Forest and land fires in Southeast Asia kill an additional 15,000 people annually when the El Nino weather phenomenon grips the region, scientists have estimated.

The deaths are caused by higher particle pollution and higher local levels of ozone, both of which are stressors for the lungs and heart, they said.

Environmental scientists in the United States looked at airborne levels of particles from fires in Southeast Asia from 1997 to 2006.

The fires are annual events, set by local farmers to clear fields or forests.

But in dry conditions, the fires can ignite carbon-rich peatland soil that can burn uncontrollably for months.

The risk was highest in years of El Nino, the disruptive weather pattern that causes drought and dryness in the tropical western Pacific but heavy rainfall or flooding on the ocean's eastern side.

During the period under study, there were three El Ninos -- a whopper in 1997-1998 that was the strongest of the 20th century, followed by a second in 2002-3 and a third that began in 2006.

At these times, particle emissions from land fires were up to 50 times greater than during the La Nina phase, when the pendulum swing of El Nino goes into reverse and the tropical west Pacific becomes wet.

During strong El Ninos, fires accounted for 200 days when air pollution exceeded guidelines by the UN's World Health Organisation (WHO) for fine particulate matter, according to the mathematical model.

These particles, which measure less than 2.5 micrometres in diameter, are dangerous for health because they can pass through the nose and throat and lodge in the lungs.

When extrapolated for the region's 540 million people, many of them living in cities close to the fires, particulate pollution caused an average rise of 10,800 in annual deaths from cardiovascular causes during the El Nino years.

Added to that, said the study, are a likely 4,100 deaths annually from ozone.

A triple-atom molecule of oxygen, ozone is beneficial in the upper atmosphere where it provides a shield from harmful ultraviolet.

At ground level, however, it irritates the airways, causing a shortage of breath, coughing and chest pains, thus posing a threat to the elderly and people with cardiac and respiratory problems.

The findings should put pressure on authorities to prevent deforestation and damaging fires, especially in the context of climate change which could accentuate El Nino droughts, said the paper.

The research, published in the journal Nature Climate Change, was headed by Miriam Marlier at the Lamont-Doherty Earth Observatory of Columbia University, New York.