"Particulate matter is identified according to its diameter. The small particulates less than 10 microns in size are more dangerous because they can penetrate deeper into the lungs," says Dr Maria Neira, Director of Public Health and Environment Department at the World Health Organization.

Analysis of the ash is ongoing and so far it is estimated about 25% of the particles are less than 10 microns in size.

People with chronic respiratory conditions like asthma, emphysema or bronchitis may be more susceptible to irritation if ash is in the lower atmosphere in high concentrations.

"Since the ash concentration may vary from country to country depending on the wind and air temperatures, our advice is to listen to local public health officials for the best guidance for individual situations," says Neira. "If people are outside and notice 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.

WHO will remain in contact with experts monitoring air quality in the affected region, and advises people to follow the latest guidance from their local health officials.