As the Scottish Environment Protection Agency (Sepa) carried out further tests on ash found in Shetland, the Met Office said that dust had been detected across Scotland, south-east England and as far south as Exeter.
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Alex Salmond said that the initial analysis had not uncovered any harmful material inside the dust and water supplies have not been affected by the ash, but the World Health Organisation warned people with lung conditions to stay indoors if the volcanic ash settles in nearby areas.
The ash from the volcano in Iceland has potential health consequences because particles which measure less than 10 microns may reach deep into the lungs.
Dan Epstein, spokesman for the WHO, said that those with conditions such as emphysema, asthma and bronchitis should stay indoors or wear protective masks if the ash lands on the ground.
Mr Salmond said: “In terms of public health, an initial analysis by Sepa of dust samples recovered from Shetland has identified no harmful material. The sample is undergoing a full chemical breakdown. Sepa and Scottish Water have also confirmed that water supplies have not been affected by the incident.
“Of course, people with pre-existing respiratory conditions, such as asthma, are advised to take precautions, including taking medication with them when outdoors.”
A scientist from Sheffield Hallam University spotted some dust on his cycle ride into work and collected samples from parked cars. Tests confirmed the material was a mixture of silicon and oxygen, calcium, aluminium and sodium -- or volcanic matter. Dr Hywel Jones, consultancy manager of the university’s Materials and Engineering Research Institute, said: “It is certainly unusual. We have examples of Sahara sand sometimes covering cars but this is clearly a different sample.
“It is essentially volcanic rock that has been melted and frozen in the atmosphere.”
Professor Ken Donaldson, professor of respiratory toxicology at the University of Edinburgh, said that ash which had fallen on to the ground was unlikely to do much harm as a very high exposure to the
low-toxicity dust was needed to have much effect on people’s health.
“Not all particles are created equal,” he said. “In the great scheme of things, volcanic ash is not all that harmful.”