The city's dental teaching hospital will submit a report to its board next month to explain how it gave patients heavily contaminated water to rinse their mouths. Prince Philip Dental Hospital's acting director, Professor Jin Lijian, disclosed the timeline yesterday as he explained the blunder to district councillors. A special task force was set up to investigate the "causes and issues" of the contamination after it was disclosed by the South China Morning Post in August. Jin and Kenny Wong Fung Kwan-yee, comptroller of the Sai Ying Pun hospital, denied there had been a cover-up although the incident did not come to light until the Post report. "It is true that two to three dental beds were affected by murky water early this year," Wong said at a meeting of the Central and Western District Council. "Now we have to do everything we can to ensure the water has no problem." According to internal documents seen by the Post , the fouling of the water continued for four months until a means of disinfection was found. The papers suggested that from mid-February to mid-June, water samples from the clinic exceeded 500 colony-forming bacterial units per millilitre - the US safe standard measurement for drinking water. A source said a student had also found coliform bacteria - usually detected in human faeces - in the water supply when conducting research in April for a thesis. Microbiologist Dr Ho Pak-leung, of the University of Hong Kong, has suggested that people with weak immune systems were particularly vulnerable to high levels of bacteria. Yesterday, Wong said that the hospital had tested the water and sought expert opinion on its safety and concluded that it would not pose any health risk. No patient has been reported affected by the contamination, and the hospital promised to disclose all future incidents affecting patients, she said. Refusing to specify the strain of the bacteria and its level found in the water, Wong said it was a strain commonly found in the environment including the water and air. Jin said the task force, set up to investigate the "causes and issues" of the incident, would look into why water being supplied to dental units through contaminated lines was not immediately halted. It would also look into the reasons why water samples were not sent to an accredited laboratory for analysis. He said it would be up to the board to decide whether the report would be made public.