Health Stream Literature Summary - Issue 50 - June 2008

The association between drinking water turbidity and gastrointestinal illness: a systematic review
Mann, A.G., Tam, C.C., Higgins, C.D. and Rodrigues, L.C. (2007) BMC Public Health, 7:256.

Turbidity, which is a measure of light refractiveness of water, is routinely used to indicate drinking water quality. Microbiological contamination is often accompanied by increases in turbidity however other factors such as silt and organic matter also affect turbidity levels of water leaving the treatment plant. Outbreaks of gastrointestinal (GI) illness have been associated with incidents where turbidity exceeds acceptable limits (limits vary between countries but are generally below 1 or 2 nephelometric turbidity units (NTU)). It is unclear however whether endemic GI is associated with drinking water within acceptable turbidity levels. This review was undertaken to determine what evidence exists for an association between drinking water turbidity levels of public drinking water supplies within acceptable quality limits and incidence of acute GI illness.

All peer-reviewed papers published before December 2006 were searched on the subject of water quality and diseases or poisonings with acute GI manifestations in a variety of databases including PubMed and EMBASE. Non peer-reviewed papers were also searched for using the System for Information on Grey Literature in Europe and a variety of websites. The reference lists of all paper that fulfilled the eligibility criteria were manually searched and results checked with experts in the field of drinking water quality. Studies were included if they investigated the effect of variations in turbidity of water from a treated public water supply, either pre-treatment or effluent (post-treatment and leaving the treatment works), on the risk of acute GI illness in the population(s) served by that supply. The review was restricted to countries comparable to the UK in terms of water supply infrastructure and incidence of acute GI illness.

Out of the 22,687 papers found, there were two eligible studies investigating the association between GI illness and pre-treatment water and eight studies of effluent water turbidity. Of the two pre-treatment water studies, one was excluded because of problems with exposure definition and potential misclassification of both exposure and outcome. In the one remaining study exposure was defined as mean daily turbidly of water entering the treatment plant and the outcome as ICD-coded admissions, accident and emergency (A&E) and outpatient visits for acute GI illness. Relative rates and odds ratios between 1.2 and 2.0 for time-series and individual level analyses were reported and several significant lags for the effect of turbidity on GI illness were highlighted. The authors estimated that less than 2% of GI illness in areas of Vancouver served by the treatment plants studied was attributable to drinking water. The authors cited references to the decreased disinfection efficiency that may result from an increase in pre-treatment water turbidity. It was difficult to draw conclusions from this study as the relationship between pre-treatment and effluent water and the relevance of this as a measure of exposure was not clear; pre-treatment water turbidity may not be a good indicator of the quality of water leaving the plant.

Of the eight eligible studies of effluent water turbidity, three were excluded for various reasons leaving five good-quality studies. Two of these studies found no association while the other three found positive associations. One study reported clear increases in telephone health service calls for acute GI illness of between 33% and 76% at around 11, 15 and 17 days after high turbidity days compared with the rate of calls the same number of days after a mean turbidity days. Two other studies based on the same turbidity data over the same time period, investigated the association of turbidity with paediatric and elderly GI illness, respectively. The authors estimated that a 0.04 NTU increase in turbidity on any given day resulted in up to a 31% increase in paediatric A&E visits and admissions four to 10 days later and up to 15% increase in elderly hospitalisation nine to 11 days later. However, it is difficult to demonstrate that certain lags are associated with specific agents, as GI-related ICD codes do not reliably differentiate between different GI pathogens and calls to a telephone health line are non-specific.

From the studies reviewed associations between drinking water turbidity and GI illness were found in two settings, in people of various ages but not in other settings. Studies may have observed different results due to differences in the mean turbidity level between settings. There were important methodological differences between the studies examined such as the level of adjustment for seasonal confounders which make the studies difficult to compare. A better understanding of the current findings could be achieved by an analysis of existing data using the same modelling approach or a pooled analysis of the raw data from these and other settings.