The smog could lead eventually to cancer and other chronic diseases if people inhaled sulphides and carbon over prolonged periods, doctors say. But the full extent of the damage may not be known for 10 or 20 years.
"Inhaling the pollutants like this is worse than just smoking which is done intermittently," said Dr Yeo Chor Tzin, a respiratory and lung specialist in Singapore. He said in the worst cases - such as Kuching, the capital of Malaysia's Borneo state of Sarawak where a state of emergency had been declared - sulphide and carbon gases were affecting people's breathing. This could result in chronic obstructive airway disease and destruction of the lungs," he said.
Dr Yeo said that in previous haze seasons he had seen a rise in cases of acute asthma and lung cancer was certainly a possible outcome of inhaling pollutants over the long-term.
Doctors say the phenomenon has not been studied long enough to be sure but there were similarities between smoking and the effects of the smog. "Cancer known to be caused by this haze will occur 10 to 20 years later. It will not happen overnight," said Hisashi Ogawa, a World Health Organisation (WHO) environmental engineer in Kuala Lumpur.
"We've never had such an experience in any other part of the world covering such a large area as South-East Asia. Normally, bush fires affect rural areas with small populations," he said.
Eye infections and anxiety may also be increased. "Eyes will be irritated ... The smog could cause conjunctival inflammation," an ophthalmologist in a Singapore hospital said. She said airborne particles could scratch the eyes.
Long Foo Yee, head of psychology at the Institute of Mental Health, said anxiety and panic could be heightened among those predisposed to it. "Anxiety and fear is a natural response to any disaster situation," he said. Some psychologists say confinement indoors and sun deprivation could trigger depression.