The World Health Organisation said Friday that an estimated one quarter of ashes from Iceland's volcanic eruption would be fine enough to penetrate deep into lungs, making them "more dangerous."
While no immediate health risk has been detected as the ashes remain in high altitudes, the UN health agency advised people with respiratory conditions to stay indoors should the ashes descend to lower atmosphere.
"The small particulates less than 10 microns in size are more dangerous because they can penetrate deeper into the lungs," said Maria Neira, director of the WHO's Public Health and Environment Department, in a statement.
"Analysis of the ash is ongoing and so far it is estimated about 25 percent of the particles are less than 10 microns in size," added the UN health agency.
The WHO advised that if people notice an "irritation in their throat and lungs, a runny nose or itchy eyes, they should return indoors and limit their outdoor activities."
"In case of increased air pollution, normal precautions are advised, i.e. avoidance of strenuous exercise by people with asthma and respiratory symptoms in days with high air pollution," it added.