By Alex Kirby
BBC News Online environment correspondent in Jeju, Korea
Scientists say they believe the dust and sand storms that for centuries have blanketed north-east Asia are becoming more dangerous to people's health.
Dust from China blows across the Korean peninsula
They think the storms are now combining with airborne pollutants emitted by human activities, and are adding to the region's severe air quality problems.
Similar dust storms from the Sahara have been blamed for spreading illness and destroying Caribbean coral reefs.
The concern has been raised with the United Nations Environment Programme.
Unep's governing council is meeting here in Jeju, in South Korea.
It was told four countries have joined in an effort to tackle the problem of the storms.
Records of severe storms here go back at least to the 16th Century: one account from the Korean capital, Seoul, in 1550 spoke of "a fog that looked like smoke creeping into every corner in all directions".
There is evidence that the storms are governed by a natural cycle. Youngsin Chun, of the Korea Meteorological Administration, said they affected Korea on 41 days a year in the mid-1940s, but on fewer than 15 in the 1950s.
She told BBC News Online: "The rate does vary, and we think the storms tend to be more frequent in warmer winters."
The trend in the last few decades has been upwards, with Korea registering 25 storm-affected days in 2002. In April that year dust levels in Seoul reached 2,070 micrograms per cubic metre, twice the level judged dangerous to health.
The World Health Organization estimates there are more than 500,000 premature deaths a year in Asia from outdoor air pollution. The storms have other effects, too, grounding aircraft, closing businesses and schools, and damaging livestock and crops.
Sandstorms are made worse by airborne pollutants
They originate in the desert regions of China and Mongolia and blow south over the Korean peninsula and Japan.
What scientists believe is happening now is that the intensity of the damage caused by the storms is increasing, and that they are combining with pollutants like soot and microscopic particles given off in vehicle exhausts and by power plants.
Researchers funded by the US space agency Nasa have found bacteria and fungi are transported in plumes of dust from the Sahara across the Atlantic. They say a fungus isolated in the dust from Africa, Aspergillus sydowii, has been shown to cause sea fan disease in coral reefs throughout the Caribbean.
China, Japan, South Korea and Mongolia (where 30% of the storms are said to originate) are pooling their efforts to lessen the impact.
Backed by Unep, the Asian Development Bank, the Economic and Social Commission for Asia and the Pacific and the UN's desertification convention, they are setting up a dust and sand storm monitoring and early warning system.