Study challenges standards for bathing water quality

The results of a Government-funded study of the health effects of sea bathing have called into question the basis of EC standards - as well as the Government's objections to a proposal to amend them.1 According to its authors, the study gives the most precise data yet on the risks of catching gastrointestinal illness and could herald a more informed approach to bathing water standards.

Studies by Professor David Kay and colleagues at the Leeds Environment Centre contributed to the WRc report on the health effects of bathing published earlier this year (ENDS Report 229, pp 10-11 ). Professor Kay is an expert adviser to a House of Lords Select Committee which has been holding an inquiry into a proposal to amend the 1976 EC Directive on bathing water quality (ENDS Report 234, pp 17-20 ).

The research involved studies of cohorts of volunteers at four beaches over four summers. They were divided at random into bathing and non-bathing groups and their activities carefully supervised. Bathers had to remain in the water for at least ten minutes and immerse their heads three times. The bacteriological quality of the bathing water at chest depth was monitored throughout the experiment so that the exposure of each bather was known. The health of all volunteers was checked both prior to and after the experiment.

The results allow a precise calculation of the risks of contracting gastrointestinal illness from bathing in seawater of varying bacteriological quality. Of the various indicators of water quality measured, the results show a strong correlation between the risk of illness and the levels of faecal streptococci. Professor Kay told ENDS that the traditional indicators measured, coliforms and faecal coliforms, did not show the correlation.

Statistical analysis of the results showed that the increased risk of disease in bathers became significant when the bathing water contained more than 32 faecal streptococci (fs) per 100ml at chest depth. This is well below the standard of 400fs/100ml in the draft EC Directive, although the currently accepted statistical methodology for deriving bathing water standards means that the figures are not incompatible.

Bacteriological counts in sea water are generally not normally distributed and there is a strong bias towards low numbers of bacteria. Geometric rather than arithmetic means are therefore used to represent the likely exposure of bathers - geometricmeans are much lower than arithmetic means, but correspond with the most probable exposure of bathers given the skewed distribution.

Both Professor Kay and Dr Edmund Pike, author of the WRc report, agree that a standard designed to result in a geometric mean exposure of about 32fs/100ml would set a 95 percentile standard in the region of 400fs/ml. However, Professor Kay doubts whether such an approach is now acceptable given the data available from his study.

The faecal streptococci limit in the draft Directive would entail "significant disease levels", he believes. For any given beach, it is now possible to calculate the risks of bathing more precisely. Risk assessments combined with cost benefit analyses of the options for improved sewage treatment are now feasible alternatives to traditional bathing water standards, he says.

However, Dr Pike is more cautious about the applicability of the study. It would be unwise to rely on the results of only one study, he said, when research elsewhere in the world had shown the importance of indicators like faecal coliforms. The study also did not consider risks to children or surfers, and measuring water quality at chest depth may be impractical for regulators. At shallower depths, water quality tends to be more variable because of the influence of sediments.

Professor Kay's work appears not to have influenced the Government's stance on the draft Directive. A Health Minister told the Lords inquiry in October that the UK will oppose even the proposed 400fs/100ml standard because it will yield little health benefit (see pp 28-29 ).

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