The assessment of beach water quality is based on the geometric mean of E.coli level in the latest five water samples taken recently. When the geometric mean of E. coli level in the latest five samples exceeds 610 counts per 100 mL and the E.coli level of the latest water sample exceeds 1,600 counts per 100mL, the beach water quality will be assessed as “unsatisfactory”.     
        
        When the quality of water is classified as "unsatisfactory", the Laboratory shall immediately notify the competent government departments to take appropriate measures.
       
       In accordance with all results of the geometric mean of the E. coli counts collected during the bathing season, with reference to the Annual Ranking System of Environmental Protection Department of Hong Kong, the beach water quality are classified into four grades, namely “Good”, “Fair”, “Poor” and “Very Poor” so as to assess the water quality of beaches in the year and offer information about long-term water quality trends.
       
        The water quality of beaches may deteriorate rapidly during or after heavy rain and usually returns to normal in about three days. Therefore, swimming at beaches during or within three days after heavy rain should be avoided as far as possible.