SCIENTISTS have issued conflicting advice over health risks posed by the volcanic dust that has fallen over Scotland.
• A plume of volcanic ash rises into the atmosphere from a crater under about 656 feet (200 metres) of ice at the Eyjafjallajokull glacier in southern Iceland
Images were released yesterday of microscopic particles recovered in Shetland from the massive plume of volcanic ash that has left air services over Europe paralysed.
Experts from the Scottish Environment Protection Agency (SEPA) are studying the rare particles to analyse the chemical make-up of the volcanic cloud.
The tiny sample was taken from a slim film of dust found in Lerwick, where the sulphuric smell of rotten eggs yesterday pervaded the atmosphere for the second successive day.
Last night, as the dust from the volcanic plume spread south over the mainland, experts from Health Protection Scotland issued a warning to the public to limit their outdoor activities as a precaution.
A spokesman for the health agency said: "It is important to stress that the concentration of particles which does reach ground level is likely to be low and should not cause serious harm.
"If people are outside and notice symptoms such as itchy or irritated eyes, runny nose, sore throat or dry cough, or if they notice a dusty haze in the air or can smell sulphur, rotten eggs, or a strong acidic smell, they may wish to limit their activities outdoors or return indoors.
"Those with respiratory conditions, such as chronic bronchitis, emphysema and asthma, may notice these effects more than others and should ensure they have any inhalers or other medications with them."
He stressed: "Any such health effects are likely to be short-term. Health Protection Scotland, the Health Protection Agency and the Met Office will continue to monitor the situation and issue further advice or updates as necessary.
"Low concentrations of volcanic dust, which may contain low levels of sulphur dioxide, are also expected to ground with the plume, though this is not expected to be a significant threat to public health."
Dr Colin Ramsay, a consultant epidemiologist with the agency, said: "Based on the evidence we have at the moment and the assessments that have been carried out, we feel that the level of risk, if there is any at all, is likely to be extremely low.
"However, a stronger warning was issued by the World Health Organisation (WHO), which urged people with lung conditions to stay indoors if the volcanic ash starts to settle.
• The Scotsman's Mark Smith is among Britons marooned by the ash plume
Chris Epstein, a WHO spokesman, said: "From what we know at the moment, the majority of cloud ash is seven to 10km up in the atmosphere. Air quality monitoring networks have not yet reported particulate on the ground but, if it does reach the ground, it may have health effects.
"People with respiratory conditions should avoid breathing in these particles. They should have their inhalers and other medication with them."
Mr Epstein said particles measuring less than ten microns have the potential to reach lower airways and cause problems in some people. Therefore, those with lung conditions should stay indoors if the ash starts to settle.
He stressed the advice did not mean everyone should go indoors and disrupt their routines.
Professor Malcolm Green, a spokesman for the British Lung Foundation, backed others' health advice.
First Minister Alex Salmond said: "In terms of public health, an initial analysis by Sepa of dust samples recovered from Shetland has identified no harmful material."
Lee Winsor, a spokeswoman for Sepa, explained that, in addition to the small sample of tiny particles found on Shetland, scientists were analysing a sample of suspected volcanic ash taken by a Sepa scientist from the roof of a car in Aberdeen.
She said: "Sepa has completed preliminary analysis on two dust samples collected in Aberdeen and Lerwick. "Preliminary microscopic analysis has shown that the properties of the particles appear to be consistent with the properties of volcanic ash, but further, more detailed analysis is required and is currently being undertaken. We have not detected any harmful material so far, but we need to do further analysis.
"Monitoring of the situation is ongoing, but the current available evidence suggests that there is a minimal risk to human health and the wider environment."
The Met Office said yesterday that it had received reports of ash deposits from the Icelandic volcano being found as far south as Cardington in Bedfordshire and in Exeter in Devon.
• Icelandic eruptions could last for a year and 'second volcano could follow'