standard for safe water is an assessment of the proportion of the population that has access to an “improved water supply.”7 8 However, an improved water supply is an engineering definition. For example, piped water or a protected spring is an improved water supply compared with water from a tanker truck or an unprotected spring.7 Importantly, improved water supplies are often contaminated with human faecal organisms—that is, they are not microbiologically safe water supplies.9 10 11 12 Thus, the reduction in diarrhoea shown by Clasen and colleagues resulting from microbiological treatment of water at point of use does not necessarily apply to improved water supplies.

For these findings to translate to improved health, water quality needs to be measured by the microbiological quality of the water that people actually drink, rather than the types of water sources. In 2006 the United Nations projected, using the outdated metric, that the world is on track to meet the millennium development target for safe water.8 The risk is that the United Nations will claim victory over unsafe water when, in fact, water supplies that are technically defined as “improved” will provide little health benefit to the population in need.

Populations with the lowest mortality rates from diarrhoeal disease have microbiologically safe water piped directly to point of use. Until such services can be provided in low income countries, point of use water treatment is a potential interim solution. However, experience of scaling up the implementation of treating household water to large populations is limited. Research is needed to evaluate whether the health gains demonstrated in the carefully controlled efficacy studies reviewed by Clasen and colleagues can be achieved in large populations at high risk for mortality from diarrhoeal disease when point of use water treatment is not provided free of cost, and when household visits to encourage use are limited.