The University of Hong Kong’s Faculty of Dentistry found that two out of five children, or 37 per cent of the 23,000 pupils it examined in the last academic year, had dental cavities.

On average they had 1.5 decayed teeth – just 0.1 tooth less than two years ago – according to the studies, which covered 125 kindergartens under its outreach dental service programme.

Although the overall prevalence rate of dental caries in kindergarten pupils had dropped from 44 per cent two years ago to 37 per cent, Dr Chu Chun-hung, from the faculty’s Community and Family Dentistry, said the situation was worrying.

In the United States, the prevalence of tooth decay for children aged between two and five was 23 per cent, and only 18 per cent for three-year-olds in Japan.

Chu said the situation was serious because so many children had tooth decay only a year or so after all their baby teeth had grown. He warned it was a misconception that baby teeth needed less attention as they would be replaced by permanent teeth.

“The eruption of permanent teeth begins when they are six years old until 12. Decayed teeth could affect their learning at school as there could be pain and fever,” he said.

Children studying in Sham Shui Po, Wong Tai Sin and North District had more decayed teeth than average – from 1.9 to 2 teeth – though the team did not look into an explanation for this.

But Edward Lo Chin-man, chair professor in dental public health at HKU, said it could be related to the parents’ educational and wealth levels. He said some parents neglected the importance of cleaning the teeth of their children as they think there are other more important things to be taken care of.

The research team found it better to use a liquid called silver diamine fluoride to control tooth decay rather than taking young children to have fillings.

Lo said this was not only cheaper, it was a much quicker method of treatment.

“On average it takes 15 to 20 minutes to refill one decayed tooth. It only takes two to three minutes to apply the liquid,” he said, adding that it was difficult to ask a child to sit tight since 90 per cent of them had never visited a dentist.

Lo said it was up to the government to expand its dental care service from primary schools to kindergartens.

In March, experts said more than 90 per cent of five-year-olds in the city with cavities would not get treatment because their parents lacked awareness of the problem. They also said there were patients who had not seen a dentist until they were in their 50s or even older.