This rare occurrence - called shallow water blackout - might have led to the drowning death of a 12-year-old boy who was having swimming lessons in January.

An autopsy indicated that Danial Syawal Jailani had drowned in a 0.9m-deep training pool at the Clementi Swimming Complex.

State Coroner Victor Yeo (correct) yesterday returned a verdict of misadventure, or accidental death.

Investigating officer Inspector Bai Kelei (correct) said Danial, a New Town Secondary School student, might have been a victim of shallow water blackout.

Contacted by The Straits Times, Mr Richard Tan, president of the Singapore Lifesaving Society, said in an e-mail that blackouts can occur in swimming pools, ponds, lakes and the sea, and may be caused by over-breathing, or hyperventilation.

He said: 'Hyperventilation increases the amount of oxygen in the body while reducing the amount of carbon dioxide.'

This, in turn, triggers an urge to breathe, increasing the likelihood of shallow water blackouts.

Deputy director Delphine Fong, from the Singapore Sports Council, said the council knows of a recorded incident in which a swimmer may have experienced such a blackout in a pool and was successfully saved by lifeguards.

The coroner's inquiry heard that on Jan 28 (08), Danial, a weak swimmer, was among a group of 60 students at a weekly lesson at the complex.

Read the full story on Wednesday's edition of The Straits Times.