A runner, a hiker and a swimmer died suddenly over the weekend as the mercury dipped to 11.9 degrees Celsius yesterday - the lowest so far this winter.

A 49-year-old hiker, Liu, died at Pamela Youde Nethersole Hospital in Chai Wan yesterday after he was found unconscious in Sai Kung on Saturday night.

He was hiking in Sai Kung with his 45-year-old wife and his 16-year-old son at 10.30pm. When reaching the Tsak Yue Wu section at Pak Tam Chung, he collapsed and fell into a coma.

His son performed cardiopulmonary resuscitation on him and he was airlifted to hospital afterward. He died after midnight yesterday.

It is understood that Liu was a construction safety officer, scout leader, first-aid coach and an experienced hiker who did not have any underlying chronic illnesses.

A 60-year-old runner, Lam, was declared dead at Alice Ho Miu Ling Nethersole Hospital yesterday after he was found collapsed by passers-by at 9.12am following a morning jog at Island House Lane in Tai Po. Police are looking into his medical history.

Meanwhile, a 62-year-old man surnamed Lee died after his routine morning swim near Tai Po Railway Pier.

He was sent to Nethersole Hospital and was declared dead at 3pm.

Cardiologist Chan Ngai-yin said early symptoms of cardiac arrest during exercise include chest pain, dizziness and shortness of breath at an unusually early time.

"The runner was at an age in which coronary heart disease is prevalent," he said, adding that exercising during winter would exacerbate the strain on the heart.

"Under a lower temperature, blood vessels will contract in order to retain heat in the body, during which the heart needs to pump harder in order to supply sufficient blood to different parts of the body, leading to higher blood pressure," Chan said.

To avoid cardiac arrest, Chan advised people to warm up before exercising, such as by stretching and starting with low-intensity workouts.

The cold weather warning was in force yesterday, with the temperature between 11.9 and 17 degrees Celsius. Tai Mo Shan was 2.1 degrees in the morning.

As families gather today for Winter Solstice, the mercury is expected to dip to 12 degrees Celsius. But it will be warmer on Christmas Day, between 15 and 19 degrees Celsius.

A runner, a swimmer and a hiker who was at Pak Tam Chung, above, died as the temperature dipped.

Three action men die in cold