The lead-in-water scare has spread to St Thomas' Primary School in Sham Shui Po, with one water sample found to contain more than four times the safe amount of lead.
The contaminated sample was collected earlier this month from a tap for washing hands in a classroom. Five other samples - in which lead levels were found to be within safety limits - were collected from drinking machines with distillers installed.
Water from the school's kitchen, where food supplied to about 500 teachers and pupils is processed, has yet to be tested.
Principal Tam Hsien-ming told parents that water samples from the kitchen would be tested shortly and distillers would be installed in the meantime.
"The school is highly concerned and has adopted a series of emergency measures," he said.
Tam said the school had taken the samples to a laboratory for tests after drinking water from a number of public housing estates had been found to contain excessive levels of lead in the past month.
Lead is particularly dangerous to children below the age of 13 because their developing bodies absorb more lead and they are more vulnerable to its damaging effects. Organs such as the brain, liver, kidney and heart can all be affected, resulting in behaviour and learning problems, lower IQ, slowed growth and anaemia.
It is difficult to rid the body of lead and other heavy metals. It can be done using chelation therapy, though this can have negative side effects.
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She developed "pAge drinking paper", which is a sturdy sheet of paper loaded with silver and copper nanoparticles that kill dangerous microbes.