Health experts say the elderly and children are at risk from predicted temperatures of 40C and above for the next three days.

The mercury hit 38.8C yesterday and the Weather Bureau forecasts 41C today, 41C tomorrow and 40C on Wednesday before cooling to 29C on Thursday.

Health Department State health co-ordinator Tarun Weeramanthri said heatwaves had serious health effects and it was important people prepared for the coming week.

"There'll be a number of sporting training events, for example, which people might want to reconsider, particu-larly for kids," he said.

"We'd ask people to look at their own individual circumstances and take appropriate precautions."

Dr Weeramanthri said visitors to Perth who might not be used to the heat as well as the elderly, young people and people with chronic health conditions were at particular risk.

Many people took to the water yesterday and emergency rescue services were kept busy.

Three people were airlifted to hospital from Rottnest Island after separate diving incidents. A 39-year-old man who was scuba diving from a boat reportedly surfaced struggling about 9.30am.

A 40-year-old woman also got into difficulty while diving just after midday.

The RAC Rescue helicopter flew both to Fremantle Hospital where they were treated in the decompression chamber.

The man was in a stable condition and the woman was critical but stable last night.

A 66-year-old male diver with chest pains was treated at the Rottnest Island nursing post and flown out by the Royal Flying Doctor Service to Fremantle Hospital. He was stable last night.

At Scarborough beach, an ambulance was called for a 21-year-old man with a dislocated shoulder.

Weather Bureau duty forecaster Shane Hicks said a surface trough had combined with a high-pressure system to cause the scorching heat.

"There's a surface trough just off the coast near Perth which is stopping that sea breeze coming in and keeping temperatures pretty warm," he said.

The Water Corporation and Western Power are also braced for above-average water and power use in the coming days.

The West Australian

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