SWIMMERS heading for the beach this summer are warned that they might suffer minor skin irritations or stomach illness, as the water quality has deteriorated. According to the latest figures from the Environmental Protection Department, five of Hongkong's 45 graded beaches dropped from Grade One to Grade Two; that means, 10 or fewer swimmers in 1,000 risk skin or stomach problems. Four beaches dropped from Grade Two to Grade Three (15 or fewer swimmers per 1,000 at risk). None of the open beaches received a Grade Four rating this time (risk to more than 15 per 1,000 swimmers). But the Rocky Bay, Old Cafeteria and Castle Peak beaches continue to be closed because they have been getting a Grade Four rating in the past. The purpose of the grading system is to inform swimmers and the general public about the state of bacteriological pollution at various beaches. The grading of some beaches may vary during the summer due to a natural fluctuation in the bacteriological quality of bathing waters caused by rain and tides. The Government warned bathers to avoid beaches during and after a storm as beaches with highly-developed hinterlands are likely to be more polluted than their grades suggest.