The earliest water quality standard for beaches in Hong Kong was set on the basis of the interim criteria for bathing water recommended by the WHO in 1977. It stated that the level of E. coli should not exceed 1 000 per 100mL calculated as the running median of the 5 most recent consecutive samples. The WHO also considered that the same water quality standard might not be applicable for worldwide conditions and recommended that epidemiological studies should be conducted locally to develop health related criteria that suited the particular conditions of different countries.

After the establishment of the EPD in 1986, local epidemiological studies were conducted and the Water Quality Objectives for bathing water in Hong Kong were also developed. In order to facilitate the dissemination and interpretation of information on beach water quality, beach rating systems were also introduced. The milestones of the water quality standards and rating systems for beaches in Hong Kong are shown in Table 3.1.

Table 3.1	Milestones of Beach Standards and Rating Systems

In accordance with the recommendations and guidelines of the WHO, epidemiological studies were conducted locally by the EPD in collaboration with local academics in the late 1980's. The studies were conducted in three phases. The findings of the studies are summarized as follows:

• Swimmers were exposed to higher risks of developing gastrointestinal, eye and skin illness symptoms than non-swimmers;
• The swimming-associated illness rates of Hong Kong beaches were generally lower than those reported by overseas researchers;
• A linear relationship between the rates of swimming-associated illnesses (gastroenteritis and skin symptom) and geometric mean E. coli densities of the beach water could be established (Figure 3.1);
• None of the bacterial pathogens studied was found to have correlation with any specific swimming-associated symptom rates.
  

Figure 3.1	Relationship between E. coli count in beach water and illness rate

A number of bacterial indicators were measured during the studies. Among these, E. coli was found to have the best correlation with swimming-associated illness rates. Hence, it was the best bacterial indicator to estimate the health risks of swimming at the beaches of Hong Kong. E. coli is also an internationally accepted faecal indicator for monitoring environmental waters.

On the basis of the results of the epidemiological studies and taking into account local factors such as disease pattern, local immunity, population behaviour, exposure pattern, etc., the bacteriological WQO for bathing water in Hong Kong was established in 1992. The WQO states that the level of E. coli should not exceed 180 per 100mL calculated as the geometric mean of all samples collected during the bathing season from March to October. Samples should be taken at least 3 times a month at intervals of between 3 and 14 days. This WQO, which is related to health risks, applies to the bathing beach subzones of all Water Control Zones since 1 March 1992.

Figure 3.2	Water Control Zones in Hong Kong

The EPD has kept abreast of new developments around the world. In 1998, the WHO has drafted a set of guideline values for marine recreational waters after reviewing 22 worldwide epidemiological studies for bathing water.

The WHO's draft guideline values use faecal streptococci (FS) as a faecal indicator, and health risks are estimated for the protection of 'healthy adult bathers' exposed to the temperate north European waters. We have reviewed the scope of applying the new WHO draft guideline values, and concluded that it would be more appropriate to continue to adopt our existing standards for bathing water:

• Through the local epidemiological studies, E. coli is found to have better correlation with swimming-associated illness rates than faecal streptococci.
• The present Hong Kong standards have been established on sound scientific basis and are based on similar approach adopted by the WHO in deriving the guideline values.
• The swimming-associated illness rate in Hong Kong at the same faecal indicator level is lower than that proposed in the WHO draft guidelines. Therefore, the Hong Kong standards are already providing adequate protection of bathers' health.
  

To facilitate the interpretation of water quality information by the public, beach rating systems were introduced after the establishment of the EPD in 1986. There are two beach rating systems, viz. the annual ranking and the beach grading systems, which reflect the long term and short term beach water quality trends respectively.

Annual Ranking System

The first beach ranking system introduced by the EPD in 1986 was on the basis of the compliance rates with the Hong Kong and WHO standards for bathing water. Beaches were classified into four ranks according to the percentage compliance with the Hong Kong and WHO standards. In light of the findings from the local epidemiological studies, a new ranking system based on the swimming-associated illness rates has been developed and implemented since 1988.

Under this ranking system, the rank of a beach is determined by calculating the annual geometric mean E. coli level of all samples collected at the beach during the bathing season from March to October. Beaches are also classified into four ranks which correspond to the respective swimming-associated illness rates. The rank of a beach shows the average water quality of the beach for the whole bathing season and hence it reflects the long-term water quality trend of a beach. The current ranking system is depicted in Table 3.2.

Table 3.2	Annual ranking system

Beach Grading System

There are variations in the water quality of a beach due to natural fluctuations under different weather and/or tidal conditions. As the beach rank could not reflect these short-term water quality changes and in order to safeguard the health of bathers, a beach grading system was first introduced in 1987. Under this system, beaches were classified into 3 grades based on the running median of the 5 most recent sampling results and some arbitrary E. coli limits. This grading system is not related to health risks.

In light of the findings of the epidemiological studies, the E. coli limits related to swimming-associated illness rates were established and a new beach grading system having four grades was introduced in 1990. The new beach grade was calculated on the basis of the geometric mean E. coli level of the 5 most recent sampling occasions. The beach grades could reflect the short-term water quality trend of the last few weeks.

In order to further safeguard the health of bathers, starting from 1999, Grade 4 (the worst grade) is also given to a beach when its last E. coli reading exceeds a high figure of 1600 E. coli per 100mL irrespective of the geometric mean. This enhanced grading system implemented until now (Table 3.3) would provide additional information on the recent deterioration of beach water quality.

Table 3.3	Beach grading system