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SUMMARYDaily and hourly variations in microbial indicators densities in the beach-waters of Hong Kong have been described. The levels of Escherichia coli at a number of beaches was observed to be influenced by tide, and for staphylococci, by bather numbers. The tidal influence was most obvious during spring tides; and for the effect of bathers, during neap tides. Both organisms are present in high densities in external sources of faecal pollution of bathing beaches, with the average staphylococci to E. coli ratios being 0·04–3. Staphylococci may serve as an indicator of bather density and the risk of cross-infection amongst bathers (rather than as another indicator of faecal contamination) when the average staphylococci to E. coli ratio for a bathing beach is considerably higher than 3. The variability of microbial indicator densities means the routine sampling of bathing beaches should be carried out on weekend days with maximum numbers of swimmers exposed to the water, and spread throughout the bathing season.

A prospective epidemiological study was undertaken in Hong Kong in 1987, in which 18,741 usable responses were obtained. It showed bathing in the coastal beaches of Hong Kong poses an increased risk of developing gastrointestinal, ear, eye, skin, respiratory and total illness. Swimmers immersed in the more polluted beach waters are exposed to a significantly higher risk of contracting swimming-associated gastrointestinal, skin, respiratory and total illness. E. coli was found to be the best indicator for swimming-associated gastroenteritis and skin symptoms amongst the bathers, and a linear relationship could be established. Staphylococci was a good indicator for ear, respiratory and total illness, and should be used in complementary to E. coli. Beach water quality objectives for both E. coli and staphylococci have been proposed. A 4-tier classification system (rather than a single acceptability criterion) based on swimming-associated health risks has been developed for the beaches of Hong Kong. Information on the bacterial water quality and health risk levels of individual beaches is reported to the public both annually and fortnightly, so that beach-goers can choose where to go for swimming based on health effects data.