This year's haze was among the worst in the past decade and health experts say the effects of repeated exposure grow over time, and can even stunt lung growth in children.

"At levels of pollution much lower than what we are talking about here, young people in the most formative stage of their development experience stunting of lung growth," Anthony Hedley, professor of community medicine at University of Hong Kong said.

"I think that is a very likely outcome of these repeated intermediate to long-term exposures of the populations in Indonesia, Malaysia (and Singapore)," he told Reuters.

The fires, lit deliberately by farmers and plantation owners on the Indonesian islands of Sumatra and large parts of Borneo to clear land, emit particulates, sulphur dioxide, carbon monoxide, nitrogen dioxide and ozone, all of which can injure the heart and lungs.

This year's haze brought air pollution indices to worryingly high levels and forced many to stay indoors.

In Singapore, the government has advised people to avoid strenuous activities outdoors and to stay inside air-conditioned environments. Complaints of asthma and lung, throat and heart-related illnesses rose to nearly 15,000 in the first week of October, 600 more than in the same period a month earlier.

The region has not seen such a prolonged period of high pollution since the haze of 1997-98, and fires are still burning in southern Sumatra and parts of Kalimantan, although the air in Singapore and Kuala Lumpur has cleared in the past week.

Street sweeper Mohammad Soleh in Palembang in south Sumatra complained: "I can only work for three hours instead of the normal six hours. Even if I have a new mask every day, it doesn't help. I have to cover my face with a shirt."

"Even that can only last for an hour. After than I have to take a rest to catch my breath and soak my face.

In parts of Kalimantan, where the haze remains thick, children went back to school on Wednesday wearing surgical masks after a long break to celebrate the end of Ramadan.

COUGHING AND WHEEZING

In 1997-98, the haze blanketed a vast area, including southern Thailand, and cost the region billions of dollars in medical costs and damage to the tourism industry. The fires burned for months in part because of a severe drought caused by El Nino.

Every year since, the haze has returned in varying levels of severity, causing acute eye and skin irritations, upper and lower respiratory symptoms, coughing and wheezing.

Young children, the elderly and those with pre-existing heart and lung problems suffer most, experts said.

Mazrura Sahani, environmental health expert at Universiti Kebangsaan Malaysia, studied death records from 1996-2000 in Malaysia's heavily populated Klang Valley and found that deaths from respiratory problems rose by 1.1 percent for very tiny increases in air pollution of the sort produced by haze.

The smoke throws dangerously small particles into the atmosphere. Measuring less than 10 micrometres in diameter, or PM 10, more than 1,000 of these particulates can fit on a pin-head and can seep into lung tissue.

"My study has shown that during the haze, the most prominent pollutants are the PM 10, which are also among the most dangerous because it goes into our lungs," she said.

Hedley said forest fires throw out "ultrafine" particles smaller than 0.1 of a micrometre that even masks cannot keep out.

David Hui, head of respiratory medicine at the Chinese University in Hong Kong, said particulates of less than 5 micrometres can penetrate deep into the lower respiratory tract.

"It can cause inflammation in the lungs and small airways and result in scarring and prevent normal development of lung functions," Hui told Reuters. (Additional reporting by Mark Bendeich from Kuala Lumpur, Crack Palinggi from Palembang and Mia Shanley from Singapore)