HALF of 24 water samples taken from a public swimming pool in Sha Tin have been found to contain amoeba, and officials are at a loss to explain why. The tests were triggered when it was discovered that a boy who had visited the Hin Tin pool had later died of brain inflammation. The findings were unveiled in a Health Department report handed to the Regional Services Department yesterday. Referring to the amoeba found in one sample collected before July 20, principal health and medical officer Dr Leung Ting-hung said: ''Although it [the amoeba] was non-pathogenic [harmless], we still find it difficult to explain its existence, given that the water was chlorinated to an international standard.'' He assured the public the water in public swimming pools was being monitored. The Hin Tin pool was closed on Monday, when the result of the first sample was known. The types of amoeba found in the 12 samples, collected while the pool was open between July 20 and July 23, have yet to be identified by the department. At the Regional Council meeting yesterday, both Dr Leung and his colleague, Dr Leung Pak-kin, would not say whether the findings meant there were weaknesses in the chlorination system. Councillor Yeung Fuk-kwong asked: ''You have always claimed that the pools met international safety standards, so how come we can still find micro-organisms in the water? ''Does that mean that we have to face that threat in all the pools?'' Another councillor, the Reverend Fung Chi-wood, questioned the effectiveness of chlorine in controlling amoeba. ''Do we need to find other ways, and what are they?'' he asked. Both officers sidestepped the question. Dr Leung Ting-hung said the water would be safe for swimming if the amount of chlorine was maintained at 0.5 to one part per million and no harmful micro-organism could exist. But he admitted the regular water test, conducted once a month at the pools, did not cover the test for amoeba, some of which were harmful. Lee Wing-lok, 15, died five days after visiting the Hin Tin pool. Naegleria fowleri, a type of amoeba, is believed to have caused the boy's death and is often found in polluted streams. The Deputy Director of Regional Services Administration, Tang Kwok-bun, said his department was looking into possible defects in the structure and drainage system of the pool. The Electrical and Mechanics Engineering Department and the Architectural Services Department have been called in for inspections. Mr Tang said officials would wait for a more detailed report on the water samples as well as the test report on the water from all public swimming pools before deciding when to re-open the pool. The other 15 swimming pools under the Regional Services Department will remain open until further instructions from the Health Department. The findings have also raised concern over the water quality in pools managed by private organisations. United Democrat Albert Chan Wai-yip questioned how the Government could be sure that no pathogenic amoeba would be found in private pools, which are tested only once a month for chloride levels.